Help figuring out miss in an HEI 350?

[cadillac_al]

I was diagnosing a miss on my K20 and it seemed to be on both sides of the engine with the same rhythm. I have a very free flowing exhaust so it is very easy to hear the misfires on either side of the truck. I was thinking it could be my 20 year old Accel spark plug wires so I ordered a tune up kit from rockauto and was waiting for it to be delivered.

After reading this thread I thought I would check that rear vacuum tap to see if anything could be leaking. Everything was fine and only my trans is plugged in there and the hose was not cracked. While I was there I unplugged my vacuum advance to verify that it was still working. When I unplugged it my misfire completely disappeared. I couldn't believe it. I plugged it back in and the miss came back. I did this about 3 times. While I was playing I put the vacuum gauge on it and this engine runs so sweet it pulls 21 inches of vac with or without the vacuum advance. I now put the vac advance on ported vacuum and the truck appears to be very happy.

This will be the first engine I have ever had that did not like full manifold vacuum to the vacuum advance. Maybe, just maybe, my ignition is still weak and can't handle that much advance at idle. My vacuum advance can is for a truck and only advances about 10 degrees so the vacuum advance isn't excessive. I just thought I would add this here in case somebody else is trying to diagnose a miss and it isn't a vacuum leak at the rear manifold vacuum port. Try unplugging the vacuum advance for the fun of it and see what happens.