If you are still getting the misfires after all that work on the ignition, then I would move on. If it is misfiring on all cylinders I think you could rule out valves(unless they are all adjusted wrong, but then the problem would happen constanty) So I would most likely think it a carb issue. Perhaps your idle adjustment is to lean? What type of Carb do you have? I would assume a rochester, my 3100 with a carter used to sputter at idle, and I cannot remember if it was to rich or to lean. try that if you have not, also maybe pull the carb and dump it out and try spraying carb cleaner in the idle port on the ID of the carb. let it dry, if worse comes to worse rebuild the carb again.
Maybe take the spare carb and the one you are using and compare the idle componenets of each(use the shop manual to identify everything) see if something is different.
You will get it. Sometimes I overlook incredibly easy fixs, until I either take a day away from it, or pull a spare and compare it.
Good luck.
