I REITERATE that spark has NOTHING to do with compression. Whatever
"unless the cylinder walls were washed" means is beyond me. Spark is
spark and while a weak spark can be quenched by compression, if you
don't have compression in a 2-stroke it's due to a physical problem
that is allowing the air/fuel mixture to escape either into the
crankcase or to the outside air, which could include piston holes,
blown head gaskets, broken rings, badly scored cylinder walls, etc.,
but a lack of compression and whether you have a spark or not are
UNRELATED. You need a proper fuel/lubricant/air mix, compression and
spark (at the right moment) for a 2-stroke engine to run.

In order to improve compression, you need to lessen or eliminate places
where the fuel/air mix can excape. Check the head gasket, inspect the
top of the piston and look carefully at the cylinder walls; it should
be fairly obvious what the problem is.

Before I tore down my engine, I would carefully redo the compression
test, and then do it again... The individual who 'tested' it may have
been looking for a lower price and fudged on the results. Just a
possibility. The compression should be within approx 10lbs of each
other or less. Closer is better. For example, if you have an older 4
cyl OMC outboard like I do, you might find something like this:

Cyl 1: 100lbs
Cyl 2: 97lbs
Cyl 3: 101lbs
Cyl 4: 99lbs

This would be acceptable.

However something like this would show a problem:

Cyl 1: 100lbs
Cyl 2: 97lbs
Cyl 3: 101lbs
Cyl 4: 79lbs

If you've run the test several times and this is what you see, the
engine may run (sorta) but Cyl 4 has a serious problem. Remember,
always do compression tests with the throttle wide open.

Compression testers are cheap and anyone who has an engine in their
garage (lawnmower, outboard, inboard or automobile) should have one.



trainfan1 wrote:
Chehalis Jeff wrote:
To cut to the chase here, whether a spark plug sparks or not has
nothing to do with compression.

Unless the cylinder walls were washed ...

Rob