87 KDX 200 melted piston. Ideas

[Corndog26j]

Have an 87 KDX 200 I’m working on. It had not ran in many years. Rebuilt carb and minor cleaning with some new parts. It was running great.
Riding it a couple weeks ago and it just popped and quit. Loss of compression.
In tear down of the top end I find the exhaust side of the piston is eaten away with a hole in it right around the rings. Almost looked similar to detonation damage but only on the exhaust side.
Aluminum was deposited on the exhaust side of the cylinder wall and exhaust port. Going to hone it out today and check. Will have a new piston here soon and will have the machinist bore to fit ……..

The crank appeared to have sufficient oil. Even the intake side of the cylinder was fine. Looks as if the exhaust side just overheated.

I found the linkage for the “governor” was not operating…. There is a link missing on right side going to the lower end. (I’ve worked tons of 2-strokes in my days but haven’t had any time with this type system).

So my question. Why did this burn up only on the exhaust side? The intake boot does have some surface cracks but don’t appear to go all the way thru. (I’m sourcing a new one). Crank seals? Stock jetting —does not appear to be lean. Timing? Or riding it too hard?
Trying to set it up for mini moto track days. It will be high rpm riding. Ideas please.

[Molly's 70]

What does the spark plug look like? Pictures?

[Corndog26j]

I’m not certain how to attach photos on here.

[Corndog26j]

The plug is a darker gray/black color. I’d say the mixture is good. There is some small particles on it from the piston.

When I sat the cylinder on my workbench on its side, I had a good amount of oil oozing out of the exhaust side port. I’m guessing this has the exhaust valves to act with governor functions. This “exhaust valve” system is not fully connected on this bike. I was not aware of that until the teardown.

I need to read up on this system.

[Chuck78]

Too rich at part of the throttle range, too lean at other throttle/RPM ranges perhaps? Powervalve not functioning properly definitely could allow this to happen.

[Molly's 70]

I'd check my carburetor jetting against stock jetting to see where I was at.