1981 toyota corolla sr5?

i have a 1981 toyota corola. with an sr5 drift machine engine, custom headers, lsd, webber carb. and a stage 3 clutch. .... now my problem happens to be my engine when its idle. it is constantly and 3 rpms i tried adjusting the throttle but when it gets as low as 2 or even a little under it shuts off. what could it be? any advise would be new