Hitchcocks Forum

Hi again. Anyone know the correct (or a good starting point) main jet size for Amal Concentric Mk1 carb on a 2006 iron barrel 350 with K&N filter and a nearly straight through exhaust?. I've tried running with a 160 but it badly lacks power on abaout 3/4 and above throttle. Cheers - Jools

Your bike is not very unlike mine, although I have a 1953 Model G: Iron barrel, 350 cc, Amal mk.I, K&N type filter and nearly straight through 50-ies exhaust. It came to me with a 160 or 170 main jet. However, I found it didn't happen very much from 2/3 to full throttle. I have tried 180 and 190 main jets and the 190 is the best, I guess. It didn't do miracles, but I felt a small improvement, at least I want to believe so.... And as it doesn't run too rich and have no bad effects, I keep the 190 jet. .-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.- What made a somewhat bigger difference, to my surprise, was fitting a long bellmouth with mesh in place of the filter. That makes most difference at full throttle and high speed. It seems that even if the filter is very little restrictive, the shape of the intake is not good. The bellmouth makes the flow better. Again: I think so. But the theory also says so. So what I am looking for now is a non-restrictive, well shaped filter to fit over the bellmouth, as I don't like to run the engine without the filter except on very "clean" roads in good weather. .-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.- Hope this helps! Regards, Anders F. R.

Hi Jools, I've also got the same set up as you - 350 Classic 2008 model with Amal carb, KN filter, less restricted exhaust. Mine runs ok on a 200 main jet. It's often been said that no two bikes are the same, so it's probably not a bad idea to experiment with a few different sizes, though as you've said, a 160 is probably a bit on the small side.

Think I know what I'm aiming at now then. Thanks for that.