Amal GP2 carb Air Valve

[Cobra Classics]

On our 67 Silverstone rebuild I've noticed the Amal GP2 carb is minus its Air valve set up and cable. I've noticed in some images of other Silverstone's that some have it and some don't.
Is it just a matter of choice or is it essential to the carb.

Thanks
Trevor
Cobra Classics

[Stan Nicholson]

This system is listed in the Amal GP2 data sheet under "Compensation". The air slide controls the primary air flow through the slot and then, via the air jet, atomises the fuel passing from the needle jet. The data sheet goes on to say "as the engine supply increases or decreases at a given throttle opening with a varying load, so compensation takes place."
The only purpose I used the air slide for when I was running with the GP on my Silverstone was to richen up the mixture by closing the slide gradually to see if the main jet size should be increased. I gather that from fully open to closed was several main jet sizes.
So it is a useful addition but not essential.
If you are interested, and send me a PM with your email address, I can send you a scan of the Amal "Description of and Tuning Instructions for GP2 Racing Carburetter" (6 sheets). I picked this up at one of the Earl's Court motorcycle shows in the mid-60s!
Stan

[Kaffman]

I know from period photos of Tom Philips on the RAS in the 1963 TT and detail related by Eric Stroud, the machines tuner that the air control was used to compensate for altitude, weather and temperature changes around the course. Real seat-of-the-pants tuning.

[KevinLS]

I think I've mentioned this before at some stage, but when I was racing Bantams it was common practice to close the air valve on a long straight to check if the engine would four stroke. If it didn't then you pretty much knew that the main jet was undersize and you could expect the bike to seize when you shut off for the next corner .
Other than that I don't think the valve added a lot of value and I ran my Silverstone without it.