On 11/07/2020 23:49:20, T i m wrote:
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

In which case the engine is more likely to stall under heavy load; where
the valve stems will expand more with temperature.

The OP said "will suddenly stall without warning, usually when cutting
short sparse grass, rather than when it's working harder to cut denser
or longer grass."