1991 Mercury 40 hp 2 cycle 4 cylinder not getting fuel to bottom cylinder

[alvargo]

I'm having an issue with my Merc 40 hp 2 cycle 4 cylinder where it appears the bottom cylinder is not getting fuel. When I pulled the plugs, the bottom plug was perfectly clean, like it came out of the box. The other 3 plugs looked normal. Compression is even 125 pounds on all four cylinders and all four cylinders have a very strong spark. When I pull the plug wire on the bottom cylinder while the engine is running, there is no change to engine speed or sound. When I pull the plug wires on any of the other three cylinders the engine stumbles and stalls.

There are two carbs on this engine and to my understanding the top carb feeds the top two cylinders and the bottom carb feeds the bottom two cylinders. I'm thinking since there seems to be no issues with cylinder three, at least some fuel should be getting to number four?

The service manual mentions checking the reed valves which appear to be buried in the center of this engine? I don't know much about reed valve operation and the manual just has one paragraph about them in the powerhead tear down instruction.

Is it possible to access the reed valves on this engine without removing the powerhead?

Could the carburetor that fuels two cylinders only feed one and supply no fuel to the other?

All ideas will be appreciated

Thanks,

Allan

 

[racerone]

Several possibilities.-----Bad lower crankshaft seal.------Reed valve could have something stuck in it.-----Bad sparkplug.-----Water getting into the lower cylinder.-----Not a lower carburetor issue !!

 

[alvargo]

Several possibilities.-----Bad lower crankshaft seal.------Reed valve could have something stuck in it.-----Bad sparkplug.-----Water getting into the lower cylinder.-----Not a lower carburetor issue !!

Thanks for the reply. I discovered the problem while changing the spark plugs, the bottom plug looked like it never fired. Put new plugs in, took it for a ride and when I pulled them again, the bottom plug looked like it came out of the box while the other three plugs showed normal evidence of combustion. There is no evidence of water intrusion. I used a bore scope to peek into the cylinder and it looks normal from what I can tell.

I'm guessing I've been running this engine on 3 cylinders for a few seasons. The engine starts, idles, accelerates normally, but I always thought it lacked power for a 40 hp motor on my 17 foot aluminum boat. A recent trip where it started to stumble and stall, led me to start with the plugs.

I squirted gas/oil mix into the carb while running on the hose for a while and it just loaded up the engine. Pulled the bottom plug again and no evidence of fuel on the plug.

How do I determine if the lower crankshaft seal is bad? Is there a way to tell without removing the powerhead?
The reed valves are buried under the crankcase cover so it looks like either option will require dismantling the powerhead.

Thanks again,

Allan

 

[racerone]

There are ways to check , but your best approach is to remove powerhead.-----Remove lower bearing / seal housing.----Then remove bearing and look at the reeds.----Inspect exhaust housing / tuner for blockage.

 

[alvargo]

Thanks for that advice. My obstacle now is the powerhead mounting nuts are partially disintegrated. The studs look like high quality stainless steel, while the nuts are all rusty. I left them soaking overnight with PBlaster, but I'm not confident any will come off without a struggle. I'm thinking of using a Dremel tool to carefully cut into the nuts and try to split them.. Is there a better way?

 

[alvargo]

Just to follow up, the lower bearing and seal were bad. I replaced the lower end cap assembly and the engine now fires on all four cylinders.


Thanks racerone for all your advice. Your approach was certainly the best way to go.

Allan