My 340 getting 6 mpg

trapster said

I think I am getting about 8 in my 64 dart with 340 3.55 gears 10.5 to 1 CR 750 vac sec 284 484 cam

one thing to consider is everytime you stop the gas in the Carb evaporates pretty quickly.

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Only if there's something wrong with your carburetor or you have a major heat soak issue.

I've never had that issue with my Holley double pumper, even in 110° temperatures. Even the Edelbrock I run on my truck takes several days.