I have a mower 6.0 HP Briggs Stratton engine. I let it sit over the winter
with gas in the tank. I did however add fuel stabilzer. I fired it up for
the first time this season and it is not running well. It runs fine for
about 30 seconds then lowers the idle then back up then back down. Not a
stable idle. I have changed the fuel and cleaned the plug and even cleaned
the carb, although I did not take the carb apart I did clean it up inside
and out with gumout carb cleaner.

My questions are should I replace the plug and/or carb?
The carb is only 22 bucks, so I would rather not rebuild it.

What may have caused this?
I added the fuel stabilzer to my genny, pressure washer and tiller, stored
them with the gas in the tanks and have no problem with them, its just the
mower.

Thanks for any help on this

Searcher

What kind of governer does the engine have?
If it's flywheel fan type, kick the mice out of the housing.