How do you determine correct ignition timing?

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.
--
Scott Gardner

"A government that robs Peter to pay Paul can always depend on the support of
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Pragmatically one can advance the timing until the point at which the engine "pings" or preignites its fuel. Then retard a couple of degrees from the "pinging" setting. Retarding the timing further will make starting easier but power and economy suffer. It may interest you that old gas marine engines' timing was "set" (at cruising speed) by turning the distributor to achieve maximum engine rpm and then backing off (retard) slightly. Done.
If it were my car I'd set the ignition to its 4 degree ATDC specification to see how it runs. You won't hurt anything.
This engine's relatively poor fuel economy could be due to: needing new plugs & wires, rich carburetor setting, incorrect ignition timing, incorrect operating temperature and needing its valves adjusted. It's impossible to opine on the fuel use or timing until some of the other possibilities are excluded.
Good luck with it.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.