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Thread: Back Firing Mower , Will it hurt if I ??

  1. #21
    Boolit Master
    Join Date
    Jun 2012
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    West Tennessee
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    2,160
    Redneck1 is exactly rite on this one! Is just unburnt fuel in the muffler. I had a zero turn with a Briggs and Stratton that did the same thing for about 14 years and around 800 hours until it finally wore the valve guide to the point it was not repairable and the engine be had to be replaced. Heck, when I was a kid and school buses had gasoline engines, we had a driver that loved to turn off the key for a few seconds so the exhaust would fill with unburned fuel, kick the key back to ignition and cause a huge backfire! Choking it on shutdown will also put fuel in your oil and degrade it's propertiesas well.

  2. #22
    Boolit Grand Master Tatume's Avatar
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    Jan 2010
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    VA
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    I've been following this from the beginning, but haven't said anything until now. What the OP is describing is not back firing. It is commonly called "run on." It is caused by a source of ignition other than the spark plug, often carbon deposits in the combustion chamber that continue to glow red hot. Sometimes an engine will also back fire as it runs on, but this is a secondary symptom.

    Back fire is ignition outside of the combustion chamber, in the intake system (carburetor) or exhaust system (muffler). Back fire requires that fuel/air mixture be present in these locations along with a source of ignition. High schoolers used to induce back fires in the exhaust system by coasting with the ignition off, then turning the ignition back on. This would sometimes blow the muffler off. Back fire in the intake system sometimes causes a fire supported by the fuel system, which can be catastrophic. This sometimes happens when working on the engine and starting with the air filter off. Smothering the fire with a towel or cranking the engine to suck the fire inside can save the vehicle.

    In any case, the OP is not experiencing back fire. Here is some good information on the phenomenon.

    https://www.howacarworks.com/ignitio...ing-running-on

  3. #23
    Boolit Master
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    Jun 2012
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    West Tennessee
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    I respectfully disagree that the OP is a describing a "run on". Any of us from the pre fuel injection days know about real "run on" where you turn off the ignition and the car sits there and sputters and clunks along a few seconds before it dies. He says a single popon killing the ignition at full throttle. Fuel injection stopped run ons by killing the fuel at the same time as killing the spark. A naturally aspirated engine continues to pull fuel even with no spark. That, and catalytic converters, are why you can't kill the ignition and cause the popon today's cars.

  4. #24
    Banned
    Join Date
    Jan 2010
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    ohio
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    I'm done ... some people can't conceive of being wrong and have gone to great lengths to make a mole hill into a mountain .

    So to the op if the engine is running good otherwise don't worry about the backfire when you shut it off and forget to idle it down it won't hurt anything . Worst case is it someday splits the muffler .

  5. #25
    Boolit Master
    Mal Paso's Avatar
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    Jun 2010
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    I never see valves mentioned. It's one of the few adjustments left after points were eliminated and "they" made carburetors nonadjustable. Valve stretch is more common in small engines than in cars. It's the second thing I look for after vacuum leaks. Valve stretch causes tight valves (valves not closing) causing performance issues and burned valves.
    Mal

    Mal Paso means Bad Pass, just so you know.

  6. #26
    Boolit Grand Master Bazoo's Avatar
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    Dec 2013
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    Cecilia, Kentucky
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    I've been choking my mower out for years... its and old murray with a 11 horse briggs. It always used some oil.. and still is the runningest sumgun you ever seen. I say it wont hurt it. All it is doing is closing the butterfly valve at full throttle.

    It backfiring wont hurt it either, less it blows the muffler apart. I had an old mower once, I took the muffler off, and put on a short piece of iron pipe. I used to mow at night and it was cool to see it spitting fire. Mowing at night is mighty peaceful out in the country.

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