September 22, 20169 yr Had my exhaust exhaust for a repair and peeked inside the cylinder. Took some photos of it at BDC, mid way through and TDC. When it was at TDC (see third pic below) I saw this browning on the skirt. Is this normal? Or is this something I should be concerned with? Recent compression test showed 155 PSI so I don't think it is blowby. Plus why would the middle of the piston skirt be okay, but the bottom have blow by? No noticeable issues except that I do have a slight knocking noise in the engine (not clutch related). I've been trying to track that down and wonder if this could help explain it.
September 23, 20169 yr It could be staining from exhaust gasses, as the piston closes the port it will create a slight vacuum pulling gasses back from the pipe, there are also exhaust gas pulses exiting the pipe which create a negative charge which travel back up the pipe preventing excessive unburnt charge escaping into the exhaust improving burn efficiency. Theres more to it than this, 2strokes aren't so simple after all, they probably shouldn't work, there's black magic at work which is why I like them. Anyway, if it's knocking check the con rod, piston and main bearing clearances/condition.
September 23, 20169 yr I once had a similar "browning" effect, not on the piston itself, but definitely caused by it being "agitated" a little too much in the barrel
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