What kind of a throttle setup are you using?
While others have provided some useful information, gassers are extremely sensitive coming off idle. If you have a geometry issue with your throttle linkage or a poorly centering throttle servo, that can also cause the idle issues but I'm more inclined to think it's a low speed needle setting problem.
The other thing as mentioned, and I don't know how familar you are with gas engines, but once you get close on the needle settings moving them to their final position is more like you "think" about moving it... lol They are not like glow engines where you can move the needle a quarter turn or something....
