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26309Re: [classicrv] Re: Onan generator in 1984 Allegro

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  • ned.crv@edwordsmith.com
    Jan 1, 2007
      J Carlisle wrote:
      > Happy New Year all,,
      > Since generator questions came up, I have one. On a 67 Travco, I
      > have an Onan, orginal that is powered from a small 4 cyl flathead
      > engine. The previous owner said when he went to start it one day
      > there was a loud pop from around the switch box, or just behind it
      > somewhere.

      Isn't the air filter back there behind the controls? A backfire at the
      carb would be heard at the air filter.

      Personally, I would ignore one backfire, but carb backfires have the
      risk of sending flames into the engine compartment, and that's where
      your fuel lines and filters and stuff are, right.

      Carb backfires mean you have fuel being ignited while an intake valve is
      open. This might be due to mis-set ignition timing, but since the
      Onan's ignition timing is fixed and can't be adjusted (if I remember
      correctly), that would be the last place to look. A better bet would be
      to do a leakdown test. Briefly, this test pumps compressed air in
      through a test fitting screwed into a sparkplug hole. If the cylinder
      doesn't hold the compressed air, you determine where it is leaking by
      listening to the exhaust system and the intake system. A leaky intake
      valve will sound like a hiss at the carb/air filter. If that's what
      you've got, you can probably fix it with some wrenches, a few gaskets,
      probably a new valve/spring/guide, and a little bit of work by a
      machinist to prepare the cylinder head.

      By the way, I saw a leakdown tester at Harbor Freight for $39 a couple
      of months ago. They also sell a portable compressed air tank that you
      refill from a gas station air pump. With these two things and a
      sparkplug wrench, I think you could do a leakdown test on a generator
      cylinder. Being a small-budget shade-tree mechanic myself, I made my
      own leakdown tester several years ago from parts I got in the air
      tools/compressor department at the local home fix-it store. Here's a
      link to a good description of how to make the tool and use it:


      Drilling the 1-mm hole was the hardest part. The tiny drill bit is
      commonly available, but boy howdee, they break easily. It took me three
      tries to drill through the epoxy plug without breaking the bit. Of
      course, a drill press would help, but anyway who has a basement big
      enough for one of those? You might ask at the autoparts store where to
      find a machinist to drill it, if you don't want to hassle with doing it

      Good Luck!

      Ned Bedinger
      Southworth, WA
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