Seff,
I have seen silencers cracked and broken inside and the packing will actually be hanging loosly in the canister.
If there is enough "hanging" in the pipe center it's just like stuffing a rag in the exhaust or holding your hand over it.
If this is the case, getting the rpm's up enough to overcome the resistance could be part of the problem.
If the silencer checks out, we can work on that off Idle to 1/8 or 1/4 throttle.
Off Idle to 1/8 your working mostly the pilot and airscrew.
You say you don't have any trouble getting her started or to idle.
That would indicate you are close on the pilot.
as a general rule More than 1-1/2 turns open on the air screw usually means you can drop one size on the pilot.
(that will lean out the whole range of that circuit.)
Less than 1-1/2 turns indicates you need to richen up the pilot (as less turns on the screw allows less air to mix with the fuel drawn through your pilot circuit)
The Clymer I have pg 336 table 6 says stock jetting is
Main 168
Slide cutaway 7
Jet needle N82M /3
Pilot #60
Air screw (Turns out) 1-1/2
Float height 15-17mm
Fuel level 0-2mm
Do you know what pilot you have in the bike?
I'll try to get some more jetting explanations in my gallery .
http://www.kxriders.com/coppermine/displayimage.php?album=lastup&cat=11045&pos=0 Tuck\o/