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I am so excited !! I have been looking for GP carbs for my 1966 BSA Spitfire at a reasonable price for over a decade.. Shockingly this pair was on Ebay and promoted on facebook Spitfire group page so I was sure they would go for the normal price or more but I won the auction at half price so now my search is complete.


Aren't they pretty.

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Originally Posted by wadeschields
Aren't they pretty.


Wade,

Good for you beerchug and they absolutely are!!


Jon W.


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Dont ya love it when a long search finally winds up.

Good onya!


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Spitfire GPs are like hens' teeth. You did well there.

In practice, of course, 932s work better, with the GPs kept for special occasions.

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Originally Posted by wadeschields
I am so excited !! I have been looking for GP carbs for my 1966 BSA Spitfire at a reasonable price for over a decade.. Shockingly this pair was on Ebay and promoted on facebook Spitfire group page so I was sure they would go for the normal price or more but I won the auction at half price so now my search is complete.


Aren't they pretty.
Good on ya.


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Originally Posted by Shane in Oz
Spitfire GPs are like hens' teeth. You did well there.

In practice, of course, 932s work better, with the GPs kept for special occasions.


Now to spend the next 10 years getting the bike to start easily and run well with them fitted eh?
Some blokes like a load of hassle................

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Originally Posted by NickL
Originally Posted by Shane in Oz
Spitfire GPs are like hens' teeth. You did well there.

In practice, of course, 932s work better, with the GPs kept for special occasions.


Now to spend the next 10 years getting the bike to start easily and run well with them fitted eh?
Some blokes like a load of hassle................


Yeah my bike does run great as it is so Im not in a hurry to change them out just yet . But at least I have them , as Shane said, for special occasions...


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I ported a head for some of those. I wrote down specs somewhere the ports are/were just under 30mm with 42mm valves. It has very high gas speed. Because they are no bigger and flow better. The thing is 30mm Concentrics knock the flow as it's sensitive to disturbed air, much more so than the std head. (as in it still flows more but looses compared to a bell which flows quite a bit. And fighting to get it then lose lots on the carb sucks.) The Concentric isn't good at speed though 32s on the same little port are better, but I think smooth bore 30s wouldn't lose so much flow and be very fast. Which is particularly good probably.

That speed will draw more air and fuel and probably require smaller/leaner jets. It may help the carb work better perhaps. If the needle position and spray tube ordinarily function better with more draw on them it may improve their operation? Solving the fuel level and idle would be the next thing.

Does it not have an idle screw to set the slide height? Because if that was the case maybe a threaded rod down through the top could be set to stop the slide going right down. If the screw part is just the outer piece? The cables would be no good for doing that. A hole in the slide could do it but doesn't give easy adjustment, and transition might be weird, or not?

Last edited by Mark Parker; 05/09/22 1:08 pm.

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Great score Wade, your pictures bring back memories. I had a motor with a GP2 on it, and and at first,when not in use I plugged it with the usual bathplug. This didn't seem a good idea to me, as junk could sit in the bellmouth beside the bathplug. What I ended up with was a sort of rubber cup that was originally used to protect the cut end of a big underground electrical cable. It fitted snugly and did a much better job of protecting the carb.

A mate of mine had a MkII Spitfire that had won the very first race at one of the Sydney tracks.


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They are absolutely Mint Wade. A great find!!

All you need now is the frame bracket for the remote float bowl and seemingly impossible to find the rubber isolastic mount. The bracket Can be made with some flat bar and some patience in shaping it.


Now let’s all have a beer beerchug

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Congrats Wade, you now enter a unique cadre of enthusiastic GP carb owners. I love to run these carbs once set up they make beautiful power range that just keeps on pulling.
Setting them up according to your instructions is pertinent: The float perfect and the cables to set up the slides are the big hitter. Yes they will idle and that is all in the cables. Keep an eye on that cable adjustment because they will change and make sure they are lubed. You might have trouble finding the correct cables.
I’m planing a 64 Spitfire Hornet replica using the GP carbs and original ET ignition with 6CA points plate. Have to finish my West Coast Hornet first.
Good luck,
Richard

