78 Mercury Dead Cylinder

[sm26890]

Hello

I have a 78 merc 800. It idles a little rough, and likes to die when I put it in gear. I have pulled the plug wires while it's idling and it seems the #4 cylinder is dead. Has 120 psi compression. When I pull the plug wire off, it has tons of spark. I have checked the resistance on the wire and it is 0 ohms. I have switched the spark plug with another cylinder but the #4 remains dead. Is it possible that the lower carb could be dirty and firing the #3 but not the #4? This is the only thing I can come up with. I spent 10 years as a diesel mechanic but have little experience with outboards. Can anybody lend some advice here? The motor has good power once it's up and going, however at some rpm ranges it does tend to sound like it's got a miss in it. I also recently had the power head off to replace the exhaust manifold and water jacket manifold gaskets. The pistons and such looked ok, didn't see anything that looked out of place. I talked to the previous owner and he never had an issue with the boat. He claims the gas is probably about two years old. I was going to replace the gas as well and add some sea foam. Any thoughts?

 

[Chris1956]

Re: 78 Mercury Dead Cylinder

Does the #4 spark plug look real clean? If so, maybe water is getting into the crankcase thru the lower crankshaft seals? If cylinder 3 works, it would be hard to see why #4 won't fire. They share reed valves.

 

[sm26890]

Re: 78 Mercury Dead Cylinder

It looks just like all the other plugs. Like it's been firing but not very well. Somewhat wet and oily. I dumped the gas and am going to get some fresh gas and add a little sea foam to the mix. We'll see what happens then.

 

[emckelvy]

Re: 78 Mercury Dead Cylinder

The reed valves between the 2 cylinders are specific to each cylinder. The reeds at the top of the reed block serve cyl #3 and the ones at the bottom, #4.

You may have a bad lower crankshaft seal, which would cause #4 to drop off like that due to loss of crankcase compression.

If you had a bad reed valve in the #4 side, it would spit fuel back into the carb. It'd also be real doggy upon takeoff, but might pick up OK once underway, since a bad reed is more evident at idle and lower speeds.

Since the lower carb feeds both cylinders, if #3 is running OK then #4 should be getting enough fuel as well. I can't recall if the fuel pump takes vacuum off #4, but if so, there could be a possibility of flooding out #4 if there was a pinhole leak in the fuel pump diaphragm. Something to check, at any rate.

If you pull the powerhead, you can remove the 3 bolts holding the crankshaft's lower seal/bearing carrier assy and pull the assy out with a harmonic balancer puller or other suitable tool. The 2 seals are pressed in with their lips facing downwards. There's an O-ring around the carrier that will also need to be replaced. Then the carrier taps back in evenly, note the 3 holes are unevenly spaced, so there's only one correct orientation for the carrier.

Unfortunately, to get at the reed valves you'll have to split the crankcase, but unless you see fuel spitting back from the carb, I wouldn't be inclined to go there yet.

One last thought, a reed block with worn-out labyrinth seals can cause poor idling, but that would also affect #3.

HTH & let us know what you find..........ed

 

[Chris1956]

Re: 78 Mercury Dead Cylinder

yes, fuel pump diaghram failure can flood #4, if I remember correctly.