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