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#861473 10/23/21 8:23 am
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Is anyone running GP2 carbs on a stock Spitfire? What compression do your run?

The GP2s are just under 30mm 1 5/32" around 29.3687mm? This head is just under 30mm round entry. But it flows quite a bit with a std valve.

[Linked Image from i.postimg.cc]

I'm kind of tempted to put 42mm valves in the head but it's already pulling 44"w just slightly in the manifold, with std 40.5mm. 400fps happens at 38" and it may be faster further in, but the port is that little the probe tube disturbs it. It should make quite a bit of power, theoretically around 70hp if it could do it.

It would be fun setting up those, though I'd want to already own some not buy them. And I'd be doing miles. But I'd run them on a good breathing head.
Good 34pwk are less than $50each but they are not GP2s.

[Linked Image from i.postimg.cc]


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Mark Parker #861477 10/23/21 11:37 am
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Hey Mark, I bought GP2's on Ebay for my 'Production' speed work.. I did a fair amount of street riding with them also. They worked pretty good but had to fiddle with the cables often to maintain decent idle. Probably around 10.5 or a bit more, 473 cam, good head, std pipes with standard looking mufflers. Best timed speed 125.
PS a couple of years later re-ebay'd.

Mark Parker #861480 10/23/21 12:40 pm
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Yes, I can just imagine riding somewhere getting lunch coming back and no carbs. They are a fortune. But they do look the business.
I like the ace bars and rear sets.

[Linked Image from i.postimg.cc]


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I bought half a dozen GPs and float chambers as part of a job lot I bought recently from the estate of a deceased local bike enthusiast.
They are 1 5/32" as used I think on the 66(?) Spitfires.
I have no personal use for them so am planning to sell them off --but hadn't got around to it yet.
This thread jogged my memory so if anyone is interested please PM me and I will dig them out.

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Mark Parker #861502 10/23/21 4:41 pm
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PM sent

Mark Parker #861516 10/23/21 6:49 pm
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I also have a set available of the GP2's complete with matchbox float, cables, and even hard to find OEM bracket for hanging the float. Also all original hardware.


[Linked Image from i.postimg.cc]


Bill B...


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Mark Parker #861535 10/24/21 2:43 am
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Why this could be fascinating is because of what may be possible with these carbs. I depends just how well they flow and how smooth bore they are and how well they fuel at high speed.

Even with the std inlet valve the flow is reading very good. A Spitfire engine with 10-1+ and free exhaust I think would be more a race engine than Street/strip? So a hp calculator predicts 75hp at the crank, or 66hp at the wheel being possible around 8,000+. But the gas speed should have power right through very strong. If it's as nice as the 34mm heads. It depends just how it affects it how much more hp it gives at low and mid rpm.

Guys racing these were mostly using the GP2 but were messing with heads on the fastest ones, not unusual to spend 2 weeks on the head. Wondering why the factory guys were never down there asking why their bikes and outfits were so fast.

[Linked Image from i.postimg.cc]
147.8with this bell.

That's considerable hp for an old 650 and makes one wonder if it was possible and that easy.

Last edited by Mark Parker; 10/24/21 10:36 am.

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Mark Parker #861553 10/24/21 11:41 am
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Did some digging and found the dyno sheets. Set up as in the picture we had about 56.5 at 7,400 correction factor .96. The fuel curve was pretty wonky. Same motor with 34 TM's and megs was 55.5 same correction. The TM's had a much smoother fuel curve. That same head was refitted with beehives which provided good control for several hundred more rpm.

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Originally Posted by Boomer
I also have a set available of the GP2's complete with matchbox float, cables, and even hard to find OEM bracket for hanging the float. Also all original hardware.


[Linked Image from i.postimg.cc]


Bill B...


I think the frame hanger should be simple enough to make, obtaining the rubber mounting point might be a little trickier to obtain?


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Mark Parker #861609 10/24/21 10:02 pm
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That's right on the money Tom, it converts to approximately 64hp, factory were saying 65hp and 66hp on a dyno sheet in 1970 so those little GP2s were getting the job done. Even with their difficulties. You would expect the 34tms to give an advantage in theory, except the flow that is holding it where it is probably wasn't at the carb end.

Does the dyno curve reflect the response of the engine? Does it measure it as in it's driving from lower rpm? The trouble with that is it may not always be full throttle, but I guess there is definitely a range of full noise.

I think I've reached a limitation with the 42mm valve in this. With the bell flow is up to 156, but just the port with its sharp edge is barely increased. Speed in the middle maybe 4mm in just the port, remembering 38" is 400fps, is 52". It's making a lot of difference having the bell. A 30mm Concentric doesn't look great but a GP2 if it's smooth bore may be quite different. But this seems the limit is the carb end and would rely on the carb being smooth and acting like a bell-mouth.

[Linked Image from i.postimg.cc]

Last edited by Mark Parker; 10/25/21 6:47 am.

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There is one of the Daytona Triumph 500,s in the AMA museum. It has 1 5/16 GP's. Another reference is that Bob Leppan had 1 3/8 GP,s on the steamliner..Also, there is room to bore the 1 5/32 to 1 3/16 but I opted to keep them standard in order to resell them. I think it all depends on the mission as to what size is best.

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A std GP may work like the bell-mouth if it's smooth bore. One guy was claiming much more than 66hp from his 654-670 engine. It's hard to say what's actually possible. I know it's possible for a 654 to go in away I never imagined.


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The problem with round slide smoothbore carbs is although they work well at WOT, at part throttle there is a lot of interference to the air flow. This is mainly due to the incoming air striking two surfaces with a space between them.
This is the reason the flat slide carb was invented.

In case anyone is interested
The slide on my race bike's 38mm carb is 2mm thick and the fuel is controlled by a valve operated by the profiled edge of the slide instead of a needle
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Mark Parker #861649 10/25/21 3:17 pm
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My '66 with the GPs is running 9-1 pistons with stock jetting. I have not confirmed the actual compression, but it is the pistons were listed as 9-1. Starts easy and runs well

Ed from NJ.

