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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.



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.

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.

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.

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.

PM sent

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.





Bill B...

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.


147.8with this bell.

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

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.

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

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.

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.

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.

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
20211025_133415 by Sigma Projects, on Flickr

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.

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

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.



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.



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.

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

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

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

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.

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.

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.

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?

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.

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