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KX 65 Overheating

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What can cause a KX 65 to overheat? I have a unique situation that I'm sure has contributed to the engine overheating.

 

I installed new top end and then let my kid ride it for about ten minutes with no engine coolant. We were way back in the woods when I noticed the radiator cap was off. We shut the bike off and received some antifreeze. The bike seemed to run fine back home, but not the next ride.

 

The bike is a bit down on power, not running well, and engine coolant is running out overflow tube of radiator. I ordered another wiseco top end kit that will be here in two days. I removed the cylinder and there is no visible damage to the cylinder, piston, or rings. I'm hoping the new piston and rings will solve the running and overheating problem.

 

What is causing the bike to run engine coolant out the overflow of radiator?

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That indicates a blown head gasket or a warped/cracked cylinder or head.

You could throw a head gasket at it, if that doesn't fix it then you can reasonably assume the sealing surfaces are warped or something is cracked.  The piston and rings are probably fine if there is no sign of seizure.

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The water pump seals probably didn't enjoy running dry as well.

Edited by mlatour
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My local race shop is going to surface my head and cylinder for me before I put the new gasket on. Thanks for the tips everyone.

Turbo Dan said that the piston and rings should be fine. I'm gonna go with that, unless I have a strong argument against leaving them in. The head gasket can only be used one time, and I only have one. My son has a Harescramble on Saturday and I may not have time to get another gasket and test everything properly. Sounds like I'm doing a pretty good job of talking myself out of leaving the old piston in, lol. It has less than an hour, but I can't afford the time of being wrong.

Thanks again for the help. I'll post back with winning race results. I hope!

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Just take a good look at the cylinder walls when you have it apart.  Any damage to the piston or rings would have corresponding marks on the bore.

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@mlatour et al

It appears that the water pump seals and possible the crank seal needs replacing. Bike bellowing white smoke. I had to run a BR7ES to get it to hit top end. I usually run an 8, which is not as hot a plug.

The transmission oil is leaking out the clutch shaft and the oil is milky white in the site glass. I changed the trans oil, and got same result. The spark plug looks clean and and moist when I remove after running engine. Too clean. I'm guessing trans oil in crank case mixing with gas and engine putting pressure on clutch side.

I'm picking up all seals tomorrow and give it a good effort to have ready for race the next day.

Can the crank seal on the clutch side be replaced without splitting case? I replaced a leaking seal in the state side last year from the outside.

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Update:

I removed engine and had local shop split the case, replace crank bearings, crank seals, and water pump seals. I reinstalled engine and it sounds and revs well, but it just didn't hit top end like it should. Top end would sing for a moment, but mostly give a blating sound at full throttle while riding.

My spark plug reading was all over the place. White plug one time, and dark the next. Finally after testing over and over with different jets, the bike started getting water back into the cylinder again. White smoke, expensive blue water running out radiator overflow, and spark plug wet.

Not sure where to go from here.

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