[Andy Dingley]

On 20 Apr, 13:01, John Rumm wrote:

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.

Scoring is caused by hard grit - easily done with careless use of
petrol containers. In particular, for cheap strimmers, a rust particle
is hard enough to cause scoring. OTOH, scoring doesn't loose enough
compression to really matter, unless it's really excessive (strimming
in a volcanic ash cloud perhaps).

Pickup looks like scoring but isn't. This is inter-metallic sticking
(galling) between ring and cylinder, leading to a broad, shallow
score. This will write off a cylinder quite easily. It usually starts
from poor storage out of use. Ryobi claim themselves that they're at
risk of this, hence their recommendation to only use synthetic two
stroke oils which don't break down so easily to an acidic residue.
Dry storing over Winter is a big help, as is pulling the plug and
giving a squirt of WD40 before storing. OTOH, giving a drop of 3-in-1
instead will encourage it, as that stuff is dreadful for its breakdown
products.

Running without lubrication won't score but will give a general
pattern of wear instead. You won't see this so easily, but you'll
measure it and it certainly ruins compression. OTOH, it takes quite a
while to develop, in terms of domestic strimmer usage.

My money is on corrosion leading to some form of inter-component
pickup without any foreign solids being needed at all. A tale familiar
to Jag owners.

[andrew]

John Rumm wrote:

From what I have seen, the foreign object scenario looks most likely.
See what you think -

Looks like two incidents, the score on the intake side has been polished but
the ones that took out the piston look fresher. Are you sure there is
nothing missing from the carb or air cleaner?

AJH