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THAT'S GOOD TO KNOW. HOW DOES IT IDLE.


I APOLOGIZE FOR THE USE OF CAPS. I CAN ONLY TYPE WITH MY RIGHT HAND SO USING THE SHIFT KEY IS BEYOND MY CAPABILITES.

The Devil is in the details.

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Mark Parker #861691 10/26/21 6:49 am
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Check this, one handed photography. This is pulling 56" up the wall. 38" is 400fps. Up to 600fps and more can be used in F1 engines. I don't know if this is the fastest point but the edge of the port messes it up. The bell now allows 159.6 through that 30mm entry. It's now 147.6 with nothing and the 30mm Concentric may knock it back even with a bell. It's now hyper sensitive to how the air gets in because it may be between 500 and 600fps a little bit of disturbance blocks it up I guess.

[Linked Image from i.postimg.cc]

This shows the finished R/h port for 42mm valve, the L/h is still std valve size that I guess would have been fine, but I have 42s and they can sit a bit higher for better compression. With a bell it may feed in better and the centre not be so fast, but then more air goes through. It seems fine, a bit noisy. A far cry from what goes through a stock port.

[Linked Image from i.postimg.cc]

Has anyone got a photo of the bore through an open GP2? Anyone race a 66 Spitfire? I'd expect this would take full throttle at fairly low rpm and just keep going. If a GP2 meant 150cfm and the speed didn't peak too high who knows what it would do. It predicts 77hp at 8,000-9,700 from flow, but probably not working on that sort of speed. So probably 8200-8500 if you could hold the bars. And port speed stayed under Mach .62

If this was on a '66 Spitfire in 650 Sportsman class it may put a cat amongst the pigeons.

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Spitfire Ken,
BSA had a host of solutions for getting a Spitfire with GPs to idle. Mine has what I believe was the final suggestion. Special cables with an additional ferrule at the carb end. A threaded slotted tube threads into the carb and allows you to raise the slide just like a Concentric or a monoblock, but you also have play in the throttle cable. That works reasonably well. I also found that if I set it up with a carb tune according to the directions, I would have a major stumble right off of idle. Richening up a bit got rid of the stumble.
The bike does not idle like a modern bike with brand new carbs. Its more like a bike with somewhat worn out carbs. You can take your hand off the throttle for a few moments, but much more than that and it will probably stall. Does require the occasional blip at stop lights. Once you open the throttle it goes great. Totally livable!

Ed from NJ

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I’ve heard of people drilling a hole in the back of the slide on Gold Stars, to get it to idle.


Amateur Loctite enthusiast.
Mark Parker #862041 10/30/21 10:30 pm
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mark, you mention the air flow capacity of the GP2 carburetters, but another possible limitation to their use is the fuel flow.

the stock BSA spitfire mk2 configuration was with paired 1-5/32-inch venturis, but a single 510 matchbox floatbowl. i'm sure that this 510 was completely adequate for normal expected use, but a lot of these carbs show up on land speed machines these days. in those cases they are generally new GP3 1-3/8-inch instruments, and invariably appear with twin matchbox float chambers, whether necessary or not.

i'm curious as to what the actual airflow is with a GP2, and whether the fuel flow keeps up at wot. i have no experience with them but i have just acquired a pair and will be experimenting


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I guess that may come in to it. I tend to think the carbs are designed to work hard and well, I'm not sure how they designed and tested them. But on an extra efficient head and motor they may actually come into their own. A stock heads flow isn't reduced much with a 30mm Concentric 109 to 105 or something with out a filter. But it knocks a lot of flow off these ports because the carb disturbs air. It may be the GP2s really excel at this flow rate, something the factory did not have. The change in valve by 1.5mm really upped the air speed it will do something, hopefully on the plus side.


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i dont know.

size for size they were said to flow more air than anything at the time. buy ive read that thrir clumsy transitional circiits were what led racers to replace them. certsinly the awkward idle adjustments and expensive parts were a major issue for street use. still are.

but the fastest mile bikes right now are alp sungertekins, and he runs GP3s himself and specifies them for the bikes he builds.

Last edited by kevin; 10/31/21 4:34 am.

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Originally Posted by kevin
i'm curious as to what the actual airflow is with a GP2,
My flow bench measurements found 8% greater air flow for a 1-1/2" GP than for an identical-size 38 mm Concentric. This potentially would give an engine 8% more h.p.

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Originally Posted by kevin
but the fastest mile bikes right now are alp sungertekins, and he runs GP3s himself and specifies them for the bikes he builds.
AMAL sponsors Alp, it's likely the carbs he uses on his bikes cost far less than what we might pay? The magic is in the engine build not in the carburetor, lol..
Better than flow numbers is power and performance seen on a dyno or track...Does an AMAL GP make more power than a similar sized Mikuni or Keihin flat slide carbs?


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I was watching an old show on Brabham's cars, early 1960s, he was using Coventry Climax motors. 4cyls with two Webers. They said 280hp and the little open wheelers were doing 170mph. His alloy V8 was also very effective but it had moved to fuel injection.

Those Webers make me think of the GPs and I agree the magic can be in the engine. No matter how good the carb, if 109cfm is your maximum flow, and it may be a little higher through that bell and smooth bore, there is a limitation because it cannot physically flow big numbers into the engine nor raise gas speed over a certain thing.

Like I worked out before, if it flows 150cfm it means around 77hp on a 654, and that's driving pretty hard from lower rpm depending on how that works out exactly. The 8% difference may well change at higher speed to favour the smooth bore. Like changing test pressure with more vac means more speed. And that speed may effect how the carb works through all the circuits.

So you can raise performance throwing on some GPs but you may be no where near the potential they actually have with a head breathing so much more, and we don't know.

