This does not sound right. Vacuum advances "kick in" at idle and then decrease the advance as the manifold vacuum decreases with throttle opening. The exception to this would be if the advance were hooked to the vacuum port on the carb.Then there is a "kick in" point that is determeined by the carb machining. Maybe you should try manifold vacuum and see how that works. Be aware that the advance degrees for your vacuum advance can may be stamped on the flat metal bracket for the can.You may have too much vacuum advance (like 20 degrees; look for a 12 degree unit). Speed shops do have adjustable vacuum advancers for HEI.