This may be a silly question, but I will ask it anyway. I have problems with a 660 raptor. I did a compression test and only got 80psi. I thought about it later. ( after I had pulled it apart) How can I get an accurate compression test if the decompression mechanism on the camshaft is releasing the compression when cranking the engine. Or does the decompression pin retract under cranking revolution speed?
the pin does not retract under cranking speeds (that's what its there for) but it does retract at running speeds.
chances are your battery will not spin the motor over with the decomp defeated but to do it, loosen the left(as you sit on the bike) exhaust valve adjuster, just back it out until it is flush with the rocker arm (on the bottom of the rocker arm) and tighten the nut back down. You need to remove the slides from the carbs to get a good reading too.
The header shouldn't be a factor, the exhaust valves should be sealing up on the compression stroke. But if it's "blowing oil" from somewhere that could certainly be a problem. Check your valve lash again and look for signs of a leaking valve stem seal or worn valve seats.
80 psi with the decomp operating is about the minimum at which the engine will start and run - you likely have problems. Have you added a teaspoon of oil to the cylinder to see if compression goes up? If it does, this is an indicator of worn rings or cylinder wall.