I changed the rotor when the old one came apart (JB welded it for a spare) but with the new one the engine doesn't feel the same. I put the repaired one back and things seem better. They are a different design at the brass. Could a different rotor effect the timing advance if the contact seems to be slightly more curved at the lead end? I havent had tome to check with the timing light yet.

I have heard of caps being slightly off spec, in that the terminals are a few degrees off. Normally that doesn't matter too much - the spark jumps the gap. But when combined with a rotor that's a bit off too, it can be problematic, particularly if the "offnesses" exagerrate and not compensate. It's not that timimng is affected, but you start getting misses.

I saw this in GrassRoots Motorsports Magazine. It might be a new source of better Distributor Rotors. Have no affiliation, don't know the guys,.etc,etc.
Good Luck Bob

http://www.antiquedistributorcaps.com/

I ordered one form Kip (antiquedistributorcap.com) and they assured me it will last. At 20 bucks it should. Shipping is a killer on small parts. I will still carry the spare.