Maintenance & Technical > KX250 / KX125

Weird engine banging sound

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jabba.rides:
I have a 86 kx125 that I got at the beginning of the summer and have not had much time to work on it yet, but am excited to ride it soon. I have done a few thing like clean the carb replace the clutch discs and replace the spark plug. However I recently noticed that the engine makes this weird banging sound. I have a few other issues like the fact that the top end leaks coolant that I recently noticed. However I want to know if this sound is normal and if not what it could be. Any help would be appreciated as I am rather new to this bike and bike maintenance in general.
https://drive.google.com/file/d/0B1Y7U4BU9d14V19FT1ZHaWVGeWs/view?usp=drivesdk

Thanks, Jabba

KXDINO:
DO NOT start engine , pull top end off and inspect , coolant in the combustion chamber is a major disaster waiting to happen.and needs to be fixed.

Rocco H.:
Hello. I had the same problem with coolant leaking into the cylinder, took the cylinder head off and put a very thin coat of gasket maker on the cylinder and cylinder head. That stopped the problem.

umberto:
Coolant leaking indicates a warped head. If you put it on a flat surface, such as a pane of glass, you should be able to see the warpage. I think there may be a guide on here on how to lap your head, I'll try to find it. As for the knock, that sounds bad. I would pull my cylinder off and check your rod clearances. You've most likely got some wear issues and may need to rebuild the bottom end.

umberto:
Here is a handy guide on how to check rod tolerances if you've never done it before.

http://www.kxriders.com/forums/index.php/board,56.0.html

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