John Rumm wrote:

Recap: line trimmer driving a hedge cutter attachment - started and ran
fine for perhaps 20 mins or so before cutting out, and then would not
restart - even when cold. The usual diagnostics seemed to show that fuel
was getting into the cylinder, there was a spark, and air but no joy.

Anyway, I dropped it off at a local garden centre service place for
investigation. Just got a call to say its beyond economic repair since
the cylinder is scored and hence its not drawing the fuel mixture in
properly on each stroke. They said it looks like it had been run without
two stroke oil in it.

Much as I expected, wrong or lack of oil in the mix is the usual suspect but
because of the way the simple diaphragm carbs work on these little two
strokes there are other ways to cause overheating and partial seizure. As I
said in my original post you can normally see this through the exhaust
port. The ring becomes stuck in the grove as the aluminium deforms over it
and hence won't seal the compression. Adding a small amount of oil to seal
the ring for a few firing cycles indicates this and then confirm by
removing the silencer. The worst damage is normally on the part of the ring
which passes the exhaust port. If this is not the case I would supsect
something else, like foreign objects.

As the mixture strength is set for a given rpm the throttle becomes an on
off switch in use, running from lo idle to full power. Set it to a mid way
speed position and the mixture may weaken but the major effect is there is
not enough cooling air. This became a big problem when nylon cord started
to replace steel saw blades, people would extend the cord such that the
engine could not reach full revs and the engine would overheat and seize
quite quickly.One make, from a cold country, was also notorious for forming
vapour locks in the fuel system even in our summers which effectively
weakened the mixture and caused erratic running and overheating in hot
weather.

Back then we mostly used leaded fuel and this delayed the onset of problems,
the change to unleaded lead to a spate of seizures. A solution ( which was
also prompted by the need to reduce NOx levels) was to provide a bypass jet
in the carb to prevent unintentional weakening of the mixture. A 2 stroke
actually produces more power when weakened beyond its safe mixture as
though torque is reduced the extra revs more than compensate.

AJH