Problem after warm up????
 

Well I am new to the forums. I just recently bought an 88 V-20 with the 3.7 Liter mercruiser in it. I hooked up my gas can to the fuel line and ran it that way since i havent put any fuel additive to the tank. She runs fine at idel during the warming up process and then after you let it run and put it into gear and give it some gas. It comes up to rpm but then when you let off the throttle and then put it back into idel she wants to bog down like it isnt getting enough gas. I have messed with the carb and thought that I had the right fuel mixture for the carb. Would it be anything else?

Welcome aboard. As you sound like you are already leaning that way, your problems certainly could be fuel related. Next time you have it running and it acts like it is running out of fuel take the breather off and place your hand part way over the carb. to choke it (Or Operate the choke manually). If it picks up and runs better then it is safe to assume the carb or fuel pump needs some attention. For safety keep a fire extinguisher handy in case of backfire. Actually I have never seen the carb on your motor so I am assuming there is a choke, Spare, Ferm or one of the other gurus here can certainly help.
If the boat has sat with fuel in the tank and carb. it is time for a rebuild anyway. Rebuild kits are not expensive and the carb is not hard to work on. It is possible to adjust the idle mixture screws to allow it to idle fairly well even though the jets could be plugged up. If the choke is electric or heat driven it could be enrichening the mixture on startup and as the temp. rises and the choke begins to wane off there just isn't enough fuel. Try pulling a sample of fuel from the fuel tank in the boat to see what you have. I would not just put additive in the tank and assume all is well. Is there a water seperating fuel filter? This will get you started. Keep us posted.