Crank seal ?

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Nov 20, 2001
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#1
I have a question on a bad crank seal.I was riding my 03 rm250 in the sand wash pretty hard,wide open most of the time.Jetting is almost prefect 32:1 w/race gas.After coming to a stop,the bike smelled bad and was smoking very heavy.The smoke was blue,and had a bad smell.I looked at the silincer and it had a lite gray oil flim coming out.Rode the bike back to camp,while watching for heavy smoke,the smoke cleared up and the bike ran normal the entire weekend.This has happen before on a hill clime (3 months ago).Its only happens when rode very hard.Do you think the tranny crank seal is bad? over heating?Would like to hear if anyone has had this kind of problem before.

Thanks :think:
 
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Nov 29, 1999
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#2
My experience with a bad crank seal, on the tranny side, caused the engine to suck the tranny oil into the motor, and create a ton of smoke, and also reduced the oil level in the trans very quickly. If your tranny oil level is fine after these episodes, I would not think your seal is bad, As recent a model as yours is, unless you have a rode a lot of hours, the seals should not be worn out. The smoke and discoloration you described could just be from the hard riding. I had a KX500 one time, that literally caught the spark arrestor on fire, after an extended fire road blast at high rpm. Did not harm a thing.......
 
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#4
The oil level is fine,the coolant level is fine too.Its only when i run it very hard,in high rpm.Then it does it.Smokes really bad then clears up in 1 minute.Has a bad burning smell.My 02 CR250 did it a couple of times too.Maybe should fatten the main jet a little.What do you think? :think:
 

RM_guy

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#5
If you are riding in high load conditions (sand) it is always a good idea to richen up the main. What's probably happening is you have some spooge in the pipe and silencer when you're not riding in the sand and it's burning out when you really load the engine down. Does it feel lean when you're in the sand?
 
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#6
No,it doesnt feel lean.It has done it also in hard pack up a big up.Thats what i was thinking about the oil in the pipe and silencer being burnt.
 

marcusgunby

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#7
Most of the time, a good main jet size for normal mx, is lean for extended high rpm use-i would richen it up a few sizes for that type of ride.
 
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#9
Just out of curiosity, how much oil would yhou loose if the crank seal was leaking? I am sure it depends on how bad the seal is but in general with a minor leak?
 
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#10
If it makes you feel any better I have a yz 250 that has done the same thing. Mainly it does it when climbing coal strip mining hills. Usually worse when we do alot of trailriding to get to the hills, so I'm thinking it is just burning off some residue when the pipe gets good and hot.