Build a Spitfire Kevin, a Triumph isn't the same thing. Use a stock cam and just make a bomb proof bottom end. I'll get better measurements on the other side when I do it.


mark
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Originally Posted by Magnetoman
Originally Posted by kevin
i'm curious as to what the actual airflow is with a GP2,
My flow bench measurements found 8% greater air flow for a 1-1/2" GP than for an identical-size 38 mm Concentric. This potentially would give an engine 8% more h.p.

thank you for some real numbers. have you tested other carbs with that same diameter?

that advantage of the GP-style venturi at WOT is what really interests me. but i keep thinking back to what gordon jennings wrote back in 73, in his two stroke tuners handbook. i had never heard of the GP-style carbs until i read it. after discussing air jet compensation and emulsion tube ports, jennings writes

With maximum application of these mixture compensating techniques,
it becomes possible to employ very large carburetor throat sizes relative to cylinder
displacement, which is why the sophisticated Mikuni is a better choice than the AMAL
GP-pattern carburetor despite the latter's unquestioned advantage in air-flow capacity,
size for size. A 35mm AMAL GP will flow more air than a 35mm Mikuni, but you can fit
a 40mm Mikuni on an engine that would develop the blind-staggers with an AMAL GP
larger than 35mm in throat size.

in his opinion the undisputed better flow of the GP-style at any given size could be outpaced by the better fuel metering of other carbs of a larger size. if carb size isn't restricted for some reason, then swapping the GP for a bigger instrumnt of some other design might exceed its performance.

was he right? the only way to know for sure is to try em and see


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Originally Posted by Hillbilly bike
Originally Posted by kevin
but the fastest mile bikes right now are alp sungertekins, and he runs GP3s himself and specifies them for the bikes he builds.
AMAL sponsors Alp, it's likely the carbs he uses on his bikes cost far less than what we might pay? The magic is in the engine build not in the carburetor, lol..
Better than flow numbers is power and performance seen on a dyno or track...Does an Amal GP make more power than a similar sized Mikuni or Keihin flat slide carbs?

th ecurrent price for a new 1-3/8 GP2 is US$800, and a matchbox float bowl is over US$250. thats a bunch of bucks. ive not seen any new-model GPs for sale second hand, ever. i guess its a pretty loyal market.

but see, the "similar size" question is the key. if you can cheat your way around the GP's flow advantage by making a bigger AMAL MK2 or a Mikuni or a dellorto or something work on the same engine, then it doesn't have an advantage anymore.


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Originally Posted by Mark Parker
Like I worked out before, if it flows 150cfm it means around 77hp on a 654, and that's driving pretty hard from lower rpm depending on how that works out exactly. The 8% difference may well change at higher speed to favour the smooth bore. Like changing test pressure with more vac means more speed. And that speed may effect how the carb works through all the circuits.

So you can raise performance throwing on some GPs but you may be no where near the potential they actually have with a head breathing so much more, and we don't know.

Build a Spitfire Kevin, a Triumph isn't the same thing. Use a stock cam and just make a bomb proof bottom end. I'll get better measurements on the other side when I do it.

youve hit two things, mark. no matter how much the carb flows, if the limitation is in the ports the carbs wont help until the ports are fixed first. and maybe the 8 percent advantage on a flow bench becomes 10 percent at actual working conditions, where the air has non-STP pressure, temperature, hmidity, velocity, and so on. im way out of my depth talking about that stuff, and wouldnt know what to do with a flow bench if i had one.

i like the 650 triumphs, but their cylinder heads were designed by the druids. i really like the spitfire idea, too, but i think i'd have to wait on a winning lottery ticket to get one.


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Originally Posted by kevin
i keep thinking back to what gordon jennings wrote back in 73, in his two stroke tuners handbook ... was he right?
Following is something I posted five years ago:
------------
Back in the '60s I used to look forward to Jennings' articles because of the technical content. Unfortunately, the more I learned the less Jennings knew, and by the end of the '60s I realized his articles only had looked good by comparison with other ones because the others lacked any technical content at all (in the land of the blind, the one-eyed man is king). Anyway, in the first article in that series he wrote what he said was everything anyone needed to know for how and why to switch to an automotive coil. However, the next three articles all basically started with Jennings explaining that he had learned a lot since the last article and the current one now has the actual story. Sigh...
-------------

Jennings did not have an engineering background, which certainly doesn't mean he couldn't have learned, but the following quote from an obituary in Car and Driver magazine says a lot about his approach to technical writing: "He liked ornate expressions and quotes, and he laced his copy and conversations with them." A phrase like "an engine that would develop the blind-staggers with an Amal GP" sounds great in print, but did Jennings write it because of the way it sounds, or because it is true? I haven't read his 'Two Stroke Tuner's Handbook' for many years, but I'd want independent confirmation of something like the above before I would believe it.

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Originally Posted by kevin
have you tested other carbs with that same diameter?
As the two graphs in one of my posts indicates, I made a lot of measurements on my flow bench to determine the modifications to make to 1036/1038 Concentrics to make them work well on a Gold Star.. Many of those measurements as a function of throttle position used 932 Concentrics because I had three of them with 'Standard', Norton, and two-stroke spray tubes, but I only measured the flow at full throttle on the 1036, 1038 and GP to see how much potential top end performance each of those carburetors had. Unfortunately, the only Mikuni VM38 that I have came without a slide so I couldn't measure it.

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Tom mentioned the nice linear fuel curve on a Mikuni flat slide... Many carbs go rich at maximum flow...So you might be flowing more air but making less power with the wrong air/fuel ratio..
The only test that matters is installed on a running engine..


