Its hard to find a tech these days that actually knows carbs. Dude clearly doesn't know his shit. Any mechanic worth his salt should be able to free up a brass screw in an aluminum housing, they don't corrode like steel. Mail your carbs to isacs papa, he's a pro. I'd do them for you, and am more than capable of the job, but I stopped buying carburated sport bikes for a reason: I end up having to do carbs on them. Figure out which cyl isn't firing, next time it does it open the bowl drain on that carb and see if gas comes out. Pull the plug and test it. At least look at it and see if the porcelain has build up on it, that will indicated a rich condition.