It may seem strange, but I set up my low speed tickover with the engine only just warm enough to enable it to start without the choke. As been mentioned, once the engine is fully warm after a good run it will run rich with these settings applied and the tickover will be found to be very fast - adjust back using the throttle stop screws to get a good even tickover as per the manual. I reckon as your mechanic, as he has said, done it with the engine fully up to temp, and the slow running screws are right on the edge and too lean to supply the fuel when the throttle is snapped shut causing it to stall. The float height, as mentioned could be the cause, but I thought it would have to be way low for this to happen, but would surely be highlighted when rapidly acellerating to high speed, high speed 80-90mph plus , with the throttle wide open as this puts a lot of sudden demand on the carburettors. The manual is very good, and has worked for everytime in the past 15 years and I now swear by AMALS.
If it was a problem with just one carb, it would manifest itself as lumpy / poor running which makes me think this will be easily sorted. The regular checks I do are, plug check to inspect colour, fingers down carb intakes to check both start to lift at same time and they both clear ( full throttle ) at the same time.
May be a mad idea but the insulators for the carbs are fitted, and not been omitted by mistake.
Hope you get it sorted and enjoy the performance as the AMALS poor fuel into the into the firey abyss