this is exactly what has been going on wiht my 2005 tomos ST...goes from 30 to 26/27 then back to 30, makes a miserable gurgling sound. however, be careful if u get into the carb. it took me a week to figure out that i litterally had to drive it off the engine (gently tapping) while everyone else just wiggles theres off, then i discoverd, contrary to the exploded unlabeled diagram in the repair manual i dnloaded, that hte only air filter is apparently in ht airbox and not in the gasket connection in the carb where it attaches (see my post on here about my carb problems). ive got 2700 miles on it and ride it daily as my general transportation.
also, dont bother the throttle linkage on top of the carb. you can get to everything in it without doing that. mine ran fine after i cleaned out the jet wiht a wire, but now it wont start at all, probably something to do wiht the airbox.
dont mean to be long, just thought you might wanna get a heads up on such since this is exactly whats g0ing on wiht mine. i also had anohter post earlier on the GD forum before i realized it needed to be on here.