Interesting responses here. I had one with the same problem. I fixed it by reengineering the butterfly advance. The butterfly was closing too much at idle. That motor ran great and idled extremely well after the mods.
But as noted above, the clogged idle system is more likely your culprit, as long as the gasket inside the bottom of the carburetor is in good shape.
Looking back, I suspect my motor had a carb from a different model on it, one with linkage that wasn’t designed for the motor.
T
