1991 Mercury 40hp 2stroke 4cy

I have a 1991 Mercury 40hp that has been sitting up outside for over 10years. I cleaned all the dirt dobber nest off. I removed the spark plugs and filled the cylinders with pb blaster and let them sit over night. The starter was rusted together and would not spin, I removed it and broke it loose using some pb blaster and a wrench. It is now spinning again, although not at what I would consider full speed but it is enough to spin the engine. I replaced the water pump. The fuel filter has been replaced too. I finally hooked it to a tank of fuel and primed up the bulb, the engine fires off but goes dead shortly after starting, also the fuel filter is not filling back up. I am guessing the next step would be to replace the diaphragm in the fuel pump and while I am at it go ahead and replace the fuel lines. Do yawl think the next step is the fuel pump or is this a carb problem? I went ahead and ordered a diaphragm kit.

I have the same engine (but 1997) and I would go after the 2 carbs first. On smaller motors I have had some success with draining everything and filling the bowls and a fuel line with Seafoam, and removing the ignition kill (you do NOT want the OB to start ...), then by priming and pulling that mix through everything. Essentially you're trying to fill all the passages up with Seafoam. Then I let it sit for a few days, drain and fill with fresh fuel (and oil if needed) and see how it goes.

I'm averaging about 50/50 as to whether I need to pull the carbs and give them a good cleaning. Always clean from small orifice to larger. FWIW I bought the Merc OEM service manual on-line for about $16 if I recall correctly. Well worth it!

Oh, one last thing ... the impeller is the WEAK point on that motor (small sized for the rated output, which is closer to 45hp says the gurus on the Scream&Fly forums) so make sure to replace it at least every other season ... even if you don't think it needs it!

They are smooth, quiet and have a great torque range for a 40hp OB, with those 4-cylinders running her.

Thanks for the info, I filled the carbs with seafoam and I'm going to let it sit until the diaphragm kit arrives. I am going to try and get by the parts house to get some new fuel line and replace that while I'm at it. Maybe by this weekend I will try and fire it up.

DaleH, thanks for the replies and the message. Do you happened to remember where you purchased the manual or the best place to get one?

I found a service manual, it is ordered but not in yet. I rebuilt the fuel pump and replaced all the fuel lines. I pulled sea foam thru the engine. I have fuel leaking from the vent in the lower carb, I assume the needle has a groove or there is still some trash in there preventing it from sealing off. Do y'all think the carbs need rebuilt or just drop the bowl and check the needle.