8Hp 4 stroke carb question

I have an 8HP 4 stroke (Tohatsu i understand) that i have replaced carburetor after years of good performance followed by bad performance and hard starting. I had cleaned carb repeatedly but did not improve starting. The replacement carb does not have the primer bulb spring valve but it does accept the one from my old carb. The new carb also does not have the vent tube protruding from the top of the carb inspection plate but it appears that there is a vent passage built into the carb body. My question is-is the vent tube necessary? The engine is hard to start and hard to accelerate. Once moving it runs great. I have adjusted idle screw and mixture screw to best performance but it still is not right. Also how do you ck for water in gas since gas floats above water?
thanks