The powered lawnmower has a Briggs and Stratton engine, normally bullet-proof. A couple of weeks back the bolt holding the fuel tank to the body worked loose and the tank was wobbling. I was aware of this because the engine started to rhythmically go fast/slow. It would lose revs then pick up then lose again... continually. As this was not normal I stopped the mowing and had a look and spotted the loose tank, re-tightened the bolt and continued. The mower still drops revs and then picks up again.
I have cleaned the air filter and plug but no difference.. so what do I to do next to get it to run at a constant speed ?

Cheers

JBL