I have a 1986 75 hp Merc 4 cylinder, 2 center bowl carburetors, triangle style fuel pump. I've had the motor since 2001, and it's always run fine on the top end but never has idled well. I've replaced the fuel pump diaphragm twice (2002, 2008) and saw a real improvement in starting and idle performance after those replacements. I replaced the diaphragm and fuel pump valves in 2002, but only the diaphragm in 2008. The carbs have never been off since I've had the motor, but it hasn't seen very many hours.
I parked the boat in April 2009 with an almost empty fuel tank and fuel stabilizer (Seafoam) in the E10 fuel that was in the aluminum tank. I tried to start it this month and have not been successful. When I tried to start it, I primed in some of the old gas and the motor sputtered in its old familiar way like it was going to start, but then it stopped trying (i.e. it spins up just fine but doesn't fire off). Realizing that not draining out year old E10 wasn't probably the smartest move, I pulled off the fuel line and hooked it up to a portable tank with fresh fuel mix. The primer bulb is hard and I'm seeing fuel in the clear fuel filter on the motor side of the fuel pump. I have spark and compression.
What is the best way to determine where my fuel problem is? Do I worry about changing a 2 year old diaphragm, or just pump the bulb while cranking to see if that changes anything? Could a stuck float be my problem? Or are my carbs just in dire need of cleaning and they finally thumbed their nose at me this year?
I spoke with a local Mercury shop, and they advised me that the carbs likely need cleaning. Question for the forum though: The Clymer manual says a thorough cleaning cannot be done unless you remove the welch plugs to access those passages. The mechanic at the dealer said this is going too far and is unnecessary. What do the experts say?