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Allan G, chaterlea25, Chris Johnson, kevin, Mark Parker, Morgan aka admin, pushrod tom
Total Likes: 13
Original Post (Thread Starter)
#861473 10/23/2021 8:23 AM
by Mark Parker
Mark Parker
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.

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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]
Liked Replies
#862176 Nov 1st a 09:58 PM
by kevin
okay. i'm in.

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

[Linked Image from]

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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] [Linked Image from]

an extra body

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original fasteners, brackets and air needles are there

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including what look like original hose clamps

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only one float chamber, but im arranging for anothrr
3 members like this
#861491 Oct 23rd a 02:33 PM
by Tridentman
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.
2 members like this
#862107 Oct 31st a 07:11 PM
by Hillbilly bike
Hillbilly bike
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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#861516 Oct 23rd a 06:49 PM
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]

Bill B...
1 member likes this
#862154 Nov 1st a 11:33 AM
by pushrod tom
pushrod tom
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.
1 member likes this
#862206 Nov 2nd a 12:26 PM
by Hillbilly bike
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?
1 member likes this
#862289 Nov 3rd a 04:50 PM
by Blown Income
Blown Income
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.
1 member likes this
#862359 Nov 4th a 02:46 AM
by Tridentman
And as many if not more comp parts went out of the back door as went through the front door.
Same as at Triumph.
1 member likes this
#862770 Nov 9th a 07:32 PM
by Andy Higham
Andy Higham
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
1 member likes this
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