Main Motorcycle: '96 Adventurer
Join Date: Jun 2011
Other Motorcycle: Yamaha TDR 125
Wrong - Mikuni BSTs are very susceptible to problems caused by airleaks, and do not take kindly to being messed with.
Check the air filter box, does it have the airfilter in there still? Is the filter clogged?
Problems from around the 4000 RPM mark and above are slide related, ie slides not raising properly or incorrect mixture from somewhere, ie an air leak. Did you re-assemble the needle valves correctly? They should be in the order - small white plastic circle spacer first, then the needle & circlip, then lastly the wider "flying saucer" plastic washer. It's very easy to get them in the wrong order, I've done it myself.
Did you use oil on the slides and guides on re-assembly? Clean it off, it hinders their operation. You can check the slide operation by lifting them with your finger (when fully assembled), they should rise to full open and return to full closed without any resistance. If they resist then there's something wrong.
Also check the emulsion tube air inlets in the rear intake mouths for blockage (the fixed jets in the middle).
I would also replace ALL O-rings with brand new correct replacements, especially the ones under the slide guides and the idle screw rubbers as they cause massive problems.
I had exactly the same problem last weekend when I put a set of Trophy carbs on my Adventurer. It idled beautifully and was great till about 4000RPM and then it just seemed to not want to rev any higher, like it was starving of fuel or something.
I changed the slides out with my original Adventurer ones and it ran great. The only difference being that the Trophy slides have only 1 tiny air hole at the bottom of the slide, whereas the Adventurer has 2 massive (circa 3mm) holes.
These holes allow air to be sucked out by air passing over them (inlet manifold pressure), raising the slides and opening the needle jets, the more pressure, the more they rise, richening the mix and allowing more fuel into the engine. If they don't rise, you get fuel starvation, and limited revs.
So I'd be looking in their direction.
Oh yes, one last thought - check to make sure the o-rings are all there under the slide lids, it's easy for them to fall out during re-assembly and you not notice it.
Last edited by ArferBrick; 09-14-2012 at 06:03 AM.