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259 2 barrel backfiring upon hard acceleration

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  • Engine: 259 2 barrel backfiring upon hard acceleration

    My 1955 Coupe runs well, but now, under hard acceleration or very very steep hills (there is only one) it has started to backfire.

    I admit that I can't remember when I performed the last tune up.

    Where should I start?

  • #2
    sounds like it is running lean
    remember today's 'gas' is 90% fuel and 10% 'filler'
    i have had to open the jets 10% on several of my old cars to get them to run correctly
    if manual choke, try leaving the choke on a little and driving that way and see if there is a difference
    that is how i confirmed my suspicions on my M5 and then reamed the intermediate jet out 10% for much better performance
    1947 M5 under restoration
    a bunch of non-Stude stuff

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    • #3
      Generally, this would be a sign of the mixture going lean. If it happens immediately, as you are depressing the pedal, it would suggest that the accelerator pump is not working properly. If it is later, at wide open throttle, it may be a weak fuel pump that can't keep up with the carburetor.
      Jim Bradley
      Lake Monticello, VA
      '78 Avanti II
      sigpic

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      • #4
        Do a complete tune up first to include distributor cap and rotor button

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        • #5
          You might also give the engine a few more degrees of timing (after verifying the jetting).
          It's common for this to happen when the timing is low. Normally....the engine will run much better with at least 6 degrees more thAn book timing.
          And...as the others have stated, it does possibly sound lean.
          Try a two jet number increase from what you have now, primary and secondary, (if a four barrel). Secondary may take a 3 or 4 number increase...depending on many things.

          While "normally"...todays fuel will not require a jet change, BUT if the carburetion was a tad lean before the new (new ?, the mixtures were changed some time ago !) gas, then yes...a little more fuel is required.
          But...10% as noted elseware is a bit excessive, so his engine musta been real lean pryor to the fuel mixture change.

          Mike

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