I'm down to 2 possibilities:


1) Sounds like an air leak in the cylinder/cases reducing the crankcase pulse...I say that because putting the choke on increases fuel on each pulse to feed the cylinder and should increase RPMs...at idle the pulse will naturally be weaker than at a higher RPM...


2) Do you have a timing light to check timing?


I'm curious...if you lift the rear wheels off the ground (safety) and crank in the idle screw, does it stay running even with the choke off?


Given a new carb, good compression and good spark (it runs) these are my last suggestions...