On 11/07/2020 20:53, NY wrote:
I have a Spear and Jackson lawnmower with a 125 cc Briggs and Stratton
engine (this one https://www.argos.co.uk/product/7597004). After a
year's perfect service, the engine has suddenly started coughing,
spluttering and backfiring, and sometimes repeatedly stalling as soon as
it tries to cut a trivial amount of grass (paradoxically, it cuts longer
grass without batting an eyelid).

At first I thought it was running low on fuel, because the uneven
running sounds identical to that which leads up to running out of fuel.
But there was plenty of fuel. And the fuel is about 1 month old - it's
not been standing around for ages for its most volatile fractions to
evaporate.

I found several things that *could* have been the cause - but weren't.
There was a build-up of grass mowings stuck to the underside of the
mower, which could have been making the blade harder to turn - fixed
that but the problem persisted. The air filter could have been blocked -
the sponge filter looked clean but was dripping in oil, so I cleaned it
in hot soapy water and gave it a *light* coating of oil as the manual
says, but to no avail. The sparking plug was coated in a thin layer of
black carbon, but the gap looked OK and there was no coking-up of the
contacts - I cleaned the carbon off the underside of the tag and the end
of the rod (ie the surfaces between which the spark jumps) but that
didn't help either.

I've got it to a state where the mower will usually run for tens of
minutes, with irregular misfiring and occasional backfires, but will
suddenly stall without warning, usually when cutting short sparse grass,
rather than when it's working harder to cut denser or longer grass. And
once it starts stalling, it does it repeatedly. My impression is that it
seems to be using a bit more fuel and that the smell of the exhaust is
stronger, so could the mixture be richer? The regulation of speed (ie
the governor) seems to be as good as ever: when the mower encounters
tougher grass, the engine note gets louder but not slower. I've tried
with the drive to the wheels disengaged (ie I have to push the mower) in
case that was a factor - but it made no different.

Any ideas? Is it time to "get it looked at"?

Jeeez - FFS get a decent cordless mower, just a quick blade sharp-up
now and again and that's it ....