HMM ! other areas possibly stale or old fuel crap from the tank has rentered the fuel system, there may be some inline filters your mechanic missed these are often located in the fuel feed at the t piece connector or some where on the carb if blocked can give the probs u metion, check all carb breathers to inc the tank breather run the engine for a bit with the cap open to see if it helps running . if not maybe carfully remove a float bowl and operate by hand a float valve and look at how the fuel flows frome the bike with care! this may help diagnostics check to see how much fuel is present in the float bowl when removed and at what point the float valves shut off fuel the manual can help here. regards craig