i have a 78 f150 with a 74 351 c 2v.here recently when the rpms build it has a misfire or maybe a backfire its a deep boom boom spit type sound other than the higher rpm range it dont do it.i changed fuel filter rebuilt carb and cleaned and regapped plugs reset timing and it still does it.any ideas

 

Look under the hood in complete darkness. Might have a wire or two jumping spark to ground.

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ill try that but would it only do it at hi rpm and not all the time

 

Wire a porcelain resistor in a wire directly from the battery to the coil and see if it fixes it. I had the same problem on a '76 with a 390. Turned out the wire going to the coil had been stretched and wouldn't deliver the correct voltage.

 

ok i might try that appreciate the help guys