Lawn mower problem

[coccinelle]

I have a petrol powered lawn mower (Briggs & Stratton engine). Last week when I used it it stalled and just stopped so thinking it needed petrol put that in and it did start up again after a while but wasn't the same. It went very slow and had a low sound to it anyway it finished the job. Yesterday, when I started it up it was the same so cleaned the spark plug which was rather dirty and cleaned the air filter. It kicked up well for a minute or two but then was the same. I changed the petrol as I was using last year's but that didn't make any difference. Anyway, after a poor performance but did manage to cut the grass it finally stopped and I can't get it to start again.
Anybody, have an idea what it could be? It has been working well all spring and summer it gets used 3 hours a week.

[coccinelle]

We think we have now found the problem: the air filter. Whether this was the original problem is debatable but the engine turns well without it and then chuggers with it in place.