On 11/09/13 15:36, JBL2527 wrote:

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 ?

It sounds like the shaking of the tank has loosened debris of some kind,
causing a fuel-feed problem. Dismantle the carburettor and check for
debris lodged in a jet or fuel passage. Check and clean the filters -
there may also be one in the fuel tank. Check that the fuel pipe is
clear, a plastic one might have reacted to the increased alcohol content
of the petrol - this problem started to surface a little while ago on
older vehicles.

Also ensure the petrol tank cap breather hole is clear.

HTH

--
John Milner