Maintenance & Technical > KX500 Original

Chronic KX500 Piston Seizing

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PBEB:
Hello everyone!

I’m having problems keeping my K5 running. It seems to consistently seize pistons under certain

conditions. If I’m generally play riding, everything is fine, but, when riding hard and fast in open terrain at ¾+

throttle in 4th or 5th gear it will seize in very short order. It's right over the first sub exhaust port. Mechanically everything is fine. Quality gas & oil. All machine work

done by reputable shops, etc. Any ideas on improving reliability? Increase piston skirt clearance? How about lubricating holes drilled over exhaust bridges?

Foxx4Beaver:
Welcome to the site.

what's your current jetting/altitude?
did you verify the cylinder and cylinder head mating surfaces were flat?...did you lap them to be sure?

PBEB:
I ride close to sea level, usually less than 2,000 feet. When I ran the stock Keihin 39mm carb it was set with a 175 to 180 main, needle, if memory serves, 3rd or 4th position. Now I run a 40mm Lectron set by Lectron. I seized pistons under both carbs. The head has been milled and reshaped so I feel relatively confident it's flat but I cannot say definitively that the cylinder has been lapped. I'll be sending it to Millennium for plating so I'll include that too. Good idea!

Foxx4Beaver:
Both pistons have a slightly steam washed look to them(but I don't think that's the case)...like the head gasket(s) may not have been sealing up completely, and allowing coolant into the cylinder.
It's always a good idea to do the lapping yourself, just so that you have peace of mind, that everything is perfectly flat, and you can see it with your own eyes.

What type/brand of head gaskets are you using?...the majority of us are runnin Cometic, and OEM.

PBEB:
My last rebuild I used a full top end kit from Cometic. I just found in my notes on my last rebuild the results from my leak down test - 16 minutes, starting at 6 psi to 5 psi. I have a 1/2 piece of glass I've saved for lapping. I'll dig that thing out and lap the cylinder when it returns. I checked the squish prior with the Cometic head gasket and it came out to 1.6mm. Does that sound about right?

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