I have a '71 with a 351C. I've added a Pertronix system. The timing at idle (approx 600 rpms) is about 6 degress. But when I get to 3500 rpms the timing goes to about 45 degrees. It seems to be smoother if I punch it vs a slow climb to 3500. Is this a vaccuum advance issue or mechanical?

 

You have to separate the two systems and set the initial timing and mechanical advance with the vacuum advance disabled.

Disconnect the vacuum advance hose and plug it. Set the timing at idle at 8-10* advanced. Rev the engine up and watch the tach and timing marks. You should max out at 30-35* before 3200rpm.

With the engine idling, plug a hose into the vacuum advance (remember to keep the carb vacuum port plugged) and suck on it while watching the timing marks. I can't recall the exact specs atm (they're in the factory service manual, which you need to get), but the advance should increase. Hold your tongue over the hose end while sucking on it and make sure the advance doesn't drop down. If it does, your VA is bad.

 

+1 on this one