Well since you said it diesels then it is probably running too rich. Try tuning you carb and ignition timing to see if that helps it. You might want to change your plugs also. Just make sure to gap them correctly.
I don't have a clue what would cause this problem. I thought it could be the points but ithas been changed with the same results. Adjusted the mixture of gas and air and it didn't resolve the problem. Adjusted the timing with same results.
