1980 70hp mariner ob

[tshaljr]

Have a bad miss. Local mechanic said it sounds like a cylinder not firing. While idling, pulled wire from top plug - engine ran very rough. Put back. Pulled wire from middle plug - ran rough. Put back. Pulled wire from bottom plug and almost no difference. Put back. Ran the same. I could hear arcing when i had the wire close to the plug. Is this an electrical or fuel problem? What is the fix/

 

[racerone]

Compression test is first thing to do.

 

[mixerjv]

Trigger could be flaking out as well firing at the wrong time, had that happen on an 87 mariner 60 (same engine, they re-rated the 70 to be the 60 when they switched from crankshaft hp to propshaft hp) Is the Trigger original? Wires corroded to heck? I would be shocked if they weren't.

 

[Chris1956]

See if the bottom spark plug is very clean, like it was steam cleaned. If so, you may have water infiltration due to bad crankshaft seals.

 

[tshaljr]

Tested compression earlier in the year and it seemed ok. Plugs are new. Local mechanic said it is the switch box. $250.00 Is there any way to test before spending the cash? Not sure what the Trigger is?

 

[mixerjv]

The trigger is below the flywheel and tells the switchbox when to fire the sparks.

Wait a minute, you said you had some spark on the bottom cylinder when you got the plug wire close to the plug, right? To me that says a health spark, usually meaning the switch box and coil are good. If the trigger is flaking out or the wire insulation is compromised it could cause such spark to be "at the wrong time" The wiring insulation on 80's mercs was awful and 30+ years just disintegrates them.

As a side note if you're keeping this engine, and you'll be working on it yourself, I highly recommend the genuine Mercury Service Manual, in addition to CDI's free troubleshooting guide:

https://issuu.com/cdielectronics/docs/practical_ouboard_ignition_troubles

page 81/82

 

[tshaljr]

Found some very corroded wiring, so, since it is the end of the season here in Western PA, gonna put her to bed. Start again next year probably with a wiring harness. Thanks for everyone's input.

 

[mixerjv]

Keep in mind that much of the bad wires will be part of the trigger and/or stator, and not part of the harness.

 

[strokendiesel002]

+1 to mixerjv. I havea '79 Mariner 80 and am in process of purchasing Stator and Pickup coil, as I replaced every other harness but the one going to the remote. I kicked myself several times when I went to work installing what I'd purchased, only to realize, it wasn't enough.
My experience: CDI seems to make decent product, the only item I bought and installed from them was the internal harness that goes through the cowl from the inside. It did require modification. While I had no issues doing this from a skill set/ tool availability standpoint, the time it took wasn't worth the $20 or $30 I saved from OEM. I am by noeans a rich man, but with 2 kids under 2, time spent repairing means no sleep or a Saturday in the garage instead of on the water. If working on a project in the off season, I'd highly encourage considering them!

Good luck! I love my Mariner 80