Petrol lawnmower, about 20 years old, Briggs and Stratton 375 engine, fixed self regulating throttle, manual drive.
Started running really rough for some reason.
On start, a bit temperamental
On warm running, rough, sounds like a bit of valve overlap/backfire
Hunting throttle
Carb assembly replaced last year, so I know it's not that. Spark plug gap is OK, I checked it last week. Oil level is good.
Thoughts
1) replace spark plug - it could be breaking down, several years old
2) clean air filter
3) Throttle regulator springs
Any other ideas?