As a worst case, one of the guys here, Ducatiman, rebuilds carburetors and has great reviews. If it comes to it, you might consider removing the carbs and sending them to him. I don't have personal experience, but I bet it's less expensive than taking the bike to a mechanic.
Also, if it does turn out to be dirt in the carbs, you should install a filter in the fuel line between the gas tank and carbs. In fact I add one to any bike I buy, if it doesn't already have one, just to prevent future headaches.
https://www.ninjette.org/forums/showthread.php?t=239678