On Sat, 11 Jul 2020 20:53:08 +0100, "NY" wrote:

snip

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.

Valve gaps? As the engine heats up and stuff expands the valve gaps
get smaller, the engine heats up on the longer bits and so appears to
misbehave when on the shorter stuff? shrug

Take the rocker cover off (often easy on such things) when it's hot
and check you still have play on both valves (you make have to turn
the engine over a bit to check both valves (make sure it's off, pull
the plug cap off, keep hands away from everything etc)).

You often need around 10 thou gap (they can be different). Easy to
check / adjust yourself but you will need a feeler gauge set (cheap)
and a spanner / screwdriver.

Cheers, T i m