KX65 Engine Problem

I used to motocross and now have 2 sons starting to ride. It's been 30+ years so bikes have changed a lot. I just bought a used 2004 KX65 and it starting frying before he made one lap on a little track. He over-reved and it locked up just like he was standing on the break. It started back up with a mild vibration when idling down. I let him ride again and it did the same thing. This time with a heavy vibration when idling down. We loaded up and went home..... I'm I looking at top end rebuild or crank. Any idea's and any pointers on how to do as cheaply as possible. I've got 2 of these things to keep up.

Thanks for any input.

Search KX65 crank here, I think I recall a problem with the cranks on those.

(upper right hand corner for "search this forum")

If it seized up, then restarted I would think the top end is toast. If the top end is bad, check jetting, premix ratio & timing.


Pull the jug and look for discoloration on the crank assembly where the rod connects...if it's discolored (looks like it overheated) that bearing probably went south - just went through the same thing. That bearing took out the whole top end on our kx65.

Repair bill to have J&M racing in Houston do it was about $900 - split the cases, new crank assembly, re-plate the jug, fix the squish band in the head, new piston and rings and gaskets all around. The guy I talked to at J&M said that bearing is a regular weak point on the kx65's.

If not, you may get off for a lot cheaper than I did...good luck and let us know what you find.

I'm betting it's the lower rod bearing. This fall 2 friends kids 65's each lost lower rod bearings. Used hotrod cranks to fix them. I think there is a ebay store that sell the crank, main bearings and piston for a good proce. Good luck. Ed

I plan to work on this weekend. I'll post my findings.

No1Clyde... you're correct, It's the lower rod baring. Got it torn down and parts on order.

As I understand it the lower con rod normally goes out due to insufficient oiling. Big K didn't leave enough room in the oiling slot. Evidently the hot rods setup has that issue fixed. The other issue common to the KX65 can be porus cases around the main bearing area; causes air leaks, sometimes intermittent.

DIRTBIKEDADX2 I'm glad you can handle the problem. I also think running 20-1 oil is a good idea and jet for it, I know a lot of people say that is too much oil but in that motor I like it. Good luck with it. Ed

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