Mariner 40HP BE 676 1979 missfiring when warm.

Hello everyone! I have a Mariner 40BE 1979 that has been running very rough since getting it a couple of months ago (attached to a nice 14ft Mckee craft) I found that the bottom (of the 2) spark plug was dead and also a bit wet. After fitting a new set of plugs it ran beautifully!.......for about 30 mins, then started spluttering and misfiring again. I removed the plugs to find that it was wet again and the top plug was dry with some soot. I know that this particular motor has contact breaker points fitted, that are a real pig to get to. I know that my fuel lines are all good and my carb is clean too. My compression is 125 psi and even in both cylinders. I'm wondering if the head gasket is leaking water, or whether its electrical. Any ideas would be greatly welcomed.....Thanks for indulging me!

Check strength of spark.---------Check fuel pump.---------Possible bad lower crankshaft seal causing poor crankcase compression.

...the fuel pump is relatively new because the previous owner had to replace it after breaking it while changing the water pump (don't ask). The cylinder compressions are at 125psi and I had a period of about 30 mins of great smooth running, so I'm with you on the spark strength I think. Which component in the ignition is most likely at fault do you think?....remember that it ran fine for 30 mins then started to misfire?....something getting warm and then the insulation breaking down possibly?..........thanks for your answer

Was it wet with water or with fuel? In the case of water, typically the bottom cylinders see it first because they're closest to where the powerhead mates with the mid section, and corrosion likes to form up in that area.

...the whole unit looks to be in great shape for it's age with no visible corrosion due to fresh water use, so I'm hoping it's not that but I do see your logic. I have just bought a puller and a spark tester, so I'm going to scrutinize the ignition over the next few days also. My friend has found a thread in which a faulty reed valve was causing a wet plug fouling problem. I will put a cigarette lighter to it the next time it is wet, thanks very much for your reply!

Sounds like you have a bad ignition coil. Typically, they will work when cold, then start skipping when the motor arms up.

Sound familiar?

Jeff