Most probably the float level is incorrect along with the powervalve sticking - its the big spring like valve in the middle of the carburettor, its controlled solely by manifold vacuum and dumps in fuel at zero vacuum (full throttle)
Your mechanic should thoroughly clean the carburettor with cleaner and compressed air and assemble it back with new gaskets/seals. The float height should be adjusted according to factory spec and every little part should be inspected for wear and tear. I usually do this on a bench/desk with a reading lamp and a magnifier fitted to it aside a cup of tea (to keep me concentrated on work) if any screws are chewed up, renew them and all threads should be completely dry on assembly or you may strip the threads in the carb body.
Usually the problem is gumming by varnish, the idle control circuits air passages close up and instead of a froth of fuel being fed into the throats a stream of fuel is fed, which causing rich running. Similarly if the fuel level inside the carb is high excess fuel will be pushed down the throats in both idle and main circuits.