submitted this once already and am not sure why it didnt go through,so I will start over.
is the vent open and clear??? if it is restricted,it can do this also.
is the fuel screen at the fuel pump clogged?? if so,clean it.
is the screen on the tank pick-up tube clean?
if this doesnt help,prime the carbs with the bulb so that it will run a minute or so,pull the fuel supply hose off of the top carb,put an extension on it and hold it in a small container,start the engine and let it idle,you should see a fairly steady strong stream coming from it,if not,the pump is weak,and it is fairly simple to repair or replace either way.
make sure you have it in water,or a hose hooked to the motor,I realize you already know that,but mention it as I have seen them run while testing,and the impeller gets burnt up in short order.