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"?