I've got a 1972 250C (M130 2.8L SOHC engine) that has had the original carbs replaced with dual Weber 32/36's, and the original points distributor replaced with a Mallory Unilite electronic model with both mechanical and vacuum advance.
I'm not sure what effect (if any) these changes would make on the required ignition timing. The factory setting for initial timing at idle (vacuum advance disconnected) is 4 degrees ATDC. When I bought the car two weeks ago, the initial timing was set to 7 degrees BTDC, which is eleven degrees more advanced than the factory setting. The engine runs fine, but sometimes it takes a few seconds to start, and the fuel economy is pretty poor (13-15 MPG with mixed city/highway driving).
Mallory recurved the distributor at the factory to match the factory-specified advance curve of the old points distributor, and the vacuum advance is adding the proper 13-17 degrees of advance, so the only thing that seems out-of-whack is the 11-degree difference in initial timing.
Next week, I'm sending the car to an old-school mechanic that I trust for a tune-up, but I was wondering how you actually go about determing the optimum ignition timing for a car that's had significant modifications.