'does' sound like a carb problem rather than a twistgrip one. Might have to think outside the square if the normal tricks don't work. Sounds like you've covered the obvious so ummm, is it really sticking or not say a partially blocked pilot jet, then idles been turned up to compensate [when cold] then it warms up & won't slow down far enough at the first corner [ok maybe not that]!! If it's physically sticking you should really be able to feel it just by drawing the slide up & feel it return minus the spring pretty much. I'm assuming it's a heavy brass slide? but if those big carbs had an anodised or teflon coated lightweight slide well--ditch it & get a chrome/brass heavy type. [or get a later 'flat' slide carb if allowed in vmx--think not]. Finally, hasn't got a bad case of a leaking crank seal on the magneto side?--or sucked a base gasket in causing very lean running, maybe sort of running on when throttle shut off at end of straight etc. All clutching at straws ideas without being 'johnny on the spot' but hope you get it sorted. I'll be having nightmares of my 490k sticking on full song now. My thought for the day. Cheers.
ps, not really relevant but going back 45 years to my DT1, if the 'bolt up' type carb flange wasn't flat or had been overtightened & warped things, the slide [anodised type] would stick just from the carb body going out of shape--plus the anodised slides don't like water or moisture, had that problem back in the YZ80 days so fitted the heavy slide from one side of the yamy R5 350 twin & no more problem.
