[eldplanko]

I have a 1975 Mercury 402 (2-cylinder), that is only running on one cylinder. It starts right up, ildes, and revs to full RPM on only the upper cylinder (briefly on the muffs), but as soon as I disconnect the upper plug wire, it dies. I have spark on upper and lower, and further confirmed the ignition system was ok by swapping the upper and lower coils... same result.

As background, this engine has been run in salt water, and has recently sat for 2-years unused. I've recently cleaned the carburator, inspected the fuel and water pumps. Also replaced the thermostat, which looked like it was the original and non-functional. There is good compression on both the upper (125-135 psi) and lower (120-130 psi) cylinders.

I pulled the powerhead, and removed cylinder cover, exhaust and transfer plates to inspect the cylinders and water jacket. I also inspected the lower driveshaft seal. There was a corroded hole in the lower portion of the exhaust plate divider that looks like it would allow water to enter the exhaust ports on the lower cylinder. I replaced the plate divider, scrubbed all water passageways and sealed up with new gaskets. I also cleaned up the exhaust plate where powerhead mounts, and sealed up with a new gasket. I used OMC gasketsealer on all gaskets. Same results as above, lower cylinder doesn't fire, and plug is wet with water.

Any ideas on how else water could be getting in.

[Laddies]

If you use a induction meter on the bottom cyl. I think you will find there is no spark, if so the switch box or the wire between the box and coil is bad.

[eldplanko]

Is it normal for the lower plug to be wet with water?