At a loss... HELP

moper said
That's all tuning. You need to bring the inital timing up to 18*. Increase the centrifugal to give a total of 38*, and get the lightweight spring to get the advance moving earlier. Then slow the idle speed down to 6-700 using the idle speed screw. Then reset ti idle mixture screws to the best hot idle vacuum reading, then back them out 1/8 turn. Then see what it does in gear. It sounds like not enough initial timing, and a lean carb at idle.

I don't agree. Most slants do not like that much total timing. 28-30 usually works best.
I'm wondering if the timing is changing at too low an rpm. It could be that at the in neutral idle rpm, the centrifugal advance has already started, and when the rpm drops, when put in gear, the centrifugal advance decreases, which lowers the idle speed to the point of stalling.