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KX65 doesn't run right...help please.

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Symptoms:

1) Starts and runs good with fresh/clean plug. Stops running after 10 minutes or so when it gets good and warm. Will not restart easily and doesn't like to idle.

2) Once it is running bad, it won't accept revs...it will bog and die or backfire.

3) Swap to new plug and it will run good again for a short period.

I have 2 KX65s so I've been able to try a lot of stuff. Here is what I've done:

1) Checked all electrical components with multi meter and swapped the entire setup from the good running bike to the bad. Magneto, stator, coil, kill switch, CDI and plug...the entire assembly.

2) Rebuilt entire motor to replace crank seals...installed new crank and bearings while there. New top end as well.

3) Pressue tested motor...holds 6psi for 6 minutes easily.

4) Checked compression - 150 psi

5) Cleaned carb

6) Dumped all gas and mixed new.

7) Swaped out heads from other motor.

8) Yes cylinder and head are torqued at 18 ft-lbs

9) Swapped out exhaust systems

10) Checked head and cylinder for warp.

Here is the only thing that has me scratching my head. When I pull the coolant drain plug (cap still installed), it will drain around 30% of the coolant. Typically, a bike will drip a few drops of coolant until I remove the rad cap since it is a sealed system. I pressurized the cooling system but didn't have a gauge. I pressurized it until the cap released and then let it hold pressure for 5 minutes. Then I took out the drain plug and it still had pressure in it.

My current theory is that antifreeze is getting into the combustion chamber through a micro crack in the cylinder. I can't just put the other cyinder on though since it is a C and this one is a B.

Any ideas...what am I missing. I've been at this thing on and off now for 2 months. I'm stuck.

Thanks,

Dean

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What's the plug look like? I know you said you cleaned carb but did you check jetting? If you have 2 of them throw the other carb on and see.

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Just a stupid question, have you tried running it with the gas cap cracked open to allow air to get in the tank (maybe you have a bad vent), 10 minutes sounds just long enough to cause vapor lock issues in which case your bike will stall and restarting will be hard until you crack the gas cap.

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I've swapped the carbs already and the carbs are stock...190 main. The other bike runs well with both carbs.

I tried running it with the fuel cap off...got some gas in uncomfortable areas and said some words that I shouldn't have. Still didn't work though.

Thanks for the ideas.

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have you tried checking the exhaust for blockage? how about your air filter? twist off the sparkplug cap and see if the high tension lead is corroded. that happend to me on my old kx. the wires were green. cut back the wire and it fired up and ran.

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you could try using the piston and cyl from the C motor....after all the other work youve done that should seem like a piece of cake...

hope you figure it out..sounds like a major pain in the ass..

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After thinking about what you did already, I agree that it is probably an issue with the cylinder/head gasket/studs. Check the compression when it is hot, and look for signs/smells of antifreeze getting into the cylinder.

I had a CR125 with bad stud threads in the head, similar issues.

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