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

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.,

The Natural Philosopher wrote:

Not oil lack.

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

Yes it doesn't look typical of an overheating seizure

AJH

John Rumm wrote:

on the induction / compression stroke, the upward motion of the piston
sucks the fuel/air/lube mixture into the lower part of the crank case
while also compressing the charge in the upper part of the cylinder.

Yes
On
the power stroke, the piston is forced down, and close to the bottom of
its travel it exposes the exhaust and induction ports. Exhaust goes out
one side, and fresh fuel etc spills round a channel in the cylinder wall
from the underside of the piston to the combustion side.

Yes the descending piston pressurises the crankcase and forces mixture via
the transfer ports into the cylinder.

So if the piston and rings are intact (at least the top half anyway),
and there are no scores etc in the top bit of the cylinder, the
induction should still work.

Yes but not if the crankcase seals are blown. One of the first signs of
crank bearing wear is when it won't sustain a slow tickover, at higher
tickover the leakage is less significant.

Is any of the spark plug interior missing? If you bring the piston up on its
forward stroke just after the exhaust port is closed and then feed some
coils of starter cord into cylinder via the plug hole and then rotate the
flywheel to jam the piston against the rope does the flywheel slip?
Clutches tend to have a normal thread that needs impact to remove and
flywheels the same but opposite thread IIRC but I haven't stripped a
chainsaw that far for 20 years as it's generally a gonner by then.

Two strokes tend to be predictable from cold and then difficult when warm if
they don't restart straight away. I think some of this is due to the fact
that petrol only ignites in a limited mixture range, when cold if the
mixture is too rich the petrol wets the walls (and sometimes spark plug and
drowns it) but the remaining mix is ignitable, as long as it fires quickly
enough to blow the excess out.


There is a spark, and yet it does not
start. This suggests to me that either there is damage to the cylinder
at the sides where I can't see it through the ports, or the timing of
the spark is out.

It is known for misfires to shear the woodruff key to the flywheel which
hoses the magnets.

The ignition module seems to be a sealed unit on the part diagram. How
does it sync with the crank rotation?

By its position relative to the magnet in the flywheel that induces the
primary current. There's probably some clever solid state stuff inside that
tells a capacitor when to discharge through a coil, including some
advance/retard function


AJH

John Rumm wrote:

ok understood. The rear access to the crankcase is easy enough to
assess. The front of it is obscured by the housing at the moment until I
can work out how to take the final output and clutch off.

Fill the crankcase with parafin and see if the level drops over night.

Typical FOD is from leaving the air filter off but I have seen screws
holding the throttle butterfly come off, the middle electrode or ceramic
from the plug being lost and bits off the ring ends.

AJH

On 22 Apr, 18:55, andrew wrote:

Yes but not if the crankcase seals are blown.

This is a Ryobi. They have a habit of the screw fasteneres working
loose and so rather than a "seal" having failed, there's just a gap
opening up between cylinder and crankcase.

Strip and re-assemble with Loctite. Needs an odd-sized Torx #27, but
easy.

On Thu, 22 Apr 2010 18:55:50 +0100, andrew wrote:
It is known for misfires to shear the woodruff key to the flywheel which
hoses the magnets.

Yes, BTDT... my lawn edger was a non-working cast off from a friend, and
upon dismantling I found the key had sheared (the flywheels are pretty
soft metal). I ended up cutting a deeper groove in the flywheel and
making a new (slightly taller) key. That did the job until my homebrew
key disintegrated :-)

I tempered and quench-hardened the next version, and that's been holding
since, although the engine's still a bit temperamental (I suspect worn-
out carb)

cheers

Jules