As sinergy has already alluded too, I also think it is an air leak.

I'd agree that it's possible your throttle linkage is not opening the carb plate enough at idle. Do as he suggested but leave the ignition off and close the choke and flip to see if it starts to pull fuel, if not then it's a definite air leak or the screen is plugged in the carb or a line may be pinched

Also, if you are using a fueler valve such as the Dubro or Sullivan, remove it because they are well known to leak air