I recently overhauled an 04 quest 650 (main bearing went with less than 200kms on it) and once i put it back together, i found it had an oil leak and what sounded like an exhaust leak.  I looked for an exhaust leak and traced noise to what seemed to be engine compression "puffing" out of the front and rear output splines.  Oil was leaking out as well.  I removed rear shaft and replaced oring and seal and yoke sleeve. No different but noticed leak was actually coing out between splines of shaft and yoke. (not an ouput seal leak)  never seen this before?!?!?!?   anyone with any help/recommendations would be greatly appreciated.
Dave  

sounds as though you have serious blow-by in the crankcase.

this could be caused by the cylinder sleeve cracked or broken,
a cracked head or cracked cylinder.
or a broken piston skirt.
or worn or broken rings
or even the timing being off could cause this.
check for fuel in the oil  

Oil smells ok, i don't think there's any fuel in it.  Was very careful with head and cylinder sleeve, had them both checked at machine shop for cracks/wear. Piston was checked during o'haul and no problems. If the timing were off, how would that cause compression to enter crankcase?