Any time I read about a new member that may or may not be well versed in KLR CVK carburetor and I read 22 cent mod, and mid-range and above issues, I immediately suspect a pinched edge of the rubber throttle valve Diaphragm!
Any vacuum leakage above the rubber diaphragm and the slide can not/will not raise properly. Then with the throttle butterfly plate opened the engine sucks more fuel than air, runs rich, won't rev, fouls plugs, etc.
The rib around the outer perimeter of the diaphragm needs to be perfect.
Tucker Rocky/Bikers Choice dealers can order a rubber diaphragm. Part #482841. Intended to be sold to the Harley crowd, for the same reason!
I need to give credit to "Alpheus" over on .net for discovering this part for all of us. He even has a video of installation. I am forbidden to search for it over there, because of my past 'transgressions'.
I have not yet needed to use one, but I do have one. $16.95
I've had the same reaction to the possibility of diaphragm air leaks. However . . . I thought the effect was the opposite from the one you describe.
Without air-tight seal and full vacuum, I thought, the slide and needle would not rise as they should in response to the throttle valve opening. Since the needle cannot uncover the main jet adequately under that circumstance, the mixture would be fuel-lean, since fuel flow through the main/needle jets would be compromised (insufficient needle lift).
I thought this theory was validated, when I found an engine with a carburetor diaphragm air leak that would run only with the starting enricher fully open, fuel-enriching the mixture to compensate for the inadequate fuel flow from the "too-low" needle excursion.
Summarizing, I thought the effect of a diaphragm air leak was a fuel-lean mixture, not a fuel-rich one.
My theory/conjecture only; if in error, I appreciate any correction/alternate explanation.
BTW; Alpheus has departed the KLR fold; is now a full-up Harley man! A function, somewhat, of his longer commute into his "day job" from his new lake community home. His expertise on Keihin CV carburetors will not go entirely to waste, depending upon the Harley models whose company he keeps!