[Tommy]

Vacuum advance does indeed advance the timing when the vacuum is high (i.e., throttle not fully open). 4500 RPM with no load would likely produce a lot of vacuum and 50* total advance doesn't sound unexpected to me. That's why timing is always set with the vacuum advance disconnected and the outlet plugged.

How does the engine run under load (i.e., on the road with full throttle)? Does it miss at 4500 RPM then too? If I were looking for a problem, I'd start with a weak ignition from bad plug wires or a weak coil.