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Good find Wade! I ran across a set a couple of years ago at a great price myself, but they are problematic in all the areas mentioned. I have a set of 30mm Concentrics that I modified and sorted years ago and they are sooo much easier to live with. That being said, the GP's look terrific on the bike and work well wide open, not so much at low throttle settings. Mine were set up by a well known BSA tuner for a race bike and have some weird parts in them (they work pretty well though) and I drilled a 3/32' hole near the base of the rear of the slides for idle air (suggested by the tuner) and it idles quite nicely at about 7-800 rpm, but here's the rub, as predicted by Mr. Parker: With the recommended no. 5 slides and the idle screws set for best tick over I have a big flat spot just off idle, if I tune to eliminate the flat spot (richer at idle) the transition is great but it loads up at idle. BTW, simply sitting on the side stand idling the left carb goes rich and the right side goes lean due to the central float, the bike has to be held perfectly vertical to adjust the idle mixture. I think the transition situation can be cured with richer slides but at a cost of nearly $100 per slide experimenting might get costly! And then you have the fuel mileage aspect, I get nearly 50 mpg with the Concentrics and have yet to achieve any better than 30 with the GP's. With the Spitfire 2 gallon U.S. tank you aren't going far! The 'Cool' factor of GP's can't be denied though, just be prepared to spend a lot of time and money getting them as "good as they can be" and still fall short of a reliable street combination. I hope to see that beast when you're done, mine is still missing a lot of it's original Spitfre bits most importantly the engine cases having been ventilated by a connecting rod by a PO long ago. Oh well, you work with what ya got! Good luck Wade, great score!!!


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those look brand new

they came with the matchbox?


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BUY A SET OF 68-8731 CABLES FROM BAXTER CYCLE THEY ARE THE LAST ET OFFERED BY SA FOR THE MK2. THEY ARE AN IMPROVEMENT OVER THE ORIGINAL DESIGN AND THEY HELP WITH THE IDLING PROBLEM.


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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I already had the float and I have the bracket to hang it... The guy I got the carbs from offered to clean the float to match and it came out great. I need to look at the bracket I have to see if the parts that connect are all there. I think the bolt on top of my float looks different then the pictures I have seen of others, so I might have to source that if it doesnt work.

Great info everyone thanks !!!


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Funny really as loads of those carbs were around 'knocked off' at the time
a lot of blokes tried them and gave up, especially in England as the bloody
things filled up with water all the time when it rained. They were great carbs
in the day but really just for racing. Beezer abandoned them as they gave so
much grief. A pair of smooth bored concentric32's with proper bellmouths
were better and a lot less hassle.

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No good in dry old Queensland then, torrential rain and canoeing to the shops one day, cyclones and helicopter rescues the next, or what ever that saying is.

What's a smooth bore 32mm look like Nick? If they made them there is a reason.

I found some numbers in a book. Std head @ .384" 109.48cfm - with a bell 112.6cfm - with std 30mm Concentric 100.92cfm - blending the lug thing on the intake of the carb like Triumph tuning book 102.7cfm. They knew that modification gave them a bit more hp. So a smooth bore GP2? 108 or something? More than 103 I guess without the needle obstruction. If you give the engine what it wants you get hp.

I cannot find where I wrote the ported 30 down but with a bell I think it's around 150cfm but it's smoking through the little port. If it was 130-140 through a GP it would be interesting, especially drivability off the bottom and through the midrange and top end. Improving the port isn't just about top end power. It can be if it's just bigger.

They basically stop making power in the top end because the air runs out or your pipes and compression are letting it down. The cam isn't a big player as it's pretty good, it will make power at high rpm if the other stuff isn't holding it back.

This is it because it had one fin needing fixing.

[Linked Image from i.postimg.cc]

This is a mold from a 30mm port I tried before, probably with a std valve. But I turned it into a 34mm because they flow more. But maybe that isn't the best rout with a 654. Tom's Hornet has only 25% or so more hp and it may not be using all the air available and need pipes and compression or 750cc because a 34mm port wants a high comp 750 to have a torque peak around 5,500 in a calculator.

Rob said generally with more flow through the same carbs means it requires smaller jets and hp will be up. Why GP2s may carburate better as there is more pull on the jets/emulsion tube. If Chris's 750 required leaner jets it should be up on hp even if the stingy new dyno isn't showing it so much. But that's probably a pipe limitation as well maybe, because he's chasing a lot.

[Linked Image from i.postimg.cc]

std on the left and I think it's the better '71 port. And this isn't as good as the new 30mm. It may be the key to getting GP2s really working. Pity you do not live next door Wade.

[Linked Image from i.postimg.cc]

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Congrats on the find, they look beautiful! Personally, i'd prefer to have them on display rather than on the road, but hey...

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Originally Posted by wadeschields
I already had the float and I have the bracket to hang it... The guy I got the carbs from offered to clean the float to match and it came out great. I need to look at the bracket I have to see if the parts that connect are all there. I think the bolt on top of my float looks different then the pictures I have seen of others, so I might have to source that if it doesnt work.

Great info everyone thanks !!!

