Lawnmower Constantly Revs High and Low: Solved


I noticed earlier this summer that my 5.5hp Briggs & Stratton powered push mower was constantly revving up to a fevered pitch, and then dropping to an idle and nearly stalling. Over and over again every 10 to 15 seconds.

Quick Diagnosis (so you can get back outside and enjoy the summer weather):

The air-fuel mixture was off. In my case, due to a plugged air filter. It was gross. I can't believe the engine ran as well as it did. A dirty carburetor could also cause these symptoms.
That should look like a sponge...

Explanation (for those of you who have to know why):

A lot of newer lawnmowers use a governor to control engine speed (rather than the classic "Rabbit-Turtle" throttle lever). If the carb cannot achieve an efficient air-fuel mixture, the system just fluctuates between over-revving and almost stalling.

After washing my filter out with degreaser and warm soapy water the mower runs just like new. Moral of the story: always try the simple stuff first.

For more info check out the links below:

Very good explanation from B&G's own website:
https://www.briggsandstratton.com/us/en/support/faqs/governor-system

How-To Carb Rebuild video by DoubleWide6
https://www.youtube.com/watch?v=_pzWNmFoba0


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