Sounds like a colapsing fuel line...at that age of a watercraft the fuel lines get soft spots from age causing it to colapse as suchion from the gas runs through it...also check to see if there are any tight bends in your gas line...something may have gotten kinked. A colapsing line is difficult to find/detect and you just have to go and replace all the fuel lines. I'd try this first before messing around with the carbs.