Hi my mower has a problem. It starts perfectly and seems to run perfectly for the first minute, then starts to rev up and down. It doesn't stall but seems like it wants to. I checked and replaced the spark plug, and I also checked the air filter. So both are new and this problem is still happening. Also, this past winter I left gas in the engine over the winter. In the spring I emptied the old gas and refilled it with fresh stuff.
The problem you have is surging.
Surging on a governed engine is always caused by insufficient fuel to maintain a high speed.
Two causes
1) Air leaks
2) Dirty carb or fuel lines.
Start the mower and spray WD 40 ( or similar ) from a TRIGGER PACK all around the engine side of the carb & the manifold.
If you get a lot of white smoke r a big change in engine speeds then you have an air leak.