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Originally Posted by Magnetoman
Originally Posted by kevin
have you tested other carbs with that same diameter?
As the two graphs in one of my posts indicates, I made a lot of measurements on my flow bench to determine the modifications to make to 1036/1038 Concentrics to make them work well on a Gold Star.. Many of those measurements as a function of throttle position used 932 Concentrics because I had three of them with 'Standard', Norton, and two-stroke spray tubes, but I only measured the flow at full throttle on the 1036, 1038 and GP to see how much potential top end performance each of those carburetors had. Unfortunately, the only Mikuni VM38 that I have came without a slide so I couldn't measure it.

well there went three hours of my life

i run 35mm keihin FCR flat slides on a 650 LSR triumph. i dont know what they flow, but they have a decently open venturi at full throttle:

[Linked Image from i.imgur.com]

[Linked Image from i.imgur.com]

the old 1966 BSA spitfire ran GP2 instruments that were 30mm when the slide was all the way up, and didn't have the needle in the way. just looking at cross sectional area, the FCRs have some 36 percent advantage over the BSA GP2s. the run well at small throttle openings and large throttle openings, and burble through th midrange reversion. so they seem more or less okay on size.

but do they flow 36 percent better? how do they compare? the FCR itself is newer than the GP2-patter, but its still old in its own right. just not as old.

im curious now. what would a spitfire do with the FCRs? what would the LSR triumph do with GP2s?


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I've been down this track a bit. I compared a 34pwk to a 34vm they are very similar. Lectrons are very good. And that 35kehin looks good. But sizes and port function surprised me.
34pwks on a 650. For some reason though you sort of expect it may not pull so well down low it's actually better, more responsive. Top gear, come to a hill and touch the throttle and it's strong. Especially for a 650. It's no bottomless pit but it's very nice. So whatever is happening I think it's a combination of speed and volume.

I've had 38mm ports that flowed less through 44.5mm valves, on bigger engines. The 30mm port for a GP cannot out flow a 34mm port with the same valve it is higher speed though, and the 34mm is very high speed, that speed determines hp the 30mm is extreme speed, multi valve F1 is 420fps in the book the best 2 valves quoted around 340-360fps. It does say up to 600fps + on F1 engines so what that does I'm not sure. The 650with 34s goes nuts on full throttle and needs a very strong clutch as stock will not cope.


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Originally Posted by kevin
well there went three hours of my life
That's exactly what I said when I finished the flow bench testing, except it wasn't three...

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A 32mm port and std.

[Linked Image from i.postimg.cc]

Down the 34mm pwk. You can see the non smooth bore cut for the chrome slide, it still manages around 160cfm and excellent speed. They are under $100 the pair.

[Linked Image from i.postimg.cc]

You can see the oval ring at the manifold head joint and the darker fill on the port floor.
Just going back to a hp calculator.

With 109cfm from the std head a street strip engine estimates 51hp, with a race engine (10-1 comp and GP2s) 56hp pretty much what was quoted power from the factory. In 1968 they dropped the compression to 9-1 and fitted 32mm Concentrics and ported better 120cfm on a std motor 57hp, again right near the factory figure. So 130cfm in a 654 factory race engine predicts 67hp and the factory dyno said 66hp. It's predictions are within 1hp. But what happens when you push that to 150 or 160cfm or more?

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From my experience with the GP's the matchbox will not flow enough fuel for sustained full throttle. Triumph used two floats for their racers at Daytona and elsewhere. For ease of use and good power the NYC Matchless G50's and Manx use 44VM.......But the fastest Manx in US has a GP. To paraphrase Hillbilly, it's the total package.

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not all GPs are created equal. the spitfire's were 30mm, while the current mile machines with them are 35. the gold star stuff is mostly 1-1/2 inches isnt it?

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okay. i'm in.

bill was kind enough to put some GP2s up for sale, and so i bought them

[Linked Image from i.imgur.com]

[Linked Image from i.imgur.com]

complete set. two 1-5/32-inch bodies, one matchbox float chamber, and lots of bits

some loose slides. 3, 4, 5 and a 6

[Linked Image from i.imgur.com] [Linked Image from i.imgur.com]

an extra body

[Linked Image from i.imgur.com]

original Fasteners, brackets and air needles are there

[Linked Image from i.imgur.com] [Linked Image from i.imgur.com]

lol

including what look like original hose clamps

[Linked Image from i.imgur.com]

only one float chamber, but im arranging for anothrr


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I have a head, who has a Spitfire?

Or a stock A65 bottom end? Late model is better. Is there something you could put an engine in Kevin? A pristine Spitfire would probably have the most value, if someone has bits laying around.

Or how close are you to Pushrod Tom? I guess you are at least on the same continent. Maybe BSA owners on forums and stuff could chip in to just see how effective the GP2s can be? Tom has pushed that already but there may be more. And if that was happening maybe AMAL would help if you need bits.

MAP do rods, EdV pistons and G/box support plate and bearing. It's not like the bike bits are rare.

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This is kind of starting at the wrong end of the problem, isn't it? Fix the valve end first then work back to the carb. But even if you fix the head and get more power, you still have the flexing crank problem due to it being suspended on the far ends, then the barely adequate clutch suspended way out on the end of the mainshaft in a gearbox with only four ratios, et cetera.

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4 speeds, even stock ratios don't matter that much but it needs gearing up. If the motor pulls hard, gears are not a big thing, just put it in a tall one and go. Power does not mean rpm. It kind of does but doesn't, there is a weird disconnect. The side car 745cc beats 920s and 840s and 1200 Vincents, some 5speed, all on Methanol in a straight line I think. And Matt revs it to only 6,800. 10 starts 8 wins and a 2nd because it was jumping out of gear. Matt's been working on a better rear tyre for more grip and less wheel spin.

3rd bearing on G/box main shaft and on the crank outside the alternator, everyone knows that. Triumph racers used to have the rotors machined so they didn't weld themselves to the stator. A bearing is better. Balance for rpm. Get good clutch parts and beef it up. Steel rods. A timing side roller means longevity and ease of replacement if you could wear one out. Not hard to set up if you want that.

When John Brittain designed his vee-twin he started with the inlet ports. A bit the same as 1300hp+ two valve per cylinder pro-stock hemi engines with dual 4 barrels.

