John Rumm wrote:
andrew wrote:
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

If it is that, then obviously I want to work out how I got it wrong, and
also for that matter, how it took six litres of fuel through it to reach
the point of no return.

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

From what I can see of the rings through the ports, they still look to
be in decent condition.

See the photos he

http://www.internode.co.uk/ryobi/

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.

Just realised I forgot to photograph that, but just went to have a look.
The rings through the exhaust port look fine. No (or very very slight)
marking under them on the base of the piston either.

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

Now this is a bit of operator error that I will admit to. At the time of
the failure I was using the hedge trimmer, and with that there is a
tendency to not use full throttle all the time- having said that the
usage would have been a mixture of idle, full, and perhaps 20% partial
throttle (was hacking a pampas grass - so you spend a fair bit of time
working out what cut to make next, pulling dead and cut stuff out etc).

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.

Temp at the time was probably 14 deg C ish - not cold but not excessive.

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.

Any thoughts taking that into account and looking at the pics?

Not oil lack.

Something was inside the engine I reckon, bolt or a bit of metal or stone.,