Need help Briggs shuts off when hot

JeremyR · Jun 28, 2014

My FIL brought over a Weedeater brand mower. It is as basic as push mowers come. With 3.5hp Briggs thank mounted card with new diaphragm.It shuts down when warm. plug has been changed more than once, coil was changed with new OEM one still the same thing. I say junk the thing and find another at a yard sale but my FIL says to just fix it.

Jeremy

old · Jun 28, 2014

Simple trouble shooting should find the problem. Does it have spark when it shuts down??? Can you maybe spray carb cleaner in the carb and have it start?? Does it maybe loose compression due to a valve that is to tight?? Or one of the 2 bolts that hold the carb on loose or missing?? I have a friend who has 2 of those and I am always messing with them to keep them running but then she likes to try to cut 2 foot tall grass with then and run it set at about 1 inch above the ground so she only has to make one pass

rrobert · Jun 28, 2014

Does it have correct gas cap? Might not be venting properly.

If coil has been changed and plug is new, then I would check to make sure kill wire is not grounding out. Might as well check to make sure intake tube is snug while there. If plugs keep getting covered in carbon crust, then rings are no good.

Could be a valve seat out of place but it should stay not running if that is the case.

I did have a bad brake cable on a push mower. It controls the brake and the kill switch. At start up, the kill switch would be open. After running a few minutes, the vibration would make the cable relax, engine would sputter and then shut off but not trigger brake. The cable was no good and I replaced it with a used one. No issues after that.

joe csuti · Jun 28, 2014

Without knowing mod/type numbers of eng & age of eng there are 3 possible causes of problem.

1-If pump diaphram located between tank/carb body on older eng a common problem was/is the top of tank would warp allowing diaphram to suck air or allow gas to leak by diaphram & flood vacume passage in carb body that operates pump diaphram when eng/tank get hot,briggs sells a repair kit for this problem pn391413 a little expensive for what's in kit however there are aftermarket kits avail for 1/2 or less of briggs kit rather than spending $40-$50 for a new tank,click on link below & read text/look at drawings,instructions for checking tank apply to both auto choke and manual choke versions of carb,move mouse pointer over text/pic & click left mouse button to enlarge.

2-carb mounting gasket needs replacement or mounting bolts loose & eng sucks air after it gets hot.

3-carbon buildup on valve stems causeing valve(s) to stick in guide(s)when eng gets hot.
tank

JeremyR · Jun 28, 2014

Already tried gas cap, new coil, new plugs, cleaned carb, new diaphragm & gasket, If the weather will corporate with me, He brought over 2 more with the same engines in various states of repair. Right know I'm thinking an air leak maybe with one of the orings on the intake.

Jeremy

El Toro · Jun 28, 2014

Let your FIL take it to a repair shop, he may change his mind. Hal
PS: I have a 9hp Toro Whirlwind he can have. It has a 31-inch cut is self-propelled w/reverse for
$200.00.

old · Jun 28, 2014

As with any engine you need a good blue/white spark that jumps a 1/4 inch gap or more. Also seen the carb intake tube crack and that will cause odd problems like your having because it opens up the crack when hot and sucks air

Mark-Ia · Jun 28, 2014

As Joe stated; check the valve stems for carbon buildup. If the previous owner wasn't good at changing oil, the stems will carbon up and do as you describe.

T.R.K. · Jun 28, 2014

About the only thing you have have not mentioned is insufficient valve clearance. check the clearance between the tappets and the valve stems. When it is hot a valve may be staying open.

Need help Briggs shuts off when hot

JeremyR

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old

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rrobert

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joe csuti

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JeremyR

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El Toro

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old

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Mark-Ia

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T.R.K.

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