[url=https://postimages.org/][Linked Image from i.postimg.cc][/url

The reality is simply this; the intake port is barely bigger than stock, it's a more refined shape and air goes through easier. And 37.6% more, sort of round about, depending on the carb. So 56hp in theory + 37.6% becomes 77hp. And that may be the case except the port is small and the air travelling so fast ramming that mixture in and that's happening at low rpm as well, again boosting hp right through the range, provided it doesn't exceed Mach .62. How a smaller 2 valve engine can decimate a bigger 4 valve Cosworth right through the range. That's what this is fiddling with. I'm not sure what little GP2s will do. But 34s do seem to have this effect, it's a really fun thing.

Just to add the crank and stuff should be fine seeing Tom runs his LSR twins with very high hp.

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the GP2s will be a long term project for me. i have a 650 track bike build going on. and i may put em in my LSR mile bike to see what happens.

where is the crossover between velocity and cross sectional area? they used to say the spitfire 2as good for 125 mph, but people dispute it. toms hornet got 125 with them. dunno


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Kevin, get a supply of jets and dyno the bike with the different carbs for a comparison .....You need some real life info...
So you're thinking the 35 mm race bred Keihins are too large, or the way, flow less air than the 30mm AMAL?


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Har Har, That 125 came after years of trying how to make it work. The GP's were for the Production class. Street trim, mufflers etc. They were about even Steven with 34 TM's. But TM's much more even fuel curve. I would say that the BSA promoters did not exactly open the crate of a random bike for their speed claims! Would suggest a very carefully prepped bike. I bet the ad sold a lot of bikes! Mark ll Sexy as hell and the GP's badass!

PS Rode that around on the street a bit and the GP's worked pretty good. Memory says that I used a one number or half number leaner slide.

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Originally Posted by Hillbilly bike
Kevin, get a supply of jets and dyno the bike with the different carbs for a comparison .....You need some real life info...
So you're thinking the 35 mm race bred Keihins are too large, or the way, flow less air than the 30mm Amal?

im just curious. i agree the test for these is to put em on a running engine, get the mixgure right, and measure.

the FCRs i have are very good and pretty latge. leo goff used 35 mm FCRs to go 146 on his norton quarter mile machines, and theyve got me to 135 in the mile point five, and over 130 easily in the mile. they have me wondering whether i should have followed other toms advice and over-carburated to 40mm.

i dont have a BSA with more than one carburetor


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Originally Posted by pushrod tom
Har Har, That 125 came after years of trying how to make it work. The GP's were for the Production class. Street trim, mufflers etc. They were about even Steven with 34 TM's..

see, ^^^your work is what got me curious. did you have a rob hall head on that bike at the time? you have lectrons on your mile bike right now, dont uou?


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Mark mentioned Pro stock car drag engines. In the USA they are 500 cubic inch push rod 2 valve engines.Like Nascar, they are built to a strict set of rules....About 1500 HP at 10, 000 rpm, the intake ports flow over 700 cfm, the valve lift is around .750 inch or more. There are several racing organization with slightly different rules.NHRA rules require wedge heads not hemi...I don't believe the wedges make less power..
If the engine heads were not rules restricted the heads would be four valve for sure...
Back to Brit junk, Supposedly a too small carb working at full flow is more efficient than a larger carb working at lets say 85% of it max flow...Who knows....


79 T140D, 89 Honda 650NT ,61 A10 .On a bike you can out run the demons
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But a V8 engine is a good start for me
Think I'll drive to find a place, to be surly"
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Originally Posted by Mark Parker
I have a head, who has a Spitfire?

Or a stock A65 bottom end? Late model is better. Is there something you could put an engine in Kevin? A pristine Spitfire would probably have the most value, if someone has bits laying around.

Or how close are you to Pushrod Tom? I guess you are at least on the same continent. Maybe BSA owners on forums and stuff could chip in to just see how effective the GP2s can be? Tom has pushed that already but there may be more. And if that was happening maybe AMAL would help if you need bits.

MAP do rods, EdV pistons and G/box support plate and bearing. It's not like the bike bits are rare.

Well Mark, I'd be interested in building an A65 land speed bike, but they're expensive (they all are) and there's not much return for my money except bragging rights. I have a set of '70 cases, a set of '68 barrels and a head, even a set of T10 GP2 carbs, but not much else. I think I'd get some help from Mark Appleton at British Cycle on used parts, but most of his used stuff is well used. I'd probably spend a few grand on parts just to get it going, and the motor has to be sleeved down or de-stroked to make it eligible for the 650 class. I just got back from Colorado and the Salt Flats so I'll be in Nova Scotia until probably April. If I can get a frame, barrels ('70), and a good crank, and decide to spend the money on pistons, cam, connecting rods, and the other parts, I can do the rest. I'll be in contact with Mark Appleton soon, but so far, he won't let anyone into his used parts supply because of Covid. I'll keep you posted.

The other thing you have to remember is that in land speed racing, horsepower is king, damn the torque curve! That means rpm x torque, and the shift from 3rd to 4th determines if you can get thru the aero wall.

Tom


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Ben's building a P4 race bike that will be a 654 with 34mm head and Tom has one to test so we'll see how they go. I think ultimately they are better because they flow more and are very fast. And they may be better again on a 744 but they are not detrimental in any way on a 654. I just did this small port head because some Spitfire owners like their exotic GP2s and I'm curious.

The 34mm works very well on a race outfit here as well. You would think cubes would win out with outfits hauling two people but they do not. 745cc beat 1200cc and everything else in P3 and lowered a lap record by 4.4seconds, it surprised people and they had trouble believing its actual displacement and fuel. And the self imposed rev limit of 6,800. It seems all wrong, but is what it does. It's unusual.

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Originally Posted by Mark Parker
It seems all wrong, but is what it does. It's unusual.


its not that it seems wrong, its that it goes against what people want to seem right..

the small GP2s are small but the flow is fast. the big throats are large but the flow is slow.

slow means that it takes time to get stuff moving through a valve as it opens. on a bench with a static open valve you might not see that unless you pause to think.

it is what it does, as you say.

got to test to see.


