XT 550 crankcase oil "pumping" into air box

I just bought an '82 XT 550 that had been sitting for a number of years.
The bike is clean and drives well on the streets with no problems.
When I drive it on the freeway for more than 10-15 minutes it begins
to lose power and eventually starts smoking. It appears that the crankcase
is venting oil back into the air filter....lots of oil. After driving on the freeway
I stopped and removed the air filter and could see the oil "pumping" into the
air box. It wasn't just venting, it was pumping oil..you could watch it at idle
literally pumping oil out. After letting it cool down it seems to go back
to normal. Does anybody have any ideas what would cause this?

TIA,
Daren

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Some BMX trick rider said: "Bones heal and chicks dig scars."
http://homepage.mac.com/dsefcik
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I had the same problem with my 1980 maxim. It was because the rings were so
worn that the compression was leaking past and pressurizing the crankcase,
causing oil to literally "pump" into the airbox. Try a compression check
and see if any (or all) cylinders are low. If they are then that is the
problem, they are worn or, as Charlie says, stuck.

Good luck
Jim