Initial startup cooling system troubles

KindredSpirit said

Thanks everyone. So far I have not connected the vacuum advance. The 28 degrees was a starting setting for mechanical advance at 2500 rpm. I'm sure it goes into the mid-30s with a few more revs. Once I can trust the engine to run long enough without overheating I'll set the total advance.

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The issue with your timing being not as far advanced as it should be points to your overheating problems. Not enough advance in this no-load condition initiates combustion too late to where there's still-burning hot mixture hanging around in the cylinder during the exhaust stroke and going out the exhaust valves and heating up the heads and headers/manifolds.

It's almost impossible to have too much timing with no load at break-in RPMs, at least enough to cause problems. The engine would probably start surging and running odd before anything bad happened. Don't be scared to crank that thing up to 40* for break-in, and/or hook up the vacuum advance during break-in; this is precisely the running conditions that vacuum advance is intended for anyway.