"NY" wrote in message
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"Fredxx" wrote in message
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On 11/07/2020 20:53:08, NY wrote:

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.

Sooted spark plug does suggest carburation issues.

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.

Which suggests mixture is perhaps closer to stoichiometric with higher
loads.

Does it tick over / idle without misfiring?

The engine misfires under all loads, from idling (blade turning, but clear
of grass) up to heavy load. It's the intermittent stalling that seems to
happen a lot more frequently under light load (but not idling).

That’s crucial information that you should have included in the original.

The suggestions about the carburettor seem very plausible - particularly
to the mixture being too rich, given the increased fuel usage and the
stronger smell of the exhaust (incomplete combustion?).

Would over-rich mixture (without an accompanying problem with the governor
at light loads) explain the symptoms?

That wouldn’t normally produce the stalling on light grass but not on long
grass.

The timing could be way out due to hitting something solid and breaking
the key that has the flywheel fixed on the crankshaft. But again, hard to
see
why that would see it stall only on light grass.