If it is doing when parked, then I think your a little leaner than what retarding the spark will compensate for. Double check your timing in any case. The carb looks pretty clean, so I'm going to assume you put a kit in it, I could be wrong. 8-] Pull the top of the carb off and check your float and jet needle settings are correct. If they are correct raise the arm that holds needle about a 1/32.
Larry
