Alright, from what you describe it reads like there may be an issue in your fuel circuits controlled by the Needle jet and Jet needle in each carb. I know you have cleaned the carbs again and again, but play along here anyway because you should get back in there and check a few things before going off in another direction. I scrounged for images and found a few belonging to others (Thank you!), so I'm gonna clutter dump them here as I ramble on.
First is a generic image showing the various enrichment circuits in a Mikuni carb in relation to throttle opening percentage. This may help you to guess in which enrichment circuit(s) your issue may be while diagnosing a problem or while improving a tune.
The Needle jet in each carb is brass and should have a short portion of the jet body sticking up into the venturi (Make sure yours are not broken off
, those cutaway protrusions are critical to the operation of the circuit) where passing air can lift and bust up the liquid fuel into tiny droplets, as the pressure within the venturi is lower than atmosheric, while the air velocity is high.
Fuel moves from the float bowl up through the Needle jet where a small amount of high pressure air is allowed to mix with the rising fuel column and then that mixture enters into the swift air stream in the venturi area of the carb, due to the pressure differential created by the increase in the speed of that air. The cutaway at the top of the Needle jet provides a secondary (booster) venturi low pressure area. Turbulent (mini vortex form along/behind each trailing edge of the booster), high velocity air rips the entering fuel column into a gazillion pieces and scatters it throughout the column of rushing air. Visualize a tornado in action along the ground...
As velocity goes up, pressure goes down, (and vice versa) its a physics rule thats always true with moving columns of gases within a closed system.
There are basically three points here within the Needle jet where we sometimes find trouble. The most common problem is the tiny holes in the tubular walls of the jet where fuel is being metered into the bore of the jet (and separately, air is also allowed to be introduced into that liquid fuel just above), become plugged. Sometimes its dirt or other junk that piggybacked in with the fuel causing the blockage. Sometimes it is old and decayed fuel that has broken down into a near solid state. Other times its corrosion caused by water and/or oxygenated fuel such as ethanol. Many times its a combination of these.
Alcohols are corrosive to metals because they bring their own oxygen along with them, so aluminum and other soft metal parts are under constant attack and given enough time, all can be reduced to gunky junk. If at all possible, put ethanol free fuel in your tank only. You'll have fewer breakdowns, save yourself so much grief and expense over the long term... in short, your time can be put toward being productive, rather than being wasted solving recurring problems that are certain to happen.
The second most common problem found in the Needle jet circuit is that the high pressure air supply (at or very near atmospheric pressure) passage becomes blocked. This passage begins just inside the carb throat area where the diameter of the throat is largest. If you take the boot off your airbox and look inside the carb throat you will see the small hole where air is introduced (branching off through separate internal passages) to the pilot jet, the air screw for idle mixture adjustment, and into the Needle jet where air is introduced as mentioned above. There is generally one entry hole needed in the carb throat for clean air, but inside the carb body that relatively large hole is branched off through much smaller drilled passages to supply both the Pilot and Needle circuits. Inside, where those smaller passages are drilled
to meet the larger entry hole, is where the plugs of dirt, mouse nest debris etc., are usually found
While you have the carb completely disassembled (nothing else can possibly be removed = completely disassembled
!) for each cleaning session, always force compressed air or a carb cleaner solvent through those passages from each end of the six possible entrys/exits and verify that your compressed air or solvent makes it all the way through, coming out all the opposite ends
. There should be a heavy flow through the Needle jet circuit and a lighter flow through the pilot circuit. This is especially important on carb models that have pressed fit Needle jets, as they cannot be easily removed for cleaning without damaging them. If you find a restricted/plugged Needle jet that you cannot blow open, it will have to be replaced.
The third most common problem with a Needle jet is breakage of the cutaway protrusion at the top of the jet (as mentioned above), and/or a high amount of wear caused by the Jet needle vibrating within the bore of the Needle jet over time, causing the motor to run rich during a portion of that jets' operating range. Piston port motors are generally more likely to cause Needle jet and Jet needle wear because of the strong induction standoff pulses inherent to that design.
If the cutaway portion at the top of the Needle jet is found to be broken off or missing, the fuel will begin its entry into the venturi air stream very late in response to increased throttle opening. The fuel will eventually gush into the venturi area once the pressure drops low enough, but most of that fuel will puddle up and flow along the floor of the carb like a river
, never getting up into the turbulent column of air to become atomized by it. That little chunk of brass doesn't look important, but it is... you're not going anywhere without them.
The Jet needle is a tapered rod and little more... It plays a minor role of being a "fine tuner" of the Needle jet circuit. It plays a much larger role of controlling the rate of increase (mass) for gross fuel flow through the Needle jet, relative to throttle position. There are usually five (5) grooves cut around the top of the Needle jet where an e-clip is used to preset its depth in the bore. The most common e-clip position is #3, the center groove, so if you don't know where it should be due to lack of documentation, clip the center groove and fine tune it later.
Thats about all I got for those midrange circuits... aren't you glad I am about to shut up!?!?
If I am guessing wrong then you can flog me and move on to find the real problem. First, lets eliminate this probability though... Game on?
Tip: There are sometimes (many models have these) one or two tiny plastic washers above and/or below the e-clip on the Jet needle shaft. Watch for them to fall out and disappear under your motor as you take the throttle cable off each slide. Also beware, the little magnets that control the TSS switches can/will fall out too. Those lil' buggers can break into pieces if they drop onto something hard. Best practices include placing a dropcloth or something similar below your work area to catch all your loose parts as you disassemble and reassemble.