i just got my car on the road today after installing a new Performer RPM maifold and new MSD 8361 distributor and coil. The car idles fine but when i am cruising with the vacuum advance hooked up the car hesitates or surges around 3000 rpms. The car runs fine when the vacuum advance is pluged. The distributor has a light and medium spring in it right now with the stock 21* bushing I believe. With the vacuum pluged i have it timed at 16 intial and 37 total. Could the vacuum advanced be coming in to early? How does engine vacuum affect the vacuum advance. This is the first car I had that actually makes good vacuum, like 17 at idle. my other cars only made around 12. the more engine vacuum does it advance the vacuum advance more or something?

Thanks Erik

Have you measured the amount of vacuum advance your distributor is adding yet? To measure it just leave the vac advance connected and rev it to 3000 RPM and read the total advance. IMO the advance should add another 10-15 degrees so total would be around 50*. This seems to work well cruising for my combo.

The mechanical curve sounds a little slow though . . . I once had a sticky mechanical advance mechanism and the engine really nosed over in the midrange. I've had good luck with a curve that causes full mechanical advance at around 2400-2600 RPM.

The other possibility is that the engine is going lean at part throttle. Fuel pressure would be a quick 1st check. Then . . . if the power valve is late and the jets are on the lean side than it could cause a hesitation on throttle tip in. Remember the old thumb-rule for power valves . . . 1/2 of lowest steady vacuum reading in neutral.

Good luck.