possible cause for backfiring in single cylinder motor

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  • April 21, 2015 at 3:54 am #1281
    benchrascal
    Participant

      I’ve inherited a bunch of forty year old small Japanese outboards ,mostly Nissans ,they’re all single cylinder.Trying to get a couple to run today and after cleaning the carbs and checking for decent spark and compression the only result is something like a backfire .One has an early solid state ignition and the other is fitted with an after market Atom computer module.Tried everything I could think of but neither wants to run at all ,just the occasional snort when I pull them over.

      April 21, 2015 at 5:35 am #14417
      legendre
      Participant

        First we need to agree on what’s meant by a ‘backfire’. Are your motors making bang-bang out of the exhaust, or out of the intake? True backfires come out of the intake / air cleaner.

        Bangs from the exhaust are +usually+ the result of a rich, un-burned mixture igniting somewhere in the exhaust tract. On the other side, bangs from the intake are +usually+ the result of a too-lean mixture causing the motor to ‘spit back’ out of the carburetor & intake. And it’s generally normal for an older outboard to spit out of the intake a few times when it’s started up from dead cold.

        Many older motors – not just outboards – have fairly crude starting devices, and they can act-up a bit when cold. If you can narrow this down, people will have a better idea of what to suggest.

        April 21, 2015 at 5:36 am #14418
        legendre
        Participant

          Oh BTW, congrats on finding some older Japanese motors.. for the most part, those things are pretty uncommon. 😉

          April 21, 2015 at 9:53 pm #14452
          benchrascal
          Participant

            It is bang out the exhaust ,and then something like hydraulicing making it difficult to crank .Lots of resistance in motors that don’t seem to have much of anything wrong with them .I’m going to try to lean the starting mix right down to see if they are trying to gulp too much gas .Very frustrating because up until this point I’ve had very good luck with these motors.

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