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Originally Posted by Hillbilly bike
, Supposedly a too small carb working at full flow is more efficient than a larger carb working at lets say 85% of it max flow...Who knows....

test, test, test.


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'the small GP2s are small but the flow is fast. the big throats are large but the flow is slow.'

In a way, but a 34 on a good flowing head is fast like an F1racer not a race spec 2 valve that may be 330fps or 360 with cnc porting. It's probably over 450fps.

But not what the 30mm port is in speed, but the flow quantity is more with the 34. It may be the 30 drives the air in harder but not as much, then a big inefficient port may not push it in much at all. So efficiency is speed and quantity, just quantity doesn't do it. Unless it's at high rpm then power is still less in comparison.


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i have no experience at all to have a valid opinion, but think about inertia. when the intake valve is closed, flow stops. then it opened iand the charge waiting in the port gathers up its skirts and starts moving into the combustion chsmber.

like you say, too small is simply too small. but maybe just a bit biggrr than too small is bettrr than a lot bigger.

a smaller passgae gathers up and speeds up and past the valve pretty quick, whereas a huge slug of mixture gets going more slowly and chugs down the port.

maybe the smaller faster charge puts more mixture past the valve thsn the bigger slower charge, simply becsuse the larger charge has a lag time before it gets up to speed. maybe not.

there would be a crossover point on the curves-- at one side small is better, on the other side, big is better.

where is that crossovrr?


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It seems like the carb and port tube isn't usually the restriction, (going from 30mm GP to a ported head probably needed the 32mm Concentric but the port isn't 32mm very far in). On the '71 I have it's factory ported but still 30mm.

It's getting around the the guide and stem, turning and getting past the valve that is the trick, but getting +54% more air through a 34mm carb and 42mm valve is really radical. The port size is not 54% bigger. Using a bigger valve 65-70% more, still through a 34mm port. When you get the shape the flow jumps.

The higher flow for the size the better it is, and the work is near the valve. The cross over is probably with the 34s if you flick it right open at low rpm. But with just a bit it responds fast and wind it open and it goes nuts. Putting at 2,000 is fine, open it a bit and it instantly goes.

If the big 38mm port could flow 220cfm or something it would be faster and better. It's probably not bad as is but for Ben's 750 we are trying 34mm carbs and a smaller tube section, it's flow isn't down much at all. But the speed increase should make it go harder and earlier. He just has to get it registered again and fit it.

Just going back to the Spitfire based racers. One of the guys that worked in engine development built his own very reliable engines and they were second in the endurance championship because, they say, they were forced to ride a works triple in the final 24hr race at the Bol, before that they were leading the championship with their A65, though it was pitted against 750s. His bike was timed at 134mph in England, and he claimed it had shown 74hp. And was 670cc on +.040" pistons.

Now with the 34mm head and 42mm valve the best flow I've yet had and this is through a carburetor predicts 86hp @ 9,380+ rpm where Ben's P4 bike isn't going, and I'm very doubtful. But whatever that 650 does produce, I think it will be quite a bit, and it needs to be bullet proof. The numbers can be wrong, and exhaust and stuff needs to be good, but it should be enormous fun and should make 750cc unnecessary.

[Linked Image from i.postimg.cc]

I've got my bit done. But he has to get tank and stuff for the OIF he has.

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Well, on this thread we have the builders of the three fastest 650 cc pushrod modified production LSR bikes in the USA, maybe the world . Two Triumphs, one BSA, all have Rob Hall heads , all have large non GP carbs...it's an extensive collection of on the track experience....Who will be the first to try the small carbs....


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But a V8 engine is a good start for me
Think I'll drive to find a place, to be surly"
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I tried a set of GP2's on my twin carb headed A10 and which didnt result in anything faster than my 32mm AMAL Concentrics. Of course my A10 was raced 100% of the time in street trim including the license plate. Probably could have seen higher speeds if I stripped it down to nothing but that wasnt my goal with that particular bike.

I have given it some thought to running larger bore GP's on my partially streamlined 750cc stroker A65 but the cost of the setup is astronomical. Currently have 34mm flat slide Mikuni's and with the stock head, have netted some really good results (134mph with basically shifting from 2nd to 4th due to worn 3rd gear dogs) and cant wait to finaly see what Mark's head work does to the power.

For the smaller diameter GP's, I think they have there place more so in road racing where your on and off the throttle where velocity of the fuel charge would be beneficial. For LSR though I think, like Tony mentions, the larger more modern type carbs seem to be top runners.


1955 BSA Bantam D1 Plunger
1956 BSA A10RR Street and LSR Bike
1961 BSA C15S
1966 BSA spitfire
1969 Triumph T100C
1970 Triumph TR6R
1970 Triumph TR6C
1972 BSA Lightning LSR Bike
1974 Triumph T150V
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Ok it took a bit, they looked the same but didn't work the same. A close look and measure and adjusting it's now close enough. This left side still has studs so I fitter a 30mm Concentric, but it has no needle and that will effect it, so 146.7 if a GP flows 8% more that's 158 but it probably doesn't work quite like that. Though that's what it reads with just the bell. I didn't check speed but it's the same size. Even taking lots off for probable exaggerated readings it would be fun.

It would give good feedback to see a power graph on that LSR 750. More or less the same configuration works in an outfit. The grunt that had before is amazing. At 6,000rpm it's 20hp up on a stock CB750. 50%, if it keeps that and pushes the peak up around a 7,000 it would be unreal.


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Chris Vincent used GP's on his outfit, i spoke to him about that and he said
he tried a weber but never pursued it as he was sponsored by AMAL and they
spent a lot of time sorting them out on his bike. He said i could have his old
weber head if i wanted it but apparently it was full of devcon (i spoke to his
passenger about about it and he was laughing. I believe that he actually had
John Passini do some prelim testing and dyno work on the weber setup but
it was abandoned. Similar with ignitions, he had a Lucas racing mag grafted
onto the rear of his engine using made up timing cover etc.Lucas also contributed
to a few other outfits. George O'Dell's Windle beezer was the same story.
The BSA comp shop did heaps of stuff on various outfits 'After Hours'.

