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I may have a similar problem and perhaps a common solution. My 1975 164E with 238,000 miles in the last year or so has taken two to three turns of the ignition and cranking to start after it has been sitting a few hours. I have checked everhing: cold start, thermal timer, fuel regulator at 30 psi, trigger points,etc. but the ability of the fuel system itself to hold the required 16psi when sitting. There is a built in check valve in the fuel pump that is to prevent fuel from draining back into the tank when the system is off. I plan on buying a new pump if and when I verify that the check valve is not holding.

A new pump is over $200, In the interm I use the following starting procedure:
I crank the engine for 3-4 seconds stop then turn the ignition on until the fuel pump stops, then crank the engine again. This method usually works. Otherwise I could crank for 30 seconds or more and it would not start. It seems that I am priming the system evertime to start the engine. Then it runs great and restarts within a i hour time frame are great also.