Are you still running points in your distributor? If so maybe you are having points bounce.

Ignition is Pertronix. Replaced the one I had used for years with a new one with the built in rev limiter. Same problem. Mummert heads with the provided valve springs have very few miles on them. Doubtful they are the problem. Will look at plug gap. Set them at Mummerts recomendation as I recall but don't rember what it was. Have replaced the plugs once already with no fix. May try tghter gap. Should get generator back next week. Guy had to order a new armature. Will tinker some more next week.