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Briggs Quantum 6hp won't shut off

Posted by vinnie_s NJ (My Page) on
Mon, Oct 21, 13 at 18:14

Briggs Quantum 6hp on Craftsman push mower would not shut off when I let go of the safety. It has been slowly getting to the point where it would not shut off right away, but then this is the first time it kept running.

Here is where it gets weird. I pulled the plug wire in an attempt to kill the mower, and it ran without the wire attached! (play Twi-light Zone music).

Can anyone explain this to me? Can a bad kill switch do this?

BTW, this mower is 14 years old and has been indestructible. This engine has been incredible.


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RE: Briggs Quantum 6hp won't shut off

  • Posted by ericwi Dane County WI (My Page) on
    Mon, Oct 21, 13 at 20:02

You can still buy "glow plug" type model airplane engines, and these also run with no power supplied to the spark plug. If there are accumulated carbon deposits in the cylinder head, the carbon can get hot and act as a glow plug, keeping the engine running, if fuel and air are available. If you can get it to run without fuel and air, now that would be something strange.


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RE: Briggs Quantum 6hp won't shut off

Well, that could explain that. It was running hot. The engine was smoking after shutdown. There was plenty of oil in it and it was clean. The primer bulb also needs to be replaced.

I hope you were right about the carbon deposits. LOL


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RE: Briggs Quantum 6hp won't shut off

Well, it sounds like it is dieseling. I googled it and that is what it sounds like. I have to look at the plug. If that is OK, there is carbon buildup in the cylinder. How do I clean carbon deposits without ripping the engine apart? Can I even do that?


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RE: Briggs Quantum 6hp won't shut off

Chances are if you cleaned all the dead grass etc. from under the cooling tins that your problem would go away.


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RE: Briggs Quantum 6hp won't shut off

All you guys were right. I replaced the plug because the old one had carbon buildup. Also, I removed the top and blew out the dead grass from the fins and airflow. Man, was it clogged! That grass caused the heat to go up. The grass was also scorched. Fired right up and shut right down! I also replace the fuel line as the old one was really badly cracked.


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