Just rebuilt my motor and after sitting for a couple days it blows oil out the exhaust after just starting it up. What could be causing this? I have about 500 miles on it - could it be that the rings have not seated yet?

If the rings are not seated you would get combustion gases blowing past them and pressurizing the crankcase. But I'm not sure how that gets oil into the exhaust unless the oil is leaking past the rings when the engine is sitting.
So exactly what did your rebuild entail?

It was a complete rebuild - new P&C's, rebuilt heads, the works. That's what I was thinking - oil seeping past the rings when sitting?

Seems reasonable. Either that or valve guides. I don't really understand what happens to cause rings to "seat". I need Jake or Cap'n to school me...

Maybe a bad from the mnfr valve guide seal.

Silly Q...When you put the oil rings in did you stagger them?

if oil is seeping past the rings and pistons when it sits you must have too much oil in it or ar you parked on a really steep incline? somethings not making sense. and you havent been making it nock?(maybe busted ring and piston)?