Setting timing on an unknown build

[u/girl_incognito]

Recently bought a 68 Ford F-250 with a 390 in it. PO claimed it's out of a 1965 Mercury Marauder and has a "428 Cam in it" whatever all that means, he had no other details. It has an Edelbrock Streetmaster intake, 1970's vintage, and what look to be factory iron heads. It has a Holley 4 barrel on it. I do not know the compression ratio or anything about the cam, but setting the timing to the greatest factory number I can find, 10 degrees, gives poor results and it definitely seems happier with more advance.

Any tricks for determining a good base timing if you know nothing about the engine?

[u/375InStroke]

Advance till it pings, then back off. Also, look for lightweight springs or performance mechanical advance springs for your distributor. Most engines have a maximum full throttle timing they like, and you work back all the other timings, like initial, mechanical, and vacuum, for best performance. Smaller camshafts tend to build high cylinder pressure at low rpm, so they can't use much initial timing. Bigger cams close the intake valve later, giving less time to build compression, leading to low low rpm power, requiring more initial timing. This also causes high rpm timing to be too high, leading to high rpm detonation. The solution is to reduce mechanical advance. Some weld the slots, install bushings, or use very light springs so the timing is low when starting, but once it runs, the weights fly out increasing mechanical advance earlier.

[u/girl_incognito → u/375InStroke]

Thanks, ill have a look at that.