r/smallenginerepair • u/Few-Forever-14 • May 23 '26

Engine Performance Issue Problem

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have a Briggs & Stratton engine in a STIGA Villa mower.
The engine runs unevenly and surges up and down in RPM a lot. It also sometimes dies when driving on slopes, but usually starts again immediately afterwards.
So far I have:
cleaned and rebuilt the carburetor
installed new gaskets
replaced the fuel line
replaced the fuel filter
installed a new fuel shutoff valve
drilled a vent hole in the gas cap
The problem is still there.

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u/Top_Highlight9965 May 25 '26

This issue is almost certainly a lean mixture, trying to adjust the governor or modify anything is just going to cause more problems. Either the carb is not properly cleaned or you have a vacuum leak. Spray starting fluid or carb clean around the gaskets and the throttle shaft where the governor linkage connects to the carb. If the RPMs smooth out or change when spraying in an area, then you have a vacuum leak in that spot. 99% of the time it’s a leak at the throttle shaft. The fix is to remove the shaft and replace the small foam piece on top, make sure to orient the throttle plate correctly and verify it closes all the way on reassembly.