Flathead does not start when hot.
 

My 51 flathead starts without any problem when cold. If I drive it for 15 minutes then shut it off, it will not start unless I pop the clutch or let it cool down for an hour. Suggestions?

Could be the power valve in the carb Is bad and letting gas flood the engine.

Sounds like high voltage draw by a hot starter.

Vapor lock: Engine heat causes the fuel to boil and then vaporize in the fuel line when the engine is shut off. This was a very common problem with all makes of flathead engines back in the day.

There were all sorts of "roadside fixums." One was to attach wooden clothes pins on the fuel line from the pump to the carb, another was to wrap the line with aluminum foil.

Vapor lock also affected early OHV V8's with 6V electrical systems. You'd crank the engine until the battery went dead, what a PITA that was!

The fixum was to install an 8V or a 6/12 battery, then open the hood to let the engine cool a bit. 6/12 had a 12V starter solenoid mounted on the top of the battery.

It would start on 12V, then switch to 6V after the engine was running. I installed 6/12 batteries on several 1949/50 Cattle-Hacks and 1950 Oldsmopiles I owned.

Modern gasoline has a different vapor pressure, because carburetors don't exist anymore in production, everything today is fuel injection. Consequently vapor lock today is probably more of a problem than it was back then in carbureted engines, especially noticeable if you happen to have a big slug of winter gas in the tank come summer. The T guys always suggest adding a pint of Kerosene per tank for vapor lock, I use a couple glugs of Marvels mysterious oil in my Y-block in the gas tank. See if that helps, it seems to. Can't hurt.