Crank seals questions

[J.D.]

I just read this on a site about reading spark plugs:

Heavy Carbon

This plug has heavy carbon build up. This engine had a blown crank seal on the tranny side of the crank. Tranny oil entered the crankcase and was burned in the combustion chamber. Engines like this will billow thick blue smoke out the exhaust pipe.


Now, when I tore the top end apart on my Tri-Z, I had what LOOKED and SMELLED like tranny oil(Quaker State 10W40) on the crankshaft. But, like they say in the quote about thick billowing blue smoke, I had none except when I would first start it up cold, and getting up in the gears 3/4 to full throttle puffs of blue smoke would come out. Does this mean I replaced my crank seals for nothing? The plug that they showed for that description is all nasty and carboned up real bad, mine was never carboned up it was just soaked with fuel. Just curious......

[hondatrikesrule22]

There is no such thing as replacing oil seals and bearings for no reason,( unless you replaced them 2 days ago :D ).Even if everything was fine now, you never no about 2 weeks from now... ;)

[J.D.]

Well that is true, but I'm like this, if it looks good(not worn etc), and works good, don't replace it. My old seals looked to be fine, but since I had tranny fluid on the crank I just figured what the hell why not do it. I would have replaced the bearings but they looked brand new, and have absolutely no slop in them.

[TimSr]

When the clutch side crank seal goes, youll know it. Youll get tons of blue smoke, and it will smell NASTY. Youll see it in the exhaust long before youll see it in a visual inspection of the inside of the motor.

[hondatrikesrule22]

I am the same way,but I am funny about bearings and seal, because these machines are 20 years old. And it sucks having to take the motor back out and split the cases again. My 2 cents, good luck with the rebuild. ;)

[J.D.]

When the clutch side crank seal goes, youll know it. Youll get tons of blue smoke, and it will smell NASTY. Youll see it in the exhaust long before youll see it in a visual inspection of the inside of the motor.


Hmm, ok, explain this to me if you will. How did I have what seemed to be tranny oil on the crank, yet no blue smoke out the exhaust except on warmup? It makes no sense to me!!!

[J.D.]

I am the same way,but I am funny about bearings and seal, because these machines are 20 years old. And it Bites having to take the motor back out and split the cases again. My 2 cents, good luck with the rebuild. ;)

Thanks. I'm almost done, I'm trying to save up enough money to bore it, and buy a gallon of BlueMarble from TimSr there because I don't wanna use any more Yamalube crap.

[TimSr]

If you had case oil inside the crank area, it had to have come through the crank seal, but if you were not seeing any of the symptoms I described, you were probably looking at blackened Yamalube. Its not uncommon to get a buildup of oil in the bottom of the crank, thhough its certainly not desireable. Common causes are worn top end (blow by) especially when combined with excessive oil to gas mixtures and to a lesser extent overly rich carburetion. Some oil in there is normal.

[J.D.]

Hmm....I see. I think I was super rich then, because the plug was always SOAKED and black when I pulled it out. The oil though that I am describing was a sort of golden purplish color (almost the color of the oil and gas mixed)....maybe it was my imagination though. Also I was mixing the Yamalube at 32:1....which really should cause that much of a buildup should it? I do believe my top-end was worn. A guy looked at it and said that the bore taper was gone, and it's not egg/oval shaped, it's like perfectly round when you look at it with one really minor scratch on it. It had 140 PSI before the piston went...have you ever done a compression check on yours after a fresh top end Tim?

[burnoutboy]

I know this is an old thread guys, but I thought I have a couple of questions. I have replaced the topend on my RM250, rebuilt the carb and installed a new spark plug. I can get it to fire with ether down the plug hole, but it's very hard to get it to fire normally. It blows out huge amounts of smoke when I start it and there is oil all over the tail pipe. My plugs are also wet fouled, they are soaked in gas.

Sounds like crank seals?

[cr480r]

It blows out huge amounts of smoke when I start it and there is oil all over the tail pipe.

does it still smoke after you warm it up and ride it a bit?



I can get it to fire with ether down the plug hole, but it's very hard to get it to fire normally. My plugs are also wet fouled, they are soaked in gas.

This may have something to do with it...

[tecat-z]

My 86 z is 161 psi on a 68.25mm bore. Cylinder has never been decked and head has never been touched either. Stock oem gaskets on the top end. That is with a wiseco piston. Runs great.