thats not heat from lean.. that would be only on the exhaust side right in the center of the exhaust port and top of pistion. this is on the intake side and exhaust sides not center. could be too tight since the failure isn't near the top of piston, mostly at the middle or bottom which is tighter than the top. I think the sign to consider is that it happened right at the relief holes.. I haven't seen this before. I'd be sure that the premix is done right with really good oil.
KDX 200 Repeat topend failure
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- Start date
Bridge releif on the old iron cylinders was like .004 inch between the piston skirt and the bridge. You say that the piston to bore was a tight .002, good, but what about where them seizure marks are at? The oil holes are to provide lube to the bridge and insufficiant clearance at this point will cause seizure. Also on a cold seizure I would think its the same points. Either way the piston expanded or the bridge expanded, The manual usually covers this area.
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An assembly error caused this problem. The engine had overheated, it was not due to a clearance problem, nor premix ratio, type of oil, air fuel ratio, nor the way it was run in.
I made the mistake of having the air dam within the reed valve assembly 180 degrees out. This meant that the inlet air fuel mixture was shielded from the left hand side of the rear of the piston(inlet side) and subsequently a lack of lubrication caused excessive heat build up and final rooting of piston and cylinder lining.
The white plastic air dam is offset to receive the angled air from the carburettor rubber boot and distibute evenly to the inlet port. If rotated 180 degrees the off set is reversed and shields the left hand side of the engine.
An exspensive lesson learnt, take a bit more time in assembly (it was obvious in hindsight), or get a manual!!
The thing that caught me out ws teh lettering "DX" that I mounted right way up, whereas I should in fact have aligned the two tangs that are provided on the cylinder and mounting plate respectively.
It was through the forums help, discussion with a reconditioner for piston failure diagnosis and the motor bike mechanic for insisting that it was a lack of lubrication or reckless running in that I was able to build a picture of the nature of the failure and locate the now obvious assembly error.
I made the mistake of having the air dam within the reed valve assembly 180 degrees out. This meant that the inlet air fuel mixture was shielded from the left hand side of the rear of the piston(inlet side) and subsequently a lack of lubrication caused excessive heat build up and final rooting of piston and cylinder lining.
The white plastic air dam is offset to receive the angled air from the carburettor rubber boot and distibute evenly to the inlet port. If rotated 180 degrees the off set is reversed and shields the left hand side of the engine.
An exspensive lesson learnt, take a bit more time in assembly (it was obvious in hindsight), or get a manual!!
The thing that caught me out ws teh lettering "DX" that I mounted right way up, whereas I should in fact have aligned the two tangs that are provided on the cylinder and mounting plate respectively.
It was through the forums help, discussion with a reconditioner for piston failure diagnosis and the motor bike mechanic for insisting that it was a lack of lubrication or reckless running in that I was able to build a picture of the nature of the failure and locate the now obvious assembly error.
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