Air leak at cylinder base/case. Help!!!!

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Oct 3, 2001
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#1
Fix for air leak at cylinder base/case

Had Eric bore and port my son's KX 85 to a 100 2 years ago. After our last rebuild last summer compression was only 140-145. He rode it all last summer but for this year I want to fix the problem. There's a visible air leak at the base of the cylinder (sprayed carb cleaner with engine running, not sure if that's a great idea or not but it really shows the air leak) and the cases. I'm semi clueless: how can you determine if it's the cylinder or the cases? I guess if I pull the cylinder I can put it on a piece of glass and check it that way but I sure you guys know a better/more accurae way. Also, it it's the cases, how to do you tell. Lastly, how much to solve these problems and who's really good at this? I"m in the Pacific NW (Oregon). Any advice on this? Thanks. Dennis
 
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Sep 12, 2006
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#2
If it only started leaking after the last rebuild, I would say the cylinder probably didn't get seated down over the guide sleaves (hollow dowels around the studs) properly. Pull the cylinder and replace the guide sleaves and install a new base gasket, and I will just bet that you will be go to go. It only takes the smallest of burrs to keep the cylinder from seating all of the way down over the guide sleaves. Eric recomends putting a littel grease on the sleaves before installing the cylinder too.
 
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#3
crusty1 said:
I'm semi clueless: how can you determine if it's the cylinder or the cases? I guess if I pull the cylinder I can put it on a piece of glass and check it that way but I sure you guys know a better/more accurae way.
crusty1.

Checking the leak with carb cleaner or wd40 is the approved method. :cool:

Remove the cylinder and visually inspect the surfaces. If it has been leaking for a while, there will be a stain or discoloration in the area of the leak. If there is no obvious discoloration, take a good quality straight edge and place it on the gasket surfaces. Look at the surfaces with a light in the background. If there are low spots in the surface, the light will shine through below the straight edge. You can also take a feeler gauge and place it under the straight edge to measure the amount of distortion.

If there is a problem, I would send it back to Eric. His work is hard to beat.

I could just be a bad base gasket.
 
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#5
Great advice. I'm going to pull the top end and start from scratch. I'm going to straight edge the cylinder base and the cases. I'll also use a bead of gasket sealer under he base gasket when I put it back together. I'm shooting for a compresion in the 170s. I would send it to Eric but I checked his website and it looks like he doesn't do this type of work anymore personally. We'll see how it goes. Thanks again.