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And as many if not more comp parts went out of the back door as went through the front door.
Same as at Triumph.
DAMHIK!

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Different world back then eh? Remember the cops wandering around pitts checking
engine numbers etc? They always went to sidecars first............

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I found some conversion charts from inches of water to MPH then to foot per second. So just 4mm in the left port its 57"w. What I don't know is speeds down in the port as the tube is a bit big for the little port. 61" is 345mph or 506fps so that's fine, it could actually go to 85-95"w and only be around 600fps Mach .55 still ok but there is a cross over in making that air move that sucks power which happens till around 600fps and power falls off.

So the 30mm ports will use this ramming speed in a higher speed range. I guess somewhere in the middle is where the power gained to power used to move the air gives the best result. And where it gives the best throttle response. When that speed falls enough is when we get reversion and mixture is blown back out the carb. I've seen this on a big cammed Triumph years ago. But all that cam was doing with its extreme timing was trying to coax a little more air into it at high rpm.


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Mark Parker #862502 11/05/21 10:42 pm
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Just to add perspective; when I was young I was at a bike shop watching a very hot 650 Triumph idling. It was idling very fast and rough and fuel was spraying from the carbs. It was their race bike with a very hot cam. And we understood this cam was designed to add hp at the very top end, and this thing should come on song and go. If a std cam gave 52hp and a super hot cam gave 55hp it means an increase in air flow of 6cfm. But if your starting point is a very well designed hot factory cam you may not achieve such an increase. But to get the increase that's the extra breathing. But what if instead of increasing that air to burn by 6cfm you increase it by possibly 40cfm? And that's what this post is about.


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Mark Parker #862508 11/05/21 11:22 pm
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Don't forget inlet port length either. Very important with hairy cams.

Mark Parker #862510 11/05/21 11:32 pm
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Mark, you mean like the gas cloud on my LSR Triumph? This is commonly seen on drag bikes and cars...Part of reversion I believe..The cams are not radial as noticed by the idle..


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Yep that was the sort of thing but fast idle was doing it. Conventional wisdom would say that reversion will go when port speed overcomes it and it will go hard. But maybe that fundamental is a little flawed if you can pick up significant flow another way. That's what I was getting at.

Pipe diameter makes quite a difference as well because big headers may be needed for top end. Pipes and X connector can effect top end quite a bit. I've tried to explain that X pipe on a twin but people maybe don't get it. Size shape length make differences but a friend tested the difference with an X connector and compared it to two unconnected pipes on a dyno.

Rob said; "a cb350 that was run with an X pipe and then switched the X for a straight thru making it two individual pipes. The X pipe made lots more power up top but was better all the way thru."

I've also never had a 2 into 1 that worked as well as the X. It doesn't mean it cannot I suppose.

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Everything goes back to how much air goes in and how much is burned. Where you want the engine to kick butt. I must admit I like the rough idle of a big cam, but if it puts carbon in the intake port and on the valve just riding around it isn't good.

Nick did the outfit go better with the hairy cam or the SRM race cam? Its specs at .020" are inlet 45-78 exhaust 81-45, it says lift .372"@at the cam whatever that makes at the valve. Gary is really enthusiastic about it.


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The srm cam is much better than the previous one on the 740 as it has more lift
and not so much overlap. It makes the motor pull better across the range.
The other cam suits the 650 buzzy engine very well though, it gives more
mid range and top end than the standard cam.

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The Sifton 390 cams in my engine have about the same lift as a Spitfire but less seat to seat duration. On the dyno the engine made best power with two individual pipes 1-1/2 x 34 inch,and the shortest intakes..Exhaust cam lobe center at 107....Track performance may differ from the dyno ...The 1937 Triumph head design probably has different needs than the BSA head...


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Sorry, I'm a bit late to this party!
At the risk of saying what is old public knowledge.

The RNs were introduced to provide more air flow at full throttle than TTs, the GP was a replacement of the TT bodge that was the RN carb! (mainly in response to Norton's race shop demands), the GP venturi is smooth all the way through (line bored with bell mouth and slide chamber installed (hence the numbering seen on the carbs to avoid mixing) because of the needle location, only having slice openings where the slide passes through the slide chamber and as such only work properly (as they were intended to do) at major to full throttle openings.

At less than major throttle openings the fuelling circuit is too long for good response (GPs used to be renowned for 'open the throttle and wait for the engine') because of the path to the remote needle. A stronger fuelling signal can be achieved but requires work at the spray tube, needless to say the spray tube revision has flow implications at full throttle. The long air path is totally detrimental to the operating principles of the AMAL carbs (which date from 1918ish) so it is unsurprising it does not work properly!
The angle of bell mouth intake is (allegedly) too shallow for best efficiency, I have nothing factual to support this statement..... but AMAL alledgedly did flow work research around the intake of the bell mouth, I have seen (and used to own) very old works Norton bell mouths whose fully rounded intake lip and angle of taper resembled that of the later Concentric, so maybe some truth there??

GPs are a pig's ear to carburate properly at low throttle openings (see above), my latter day personal choice is Mr Gardner's delightfull object and they consistently produce 3 - 5% more power with better response than amals of the same size. The fuelling and metering principles of these carbs is totally different to GPs (hence the reason why they work consistently, but confuse most people!!)
The GP matchbox float was renowned for frothing and screwing the mixture - hence most race bikes of the period junked them or took a lot of trouble to anti vibe mount them.

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Originally Posted by Mark Parker
Has anyone got a photo of the bore through an open GP2?

Somewhere I have a shot taken through the GP2 of my Manx with the throttle jammed full open. It's just a long shiny cylinder with the head of a valve at the other end, no obstructions.

