JD Bob,

If the idle speed is too high as Jerry mentions, there usually needs to be some source of auto-ignition to light the fuel off after you kill the switch to cause the engine to keep running. Carbon can do this but so can a burned valve or a worn electrode on a plug. One of these items might tie in with your slight miss a mid speed.

A good tune up with a compression test thrown in might help with the diagnosis and could solve the problem.

I don't know if this engine is in your '58 or not. Some alternator installations on rigs that originally had generators will tap off the coil for field power. If not done properly, this can back-feed voltage from the alternator to the coil and cause run-on...this is just a thought.

Stuart