Strange Compression Issue - yz125

[GarbatyA4]

Hello All,

My friends 1992 YZ125 ran fine all summer, but we noticed compression was getting worse. So decieded to rebuild.

Took the piston out, looks awesome (little carbon on top)
Ring was fine, still tight ring end gap. No scratches in the Barrell.

So we couldn't figure out why it was so easy to kick it over (i could start the bike with my hand :yikes: )


I noticed 1 thing that was strange. In the Barrel, at the top between the piston @ TDC and head There is a white film/colour change. Is that telling me that the o-ring was bad and compressiong the cooling system? I did notice that the o-ring was elongated and larger than it should be.

What do you guys think?

Thought i'd ask this because the forum has been dead today.

 

[whenfoxforks-ruled]

Burning a little antifreeze would be my guess, and consequently, losing compression. Steam makes power also though?

 

[GarbatyA4]

Burning a little antifreeze would be my guess, and consequently, losing compression. Steam makes power also though?

Ya, i think the o-ring took a ****.

The white hazy film/discolouration was also around the exhaust port. So it must have been burning coolant.?

 

[Matt90GT]

You also need to measure the bore and the piston and the ring gap. Then get out the factory manual for the specs of each and the clearance required for the piston to cylinder. Most likely you are getting worn on the cylinder and rings. Time for the next size piston and new top end from there.

 

[rmc_olderthandirt]

Hello All,

So we couldn't figure out why it was so easy to kick it over (i could start the bike with my hand :yikes: )

You can start it with your hand?
Sometimes I wish my bike would have that problem......

Other than being really easy to kick over, is there any other signs of a problem? How does the bike run?

Normally when you have a head gasket leak you get a lot of exhaust gases leaking into the cooling system. When you shut off and the engine cools down it can sometimes draw water into the cylinder that can cause problems the next time you start but the more obvious symptom is that the pressure builds up in the cooling system and the radiator starts puking water.

If you are not having a cooling system problem (no overheating, no puking, no unexplained loss of coolant) then I doubt that you are having a head gasket issue.

Since you have it apart I would go ahead and put a new ring in it and obviously you would use new gaskets. My recommendation would be to run a compression test and a leak down test once you get it back together so if nothing else you have benchmark to compare against later.

Rod