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.