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  1. #1
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    May 2010
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    N.E. Arkansas (transplant)

    Default Kohler 15.5 HP issue

    On a Craftsman riding mower. It always starts fairly well hot or cold, But when warmed up with no load on the engine, it backfires and pops back through the exaust. Throw the mower and deck in gear and mow with it the backfire goes away and it runs perfectly. Stop and take everyting out of gear, backfire returns. Also theres no difference at full throttle or idle.
    Compression is good, 110 psi. plug while not new is clean and a bit darker brown than the normal tan one usually associates with a properly burning spark plug. fuel is fresh.
    I've thought about the possibility of a carboned up piston, possibly a burned or stretched exaust valve, or possibly ignition/timing or carb issues BUT All those issues should result in it running worse, not better, when putting the engine under a load.
    My current hypothesis is that the carb's on the rich side thus dumping a bit of unburned fuel into the exaust with no load, then of course burning that fuel when the engine is put under a load, but the mower hasnt acted this way until mid summer of this year.
    Also a neighbor down the road has an old rider with a 14 or 15 hp briggs that suffers the exact same symptoms.

    Anyone got any thoughts?

  2. #2
    Gold Member Oldtractorfixer's Avatar
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    Mar 2012
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    Oregon City, Oregon
    Tractor
    several

    Default Re: Kohler 15.5 HP issue

    Kind of sounds like a vaccum leak. Might make sure the carb mounting bolts are tight. Might also be a plugged up idle curcuit in the carb. I'm just guessing.

  3. #3
    Gold Member
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    May 2010
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    Location
    N.E. Arkansas (transplant)

    Default Re: Kohler 15.5 HP issue

    Spraying carb cleaner around the carb makes no changes in the way the engine runs so that tells me there is no vacuum leak. Also if there were an idle circuit issue in the carb i'd think the mower wouldnt idle. The mower's always had an inline fuel filter so the chance of it being crud in the carb are reduced quite a bit.
    I guess after things slow down a little as winter approaches i'll get serious about finding the problem.

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