oil slick

If your engine is running good, idles well and has power it is doubtful that the crankshaft seal is bad. The "black goo" you saw could just be clutch friction pad residue mixed with some gear lube from the crank and clutch gears. If the seal is bad, you'd know something was wrong. The engine wouldn't idle well, have reduced power and the plug would indicate a lean condition. If it's not broke, don't fix it.
Tom

VENICE BOY said:

I built a bike for a friend & the clutch cover was filling with oil & causing the clutch to slip real bad... I'm guessing that it's a bad oil seal?
But the engine was running so good that I didn't want to mess with it!! So, I drilled a tiny hole in the bottom of the clutch cover to let the oil out... It still runs great, & no more oily clutch!!

That's real 'cheesy' way to fix it, but it worked real well!

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Just curious, Venice, Is that motor still leaking oil/goo from the hole you drilled? The seal is open to the crankcase so a bad seal should cause a lean condition and the motor will not run very well. Could it have been excess gear lube that when hot would drip out? If the leak finally stopped that was probbaly the issue but if it continues to leak that's interesting. I got a couple of hundred miles out of an engine with a bad seal on the mag side by sealing up the cover with silicone. The oil finally got so deep behind the cover that it wrecked the magneto. I found a source for seals and replaced the bad one. Like I said, Just curious.
Tom

VENICE BOY said:

Yep, It's definitely oil dripping out, not gear lube.
I thought a bad seal would cause a lean condition too; but the plug is a perfect color, the engine has lots of smooth power & it idles real smooth...
I also thought the hole I drilled would make it run bad... but it didn't. It's still one of the best running bikes I've ever built.
I'm at a loss to explain this one... this goes against everything I know about these engines!?!?

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Here's a theory: (just a theory) If the crankshaft bearing is the sealed type, the one with the plastic or metal cover over the balls and inner race, could that be enough to seal off the crankcase against the atmosphere? The bearings on the one I had the seal go bad on appeared to be open, unsealed. The balls and race were visible behind the seal bore. Oh, wait a second...if the bearing seal held back the crankcase pressure how would it leak oil? Duh...told you it was just a theory, and not a very good one. Sorry
Tom