07 400ex carb adjustments

Ok so I'm completely stumped with my machine when i bought it it had the air box lid drilled the snorkel removed and blocked off and a k&n air filter. It has an aftermarket slip on pipe that I beleive is an fmf from the design of it. My problem is that I've had to jet it way down on the main jet and have adjusted the needle to the leanest setting to get it to run decent after 1/4 throttle(which is backwards of what I know to do) i was told to check compression and valve adjustment which I did and it's got between 100 -120 psi and valve clearance seems to be correct at .1 on intake and .12 on exhaust. What else do I need to be looking at for solutions.