It may just be the low end needle. It could be lean and starving for fuel, or rich and not keeping the plug lit. Either way just try 1/8 turn in or out.
Sometimes the regulator on the bottom needs to be adjusted. If this is a used engine (Got from some body else), they may have messed with it. Normally set it flush with its case heigth.

If the regulator is old and the diaphram is leaking it may not be keeping enough pressure at idle.

There are many variables and you just need to try one thing and begin the process of weeding out potential problems from the list.

Now if this is for a warbird racer, who needs low idle anyway.
Wide open baby, wide open!!!