I have the same problem and have yet to figure out how to correct it. The only thing I have left to try is to change the line that runs from the carb to the modulator valve. It is supposed to be a steel line; mine is replaced with a rubber tube. It was suggested to me a while back that under vaccuum, the rubber line may be collapsing and causing a vaccuum problem. Ames carries a replacement steel line; is yours rubber or steel?