pre-ignition or over run.Is this ride driven everyday??.Sounds like carbon build up on the heads or pistons from a sunday driver.Take her out on the highway for a real good runnnnn.Pedal to the metal.If you don't want to run her hard.This is from the old school.Remove the air cleaner with the engine running,reving.pour about a quart of water slowly into the carb, reving the carb.Don't let it stall.This will remove the carbon from the heads.If you look at the tailpipe you'll see pieces of carbon on the ground.Check timming,adjust carb mixture and chock ,idle.Just remember tap water is cheep
