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AMAL GP Air Slide Carb function

Brian Thompson

Well Known and Active Website User
VOC Member
Hello everyone,
While not being directly Vincent based at the moment, it will be.
Firstly, I've fitted a new Amal 1 1/2" GP carb to a BSA Goldstar. While I'm nearly ready to start it after restoring it I come across an air slide cable. I've installed the carb and have all the cables connected. BUT, What function does have this air slide have. Is it cold start or tuning? When the piston is on the cable it doesn't close right off in the bottom of the slide in the carb.
Secondly, I've been told that the bike won't idle with this carb. Is that true? The mixture screw does have a locking nut. So I guess tuning it at the traffic lights is going to be awkward.
Its just I"m going to fit 2 x 1 1/2" GP's to the 1200 Vincent when I put the engine in the McIntosh Egli frame. I have the engine running nicely on 36 mm PHF Dellorto's. But the bike owner wants GP carbs on it.
Thank you in advance.


Well Known and Active Website User
VOC Member
Just fitted a set of 1-1/4 GP 2 to the new twin racer (thats my choice of best inlet size for short circuit)
GP2 mean some improvement to steep down draft control and the pilot works like road carbs (on air not petrol) we are retaining the air enricher system (they are not a choke) for checking mixture (in our case on the test brake) as the air flow and carb orifice is not affected by its use
Here is a poor picture of the down draft when it was on the Comet with a Gardener yellow lines are UFM and red line the CL of the carb it will also explain why our tank has a big hole in the top! That was a source of puzzlement to many at Cadwell in the autumn when our race winning twin was on standard monoblocks o_O
gp angle.jpg

Mike 40M

Well Known and Active Website User
VOC Member
Sadly I have no experience on the air slide, as the GPs I've had came without them. Most racers omitted them. I don't like idling on a race bike either. Because on speedway we wanted as much engine brake as possible. Same for me when roadracing with drum brakes, which fade.
What i have read and heard is that the air slide can be used to help finding the right main jet on full throttle runs. It's also said it to be used for adjusting mixture depending on altitude. Useful when you are going up on Sneafell.
On some engines with GP carbs, you can get a decent idling on a warm engine. But I think the range of adjustment by the air slide is limited when idling.
Last edited:


Well Known and Active Website User
VOC Member
According to the gent that designed it, the airslide is both for helping with cold starts and tuning. You can tune the bike with the slide open or closed and adjust it either way. I dyno the bike with it open and then close it for a cold start. However, my bike is an easy starter and it could remain open almost all the time.

Make sure that if you tune it for open that you tighten the lever enough so it does not vibrate to a closing position during use. You will be wondering why your tuning is changing.