The following written description was provided via
http://forums.pstuning.com/index.php?showtopic=1559
Google images of "Piston wash" to see what it looks like. Same principal applies to all 2 strokes but, the below was written for snomobiles...
I've never read or heard a good explanation... so I can only share what
I've learned over the years. If you understand what "piston wash" is, then
it's pretty easy to know what to look for and what to look out for.
The first thing you need to realize is that the top of the piston is not
a uniform temperature. The area in front of the exhaust port that is
exposed to all of the hot gases exiting the cylinder is naturally the
hottest. The area in front of the transfer ports that is cooled by the
fresh air and fuel entering the cylinder is obviously going to be the
coolest. And, the center of the piston will be somewhere in between.
The second important thing is that there is some small range of
temperatures where the piston is hot enough to char the oil that comes in
contact with it, but cool enough not to melt the piston.
Putting those together, you have "piston wash".
When the engine is jetted very rich, most of the piston (except near the
exhaust port) is too cool to char the oil; and you will have large areas
spreading out from the transfer ports that appear to have been "washed"
clean of carbon by the air and fuel entering the cylinder... that is "piston
wash".
If you jet down leaner, the temperature of the whole piston will
increase... and more areas will be hot enough to char the oil. The "washed"
areas will be large circular areas, located just in front of the transfer
ports. At this point, the center of the piston is quite hot; and is
actually charring the oil on the underside of the dome, right in the center.
Jetting down further will cause the whole piston to get still hotter.
Now, nearly the entire piston is hot enough to char the oil... and the
"wash" will be just a small area about the size of a half of a dime, right
in front of each transfer port. The bottom side of the piston will be
charring the oil over a large area by now also. At this point, the area
near the exhaust port is getting almost hot enough to melt. This would be
considered (at least by me) to be jetted correctly... but near the "edge".
If you go leaner yet, the whole piston will be hot enough to char the
oil on top of it and there will be no "washed" areas left. The entire
piston will be covered with carbon, and the aluminum at the edge of the
piston, right in front of the exhaust port will actually be slowly melting
away and smearing onto the rings... more than a few seconds of this and it's
time for new pistons.
That, for whatever it's worth, is my own twisted look at piston wash.
Some things to keep in mind include the fact that the size of the
"washed" area depends somewhat on the upward angle of the transfer port. If
the port is angled flat across the top of the piston, there will be more
cooling and more wash... in spite of the fact that the area in front of the
exhaust port might be just as hot as it would be on a motor with upward
angled transfers that shows much less wash. The upward angled transfers
don't cool as much of the piston top.
Also, the size of the "washed" area has to be somewhat proportional to
the size of the piston. A "half a dime" sized wash area on a 600 triple
(very small piston) means the piston is a lot cooler than a "half a dime"
sized wash area on an 800 twin (with coffee cans for pistons)! What you're
really interested in is how much of the piston isn't clean, because that
tells you how much of the piston is hot enough to char the oil... and you
know the exhaust area is hotter yet.
All this really means is that you can't just say, "Every motor should
have a half-dime sized wash area." You have to correlate the size of the
wash area to other indicators and engine specs... and use it as just one of
your tools for jetting.
