15.5 B&S hunts and backfires

Discussion in 'Lawn Mowing' started by 74baja, May 30, 2010.

  1. 74baja

    74baja LawnSite Member
    Posts: 16

    Hi all,
    I have a Bobcat Walkbehind Classic with a 15.5 Briggs and Stratton. It runs great under light load, except it needs about 5 minutes to warm up before you try to cut at full throttle or it will bog down, then start to run rough and throttle-hunt. When there is a heavier load (taller grass) it will start to bog, the governor will kick in, and the engine will start to backfire while the governor hunts. I can see the throttle moving back and forth when this happens. It won't smooth out (even when load is gone) until I manually throttle down for a few seconds. I removed the carb and sprayed carb cleaner in all jets and passages.
    What's up?
  2. topsites

    topsites LawnSite Fanatic
    Posts: 21,653

    How's that PCV rubber tube looking, if it's cracked or corroded it might need
    replacing, whether it will fix the problem I could not tell you but I do know when
    that tube leaks air it will make the engine run like crap.
  3. underPSI

    underPSI LawnSite Member
    Posts: 235

    Sounds to me like a full carb rebuild is in order. This requires you to actually disassemble the carb and blow out the jets with compresses air. Don't try to blow it out still assembled or you'll damage the float. After the jets have been blown out let them and the rest of the carb minus the rubber parts soak in some carb cleaner overnight.
    When's the last time you performed a tune-up such as air and fuel filter replacement?

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