If the float eye bolt can't be sourced, which is very likely, you can make one like what I did here.

[Linked Image from i.postimg.cc]


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A locking ring tool can be made as I did here.

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1967 BSA Hornet (West Coast Model)
1967 BSA Hornet (East Coast Model)
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Awesome Gary thanks


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Good for you Wade! Spitfire Ken is right about the cables. Basically it is a cable with the regular ferule at the carb end and a second ferule a few inches above that. It comes with a hollow threaded tube with a split in it and a seat for the ferule at the top. The tube is threaded into the top of the carb where the cable normally enters. You thread it up or down to raise the slide. You then use the regular cable adjuster to adjust for free play in the cable. I have them on my Spitfire. It starts easy and idles like an old Triumph with slightly worn carbs (pretty good but not perfect). As for the flat spot that DWR talks about. I had that when I originally set my carbs up with a ColorTune. Eventually, I put the ColorTune back on the shelf and adjusted the mixture by ear until I no longer had a flat spot when opening the throttle. Its a little rich and can load up in very heavy traffic with lots of idling, but runs fine normally.

Ed from NJ

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What's a smooth bore 32mm look like Nick? If they made them there is a reason.

AMAL didn't make them, various blokes did them , Mick Hemmings supplied the pair
i had they were actually 33 bores once machined. A lot of blokes who raced nortons used
them, hence the Mick Hemmings connection. They were very good carbs and were
eligible under the pre 72 racing rules.

https://imgur.com/a/GYvbzcX

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Originally Posted by NickL
What's a smooth bore 32mm look like Nick? If they made them there is a reason.

AMAL didn't make them, various blokes did them , Mick Hemmings supplied the pair
i had they were actually 33 bores once machined. A lot of blokes who raced nortons used
them, hence the Mick Hemmings connection. They were very good carbs and were
eligible under the pre 72 racing rules.

https://imgur.com/a/GYvbzcX


Whats the difference in them? were they just "finished"?


Now let’s all have a beer beerchug

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Whats the difference in them? were they just "finished"?


They were fitted with the trumpets then the assembly was bored and polished
as an item. There was no step or joint and the taper was of course just a
continuation of the trumpet. The trumpets were not removable.
Snag with them was, one of mine had the progression hole blocked by the
machined material and it was a bitch to clear, doesn't really matter on a race bike
but without those holes you know what amals are like, no idle etc.
Blimey Allen that was 30+years ago now.............. I used them for a couple of seasons
then went to the weber carb.

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It needs that because the trumpet otherwise doesn't do much. Where the slide sides are might be thinner but isn't helping. Pwk are not smooth either nor VM nor TM but better, Lectron are very good, but old school GP2 no needle in the way. Not Big but on a 654 they could kick butt. If the quirks are under control.

It's not the 32 concentics on the MKIV so much that are the key to it's equal hp with the lower 9-1 compression, it's the altered port, as using that port on a Firebird with 30mm gave a similar increase at Umberslades on their dynos in 1970. 32s would help but not on their own. That head with GP2s and 10-1 would have been nice.

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Or.........It could have been the marketing dept. Sure they would not try to sell less hp. Kind of like the Hot Rod Mag test of a new Lightning Rocket where they did something like 132 with an out of the box machine!

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Originally Posted by pushrod tom
Or.........It could have been the marketing dept. Sure they would not try to sell less hp. Kind of like the Hot Rod Mag test of a new Lightning Rocket where they did something like 132 with an out of the box machine!

Whoo! On stock gearing, what was that then....10,000 RPM?

Oh, the good old days when Bob Green and Floyd Clymer were holding the stopwatches .....

Lannis


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In 1970 Umberslades built the engine for the '71 prototype photo shoot Firebird and did the intake ports to the drawing - probably the special '68 MKiv head drawing, which was listed under the wrong old part number for that bike. It's blanked and the new number stamped I think, people have them. On their dynos they reported around 8hp extra over the production line engines for that Firebird. The riders for the photo shoot reporting how lively and responsive it was. I have one '71 head like that, so they may have incorporated that and not said? Production engines wouldn't have had the dyno testing and fine tune. And the '71s performance wasn't going backwards with the new quieter mufflers though Firebirds didn't have them.

Some old things had speed. A 1950 plunger A10 was rated at 105mph top speed. Later Super Rockets with all their specs were not claiming better. What was faster in LSR an early iron A10 or a later Super Rocket based hottie? In old drag racing there was an iron A10 plunger with a blower that was super fast at the time. And a spitfire in the 12s. Factory reps even went to try and workout why.

