The problem that you are having is due to over advanced timing. To eliminate this you need to limit the amount of vacuum advance not the rate. I do this by drilling and tapping a hole in the plate and installing a screw to stop limit the amount of movement in the plate. This way you can properly adjust the rate of vacuum advance with the set screw in the can. On a duraspark dist the screw doesn't change the amount of vacuum just the rate. If you have any more questions, give me a call.
Marc Curtis
Jim Green's Performance Center.
425-774-3507
