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I have a Briggs & Stratton 500 Series engine on my push mower. Lately, whenever I am moving forward with the mower and then stop the engine begins to stall. It never actually dies but comes very close. After about 15 minutes the problem tends to go away but never completely. I can also replicate the issue by tipping the mower up on its front wheels. Any ideas what could be causing this?

The gas is one week old. The air filter is clean. The spark plug is clean. I sprayed carb cleaner in the carb and it fired up right away.

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  • If I'm understanding this correctly, the engine almost stalls when you come to a stop, right? Does the RPM pick back up right away? I mean, you stop, the engine roars down, then comes back up to speed? Or does it remain slow? Do you see any black smoke from the exhaust when this happens? This appears to be a fueling issue, but I'm trying to understand exactly so I can give you the correct advice.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Aug 7, 2016 at 14:03
  • The RPMs do come back up right away. There is no black smoke. I also noticed today for the first time that there is occasionally a smell of gas while mowing. This either wasn't present before, or it was not enough to notice.
    – aullom1
    Commented Aug 7, 2016 at 14:49

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I think your issue could be one of two things.

1) The float in the carburetor may be fuel saturated. If it is leaking and not closing the needle valve all the way, it could be causing the carb to flood slightly, which would make it slow down a bit when changing the moving vector of the lawn mower.

2) The needle valve itself is not seating correctly and letting too much fuel through when you don't want it. This is less likely, but could be happening.

Even if either of these two things aren't in play, I still believe its a fueling issue. I would bet either a carb rebuild would fix it (if you can get the rebuild to work correctly) or a new carb would definitely do it. While a new carb would be more expensive, I have been opting for this over a rebuild kit as I can never seem to get the rebuilt carb to function correctly. Getting a new carb for your small engine will save you a lot of time and hassle in the long run over the rebuild as well. A new carb will usually cost about 3 times as much as a rebuild kit, but is well worth it in the end, in my humble opinion (I always consider: What is my time worth?).

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  • I'll give a new carb a try.
    – aullom1
    Commented Aug 7, 2016 at 15:58
  • For anyone else who is looking for online manuals, Mower Parts has a nice listing that gives both the mower and engine manuals. In my case its an MTD mower with a B&S engine.
    – aullom1
    Commented Aug 7, 2016 at 19:46
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    Just got the new carb installed. The mower fired up right away, and the problem appears to be solved.
    – aullom1
    Commented Aug 12, 2016 at 1:47

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