YZ 465G Jetting Question

I have a quick question for anyone who has experienced this problem. I will describe the existing setup and then my problem.

1980 Yamaha 465g
bored +040 w/wiseco
boysen power reeds
reed spacer
DG pipe and silencer
Uni air filter
2001 or 2002? Mikuni VM-38 carb
No porting or other mods.
6000+ ft. altitude
Fuel - 50/50 mix sunoco 110 and 91premium w/yamalube2R 32:1
Main jet:380
Needle Jet:P8
Jet needle:6F16 clip at 2nd notch from top
Slide:2.5 cut to about 3.1
Pilot: 50 w/air screw 2 turns out

The performance is as follows:
Starting and idle is good, throttle response is crisp and excellent. The bike pulls like a tractor to about 3/8 throttle where it starts to sputter and won't clean up. At throttle positions 1/2 to full...well lets' just say it is a rocket ship. The bike runs clean and perfect at every throttle positon but @1/4 to 1/2 throttle. I have tried changing to a leaner clip position. The bike runs better overall except it does not seem to affect the "blubber" at 3/8. I am thinking that I either need to cut the slide further or find a different needle with a fatter initial taper but I don't know which way to go!

Any help would be greatly appreciated!

Thanks for the reply Cladoo, I don't have a chart and have no idea what needle to try. The slide I can do but before I cut it i would like to try the needle first. what number should I use?

Thanks! I'll stop by the dealer here and see if they have the literature. This baby is so close to having the perfect powerband and jetting of anything I have ridden. I sure hope I can get it cleaned up.

Thanks!