Join Date: Nov 1999
Location: Central California, USA
You need a vaccum advance for street use. For full throttle applications you don't need the vaccum advance. The vaccum advance operates under higher vaccum conditions like part throttle crusing conditions. Less during 3/4 throttle, and not at all under full throttle. If you disconnect the vaccum advance on a stock distributor, your engine will be running retarded during part throttle conditions. This will cause the plugs to run cooler which will cause buildup of carbon deposits. Also your mileage will suffer and the engine could load up so much on a long trip it could stall. I had this happen to me! Full throttle performance will not change except where the carboned up plugs will misfire if too dirty.
If you recurve the mechanical advance correctly, you may be able to get by without the vaccum advance and only loose a little mileage. The usual problem is when the mechanical advance is modified to get better full throttle performance, the stock vaccum advance has too much advance and at too low a vaccum. The vaccum can needs to be changed to one with less total advance, or limited with a stop, and have a stiffer spring inside to come on only at higher vaccum. The typical symptom of an over active stock vaccum advance is a brief ping when accelerating from a stop. The vaccum advance opens all the way when you first open the throtle, then takes a little time to close as you get into the throttle and the vaccum drops. While the vaccum advance is closing the mechanical advance is opening (on a recurved distributor) and you get too much advance briefly, causing a ping. Regards, David