You've tried all the normal stuff but the symptom persists...makes me think you have a tiny bit of grit in the pilot system passages some where.  You said that once you get beyond 1/4 throttle it goes away; that's because you now bring in the needle/jet midrange circuit to supply more of the fuel and the slight blockage in the pilot system is no longer a major issue.  
As suggested above, if you loosen the clamps on both sides of the carb and just rotate it so the carb is horizontal, then remove the bowl cover and take out jets and the carb top/slide assembly as well if possible, then with compressed air or at least carb cleaner can, blow out the pilot circuit passages.  If you have time and can get access then do main circuit and needle jet as well...might as well as long as you are between dance tunes, eh ?  

Grab the shaft the throttle cable is working & shake it.

justin_o_guy2 wrote on 10/26/09 at 08:21:26:


What does that do?  It turns freely.

OK.  55 is in.  No shake in the throttle shaft.  It's not that old a bike really.  The carb is clean clean inside.  So it's a bit odd.

Now on accel or decel I have a nice lean notch from just off idle to somewhere right around 1/4 throttle.  I can tell it's lean by running in that throttle position (a nice setting for my country road riding) and popping out the enrichment knob one click - the running settles in nicely with a little extra kick of fuel drained in.

My understanding is that this would be the pilot range.  

So.  I have the white spacer in.  Does that impact the pilot range?  If so, I can pull it out.  Otherwise, I'll pull the carb, tear it down, and run a guitar string through all the passages I can reach, blow them out, make sure they're running clean.

If that doesn't work, I'll buy some jeweler's reamers (actually, I'll get my dad to bring his this weekend) and make myself a 56, 57, 59 or wherever it ends up!  

On the other hand, the bike runs great over 1/4 throttle - very strong.  65 mph on the highway, just purrs along.

Slide should be fine - moves very freely, just cleaned.  And I routinely run seafoam.  Possibly one of the reasons the carb is always sparkly.  If it was crudded up this would make more sense!!  But it's spotless inside.

Have you checked your valve clearance?  If they are too tight, they may be leaving the valves partially open, leading to a lean mixture and incomplete combustion....

Have you tried it WITHOUT the filter since???  You could be sucking air through the filter.....  Try a straight piece of fuel line and see if it improves...

Why do I almost hate to chime in also, but I must.

I agree to clean all passages in the carb, because after all that has been done, it is not just a metering adjustment or jet size problem.

And just for info for those that don't know, (I'm sure the mechs here already know) if an engine is idling good, there is no intake manifold, or vacumn leak, nor are the valves too tight. At idle is the first place these problems will show up. Even a vacumn leak from a worn throttle shaft will show first at idle.

Also, if an engine runs good at WOT, there is no problem w/ the air and fuel delivery system, which includes the air filter, the fuel filter, the main jet, and the main gas line from the tank to the carb.