Don,
I think the shortest time to overheat an engine, from a cold start, is with no coolant circulation. That's why your getting a lot of suggestions about the thermostat not working, or vaporlock at the thermostat, etc.
IF you're confident that water/coolant is circulating properly (if the upper radiator hose gets hot, that's a pretty good sign it's at least circulating at the radiator . . .), then maybe something else is to blame.
Stupid questions first: Is the fan definitely turning and circulating a good amount of air through the radiator? Fan belt good and snug?
I agree with truckernix, that misadjusted timing would contribute to overheating, but not THAT severely. And it seems to me that if timing were an issue, you'd have other symptoms along with the overheating.
I'm not experienced enough to comment on misadjusted valves leading to overheating. Seems like too much open on the exhaust could lead to over-hot valves, but wouldn't peg the temp gage, I think. Too much closed on the intake could lead to a lean condition maybe? There again, you'd see other issues I think.
My 216 started overheating badly from a lean condition. Pulling the choke out a third cured it instantly. BUT, the choke had to be in just the right spot. So maybe you're running lean? Carb float ajusted to low? Lean condition also can be caused by vacuum leaks. Is the carb tight on the manifold? Manifold snug on the block?
Hope this helps,
-Michael
