YZ250 smoking, blowing oil, low compression

Good Day!  

 

I have a 1979 YZ250 that I am trying to get running right but I am having some issues.  First off I have already changed the crankcase oil, and cleaned out all fuel lines gas tank etc and started with the 32:1 ratio of premium fuel to Bel ray 2 cycle oil.  I also removed the exhaust and moved the piston to TDC and rings, piston and cylinder look really clean and good, however I noticed that the compression just seemed low.  Like during my kick starting a few kicks would be hard solid with compression and then a few would be easy.  Just seemed weird.  My symptoms are below and thanks in advance:

 

-Smoking like a wildfire, thick white smoke and lots of it

-Oil Spray after test ride on silencer, and rear left swing arm.  Oil is dark. not translucent

-While riding it feels like nothing is really happening like its missing in first and second and it seems like midway through second I 

get that smooth running fast motor almost power bandish, but then when I let off it feels like its missing

 

 

I was thinking that it might be running rich and that the Mikuni Carb is just dirty and is dumping fuel in the cylinder and not burning off then blowing out of my exhaust.  My second thought was crank seals.  

 

Any thoughts?  Advice?  Steps?  My manual is in the mail.

 

Thank you again 

David Lindley

 

David,

 

Have the same problem:

 

-Smoking like a wildfireicon1.png, thick white smoke and lots of it -  two things: seal on the clutch side is not doing its job anymore, radiator fluid is leaking into your cylinder and probably due to a bad O ring

-Oil Spray after test ride on silencer, and rear left swing arm.  Oil is dark. not translucent -  this is from your transmission oil leaking into the cylinder because of a bad seal

-While riding it feels like nothing is really happening like its missing in first and second and it seems like midway through second I 

get that smooth running fast motor almost power bandish, but then when I let off it feels like its missing - check your clutch plates or springs, may need to be replaced if worn out

 

Just know that these are just my opinion based on your symptoms and the same experience I have with mine. Good luck.

Did you just rebuild the bottom end? Thick white smoke would seem to me that the clutch side crank seal is letting oil by. Sounds like it just may need a top and bottom end rebuild. I know that doesn't give much advice. But if it hasn't been done and it's that old I would do it.

Edited by crimsononly

no, this is a bike that I just bought off of a guy.  Nothing has been done to it.  

David,

 

Have the same problem:

 

-Smoking like a wildfireicon1.png, thick white smoke and lots of it -  two things: seal on the clutch side is not doing its job anymore, radiator fluid is leaking into your cylinder and probably due to a bad O ring

-Oil Spray after test ride on silencer, and rear left swing arm.  Oil is dark. not translucent -  this is from your transmission oil leaking into the cylinder because of a bad seal

-While riding it feels like nothing is really happening like its missing in first and second and it seems like midway through second I 

get that smooth running fast motor almost power bandish, but then when I let off it feels like its missing - check your clutch plates or springs, may need to be replaced if worn out

 

Just know that these are just my opinion based on your symptoms and the same experience I have with mine. Good luck.

BugEyz-

 

My bike is air cooled.  So most likely we are looking at low end and crankshaft seals correct?  

oops, didn't read your post correctly that it was a '79, and yes it's the crank shaft seals and you might want to check and replace the bearings as well if it wasn't done before and if you find out that the bearings are worn out then check the crank shaft if it has any signs of wear because chances are the specs may not be true anymore then that leads you to replace it. By the way, don't forget your top end - piston may need to be replaced and cylinder honed or machined depending on its current state.

so should I start with the carb or just go for the gusto here?  I have checked out the piston, rings, and cylinder through the exhaust port and it looks really really good.  No marring, discoloration, or anything scratched chipped.  I got it to TDC and inspected.  All looks good in there.  

Edited by DAVIDLINDLEY

I suppose if you don't wanna do it all right away start with the carb. Completely break it down and clean everything. Make sure the float height is correct and put the jetting and other settings back to factory ) provided it's not a modded bike). That may be it. Although if were interpreting the symptoms the correct way, it's gonna need a full rebuild, new piston and rings. But give the carb a try first.

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