This can happen when the springs on the spark advancer are too soft - which is 100% of them today, due to years of heat-annealing. You will need to trim off no less than 1/2 turn of each advancer's spring: usually 1 full term is needed, though. Mine has more trimmed off than that, for the same reason you have as 'stuck idle' now.
Also - if you look at the timing marks under a strobe timing light and they are umping around: the shaft on which that advancer is mounted is bent. This happens almost 100% of the time when the engine is turned backwards using the big nut by the points. We see it A LOT on engines rebuilt lately. You need to re-true that shaft, if this is the case, to within no more than 0.006", while 0.002" or 0.0" is possible (and best). You can get a dial indicator and magnetic base from Harbor Freight that is good enough to make this happen. Make sure to put the nut on those threads before hitting the shaft with a small hammer so you don't damage them.
Rarely, vacuum leaks at the carb intake hoses can cause your situation: it is far more common with the first 2 items above. But, if you have all 3, then it's tricky to know where to start!
