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Mark Parker #862247 11/03/21 1:20 am
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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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Mark Parker #862249 11/03/21 1:40 am
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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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Mark Parker #862255 11/03/21 5:21 am
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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.

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

Last edited by Mark Parker; 11/03/21 8:20 am.

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Mark Parker #862268 11/03/21 11:05 am
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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....


79 T140D, 89 Honda 650NT ,61 A10 .On a bike you can out run the demons
"I don't know what the world may need
But a V8 engine is a good start for me
Think I'll drive to find a place, to be surly"
Mark Parker #862289 11/03/21 4:50 pm
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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.


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Mark Parker #862356 11/04/21 2:04 am
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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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Mark Parker #862357 11/04/21 2:37 am
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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'.

Mark Parker #862359 11/04/21 2:46 am
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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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Mark Parker #862360 11/04/21 2:49 am
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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............

Mark Parker #862375 11/04/21 8:41 am
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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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[/video]


79 T140D, 89 Honda 650NT ,61 A10 .On a bike you can out run the demons
"I don't know what the world may need
But a V8 engine is a good start for me
Think I'll drive to find a place, to be surly"
Mark Parker #862518 11/06/21 3:46 am
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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.

[Linked Image from i.postimg.cc]

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.

Mark Parker #862530 11/06/21 11:20 am
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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...


79 T140D, 89 Honda 650NT ,61 A10 .On a bike you can out run the demons
"I don't know what the world may need
But a V8 engine is a good start for me
Think I'll drive to find a place, to be surly"
Mark Parker #862705 11/08/21 10:01 pm
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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.

Mark Parker #862746 11/09/21 12:54 pm
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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.

1954 Norton Dominator 88 cafe - Yamaha wheels, Lyta tank ( project in progress)
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Mark Parker #862754 11/09/21 2:47 pm
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[Linked Image from i.imgur.com]

[Linked Image from i.imgur.com]

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


i'm old enough to remember when patriotism meant not trying to overthrow the government.
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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?

[Linked Image from 64.media.tumblr.com]

[Linked Image from i.imgur.com]

Last edited by kevin; 11/09/21 11:23 pm.

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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?


Amateur Loctite enthusiast.
Mark Parker #862788 11/09/21 11:30 pm
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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.

Last edited by Mark Parker; 11/10/21 8:38 am.

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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.

[Linked Image from i.postimg.cc]


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