Low throttle openings were not an issue as the general state of tune meant that to get any result revs had to be above 5,000.

Last edited by sammysnail; 11/09/21 12:59 pm.

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Mark Parker #862754 11/09/21 2:47 pm
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[Linked Image from i.imgur.com]

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smooth, round, with no inverted teacup above as in carburetters with solid slides

the hollow slide goes up and down in the circular cutout, like the old telescopic gas holders


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GP's, TT's, and Monoblocs are all smooth bore carbs. The hollow slide creates "the teacup" at all positions other than WOT.
Solid slides give a better airflow at all positions except WOT.
Flat slides are the best of both worlds, less disturbance at part throttle and they disappear at WOT to leave the bore unobstructed.

Look at the bikes winning classic races, you will be hard pressed to find a GP carb. most use Gardners


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Mark Parker #862773 11/09/21 8:12 pm
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whats the difference between a gardner and the old posa fuels?

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Andy Higham #862786 11/09/21 10:09 pm
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Originally Posted by Andy Higham
Solid slides give a better airflow at all positions except WOT.

Do you need better air flow when you’re deliberately restricting the intake by using a small throttle opening?


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Racing may spend more time with wide open throttle and using that maximum drive that has a nature to it. And a nice linear pick up is good.
I think we are talking about the nature of air flow at part throttle which effects the lower speed circuits of the carb. The consistency and stability of the draw on the needle compared to turbulence or a weak pull.

I bought some second hand 36mm Lectrons for maybe $100 many years ago. I could set them up very well for small throttle openings even after boring them to 38mm.

I actually checked fuel economy droning down the Hume from Sydney to Canberra, probably a constant 80mph or so, just a little faster than most traffic to be safe. 90mpg, imperial gallons though, 4.5ltr approx.

But what the carb is attached to makes a huge difference that head had big valves and I was adjusting size where it wasn't required and having restrictions in place stopping flow. It went all right to a point because it was greater flow than stock and it was very obvious but it was so less of an improvement than it could have been.

I didn't have the Lectrons on it with the data logger, the floats needed replacing. But that started logging with 48hp max, 810cc and I managed to get it to 52 and maybe 55hp going from VMs to TM Mikuni's. Big bore 60hp but power dropping off around 6,000. No air. Last one was around 85hp getting the head as it now is. That matches a dyno of rwhp. That changing was primarily the cylinder head inlet ports. And it does not die after 6,000 but goes harder and harder. It would be more now correcting mixture and freer mufflers. I don't know that that changed midrange much but it can do that like the Firebird. If I was doing it again it would be a std stroke 744 and I'd be looking for that boost in the midrange which also feels to drive top end so hard. Why we have a new head for Ben's just to see what is possible.

There is no reason a 654 RGV would need to be slow, the engine size isn't the main determining factor, which I hadn't realized, but it needs to be smooth.

Raining here so thought I'd try to match this good head, through the 34pwk its 168, the other one I adjusted and got 164 and adjusted more for 163 frown and that's the problem, working out why, with so many curves, depths and widths, maybe the faired little section to the valve guide or something is effecting the air? Ports look the same. I might try the fairings before and after the guide they are not quite the same and they should not make it worse. Tried it added 2.5 frown
potentially 1hp.

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[quote=kevin]whats the difference between a gardner and the old posa fuels?

Ron Gardner's design is the original, everything else (Lectrons included) are a copy and slight tweak of Ron's original design.

The first Gardner carbs all pre-date the appearance of others.
Ron told me he didn't patent his design because he could not afford the lawyers to police any infringements, back in the day he also wrote a "imitation is the sincerest form of flattery" letter to Lectron!
Ron's in his mid eightys now, still a character with loads of race paddock stories!

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Trying to get the 34mm port and 42mm valve on a ported head to the flow of the best head. Probably makes no difference on a 654 really. So I did one side yesterday and one today that delivered very easy in one go. You don't know because you can loose easily and you do not know what is possible. Problem is... close to the wind. Probably a sailing term for tacking that is good but close to going wrong and loosing the race.

So the last port broke through into the pushrod tunnel. It's not a disaster but needs patching. It could have easily been cast thicker. I know from past experience JBweld on it's own will crack here and now is the wrong time to weld, if you could get at it. But JBwelding an alloy plate on works fine. But it's nice avoiding it.

Anyway straight up it's 180 with a bell and 169 through the carb. It can be wrong but it's consistently reading within 1 or 2cfm. It should be brilliant on a 744 and make getting hp very easy. It predicts lots of power just off carb flow. I doubt a Triumph or Norton head could do this because of head bolt location and stuff, a triple definitely couldn't.

I put the big 44.5mm valve head on yesterday so 189.2 and 178 through the 34mm carb. An interesting thing without the carb with just the port; speed in the centre of the port is up. The sharp edge must disturb the air and block it a bit, down the port is ripping it through so speeding up the air down the centre. 54" or 470fps, a bell has more air going through but uses all the port and drops the speed there. The 34 with smaller valve around 44" 428fps.

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We tried several welders and couldn't get one to do anything with the tunnel.

Mark Parker #863298 11/16/21 9:50 am
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That makes me feel better about my attempt. It's awkward. A gun welder with a mig might get metal in there but it's not a good place. We can fit a plate. The engine will not know.

The dynamics of this stop starting flow to appreciate is that air is hitting 300+mph with 28" of vac but the motor can produce far more vac when the inlet is starting to open on overlap100-120" and that is getting it moving, at full lift it may have 20" and peak the port velocity, but that is a dynamic going on and effecting how the thing goes.

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Speedway engines are limited to a maximum 34mm carb. For a 12,000 rpm 500cc cylinder that is a tiny carb.
To get the maximum flow the inlet port is very short, on my engines 85mm from the back if the valve to the step in the carb rubber where the carb seats, the carb is only 25mm to the 2mm thick flat slide, 64mm to the end of the 74mm diameter bellmouth


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