What is the fuel vacuum on the fuel line going to the engine? If you have vapor lock as they say, the vacuum should be high. Substitute a good know portable fuel tank and hose going to the engine and drive it. If you have a restricted fuel tank lick-up, anti siphon valve, or supply line, opening the fuel cap will have no affect on it. Vacuum should not exceed 2.5 inches of vacuum. A boost pump will not fix a restriction in the fuel supply.
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