Chad, I agree with Dave Ede it does sound like pinking, try richening the mixture a little ( easiest way is ride with the choke 1/4 or 1/3 or even 1/2 on to see if it makes a difference. if it does make a difference for the better then the mixture is too weak and need richening OR the timing need to be retarded by a degree or three.
try it and let us know what the results are!
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