A Lightning Rocket would have the gear ratio, they just needed the big tail wind or the hill. Sounds like they had the optimism.
That show Harley and the Davidsons is really funny in a few places, but the speed testing and excitement at the new Nucklehead's 105mph, 'fastest motorcycle in the world' is really great, the guy probably turned around in the next state to come back, unless brakes worked on that model. The designs are beautiful. New ones sort of lost it. Worth watching if you haven't.


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Yeah, The early LR's had a 42 tooth sprocket as part of the brake drum. I have that with a 22 front on my Speed bike which recorded a best of 166+. A bit overgeared for street use :}

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Don’t forget that apart from 68, firebirds have 1 3/8” pipes and the 71 model even has a smaller gearbox sprocket so revs shouldn’t have been a problem.

Having ridden Shane’s Firebird, it has fantastic midrange compared to a stock Lightning.

The oif megas are also a fantastic way of knocking a good 15 mph off the top speed of an oif compared to a set of pre oif T120 silencers. So there was plenty of reason to not mess too much with their flagship A65.


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Interesting Allan. Certainly how they feel. Hard to tell on mine with this head but it's just very strong. Purists would wonder what the hell was wrong with it when they turned the throttle, until it shook them off I guess. The race bike's getting Firebird style pipes with an X connector. Ben's ordered some mandrel bends they will be stepping bigger for that set up. We sort of know what they can do.


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Originally Posted by Mark Parker
Interesting Allan. Certainly how they feel. Hard to tell on mine with this head but it's just very strong. Purists would wonder what the hell was wrong with it when they turned the throttle, until it shook them off I guess. The race bike's getting Firebird style pipes with an X connector. Ben's ordered some mandrel bends they will be stepping bigger for that set up. We sort of know what they can do.

When I did my spell fabricating exhausts, we had our own mandril which helps a lot, but instead of directly crossing the pipes over (your 883 will be different from my way because of how the pipes connect, this way is only for pipes that are parallel) we would make 2x sections, each having 3 bends, then mate the two together… I’ll do a sketch then upload the photo.

My freehand drawing isn’t what it used to be, but I hope this explains Better than I can. [Linked Image]

You don’t need mandril bends to make it either, just and angle grinder and a MIG welder to make the bends etc the middle bend you make, then cut it out, 50% of the pipe OD is an ideal amount to cut off to get a decent cross over.


In effect the two exhausts are then mated together over a longer distance and you get a better sharing/splitting of the gasses. The factory Siamese pipes worked in much the same way and have a big performance difference against the aftermarket crap.

Last edited by Allan G; 05/15/22 7:22 am.

Now let’s all have a beer beerchug

68’ A65 Lightning “clubman”
71’ A65 823 Thunderbolt (now rebuilt)
67’ D10 sportsman (undergoing restoration)
68’ D14 trials (undergoing transformation)

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This is Paul's 744 with exactly that type of X Allen it works really well, internally really nice. The problem with this system which was big bore is the head. I sent this system to Chris for his LSR because of its good top end.

The head has 38mm carbs and ports and because of lack of gas speed it had reversion and wasn't good till around 5,000. I shifted at 8,500 in second. It probably flowed 160cfm. What I know now is that the pipes are probably good at lower rpm with the new 34mm head which flows so much more cfm. So speed is good and no reversion, like Ben's 734 which uses the same size headers. It needs a big bore really as the inlets are 44.5mm.

With this head riding on the road was pretty useless, it needed rpm to function. That smaller port head will I expect remedy that and have more punch and more top end.



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Originally Posted by Mark Parker
It needs that because the trumpet otherwise doesn't do much. Where the slide sides are might be thinner but isn't helping. Pwk are not smooth either nor VM nor TM but better, Lectron are very good, but old school GP2 no needle in the way. Not Big but on a 654 they could kick butt. If the quirks are under control.

It's not the 32 concentics on the MKIV so much that are the key to it's equal hp with the lower 9-1 compression, it's the altered port, as using that port on a Firebird with 30mm gave a similar increase at Umberslades on their dynos in 1970. 32s would help but not on their own. That head with GP2s and 10-1 would have been nice.


That engine had +0.060 10.5- 1 Venolia pistons in it and a reasonable head, it had been worked/flowed and
had SRM inlet valves. We were clocked on it at 108mph at one circuit, about 15mph too slow but for a 650 it
wasn't bad. We both weighed about 180lb and the old outfit was no lightweight. I put it together to learn to ride
the old crate again in 1991 after a 15 year lay-off. It rekindled my dormant lunacy.

Last edited by NickL; 05/16/22 12:50 am.
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