1977 Evinrude 85hp - Intermittent power loss

[OldOMC31]

Engine = 1977 Evinrude 85HP - 85799S
Intermittent power loss. Seems electrical to me, I have gone through the carburetor and fuel pumps and they are spotless.
Seems to happen more when its cold or humid outside, but doenst always happen under these conditions. Throttle prosition doesnt seem to matter. It seems like an electrical miss with quick recovery when it stumbles. Happens for less than 30 seconds, then the motor runs flawlessly for awhile at all throttle setting.

My assumption is either coils breaking down or powerpack. I have a multimeter, but little else to diagnose electrical issues.

What would be the prudent next steps to diagnose here?

I cannot see how they come apart without possible destroying one, but can the spark plug wires coming out of the coil be replaced? Or do i need a new coil to get replacement wires?

 

[racerone]

Replace boots on plug wires.-----Or test run with a timing light 1 cylinder at a time.-----Might help pinpoint the problem cylinder.

 

[OldOMC31]

Will give the boots a shot. Coils are original to the motor. Do the boots tend to deteriorate before the wire insulation and cause issues?

I realize these are pushing 50-year old components. If the boots don't solve the issue, I'll look into getting a timing light to try to identify a problem cylinder. I hate intermittent electrical issues because most of the time it runs beautifully. Almost like you have to always bring test equipment along to diagnose on the water if the issue pops up that day.

 

[McGR]

The plug wires can be removed and replaced. They more or less unscrew from electrodes within the coil housing.

 

[jimmbo]

Misfires on these engines have been known to happen when the a Spark jumps from the Wire/Boot, to the Cowl Latches on the back of the Cowl.

 

[OldOMC31]

New plug wires and new boots for all 4x coils this weekend. Started on muffs in the driveway and idled well for 5 minutes. The real test will come now over the next 6 weeks of running it this year as this problem was/is intermittent. Hoping for the best, but will update if I still have this electrical miss.

 

[OldOMC31]

Update on this 1977 Evinrude 85 85799S.
Issue is still intermittent. Did find something last night. Had the motor running on muffs in the driveway. Seemed to be idling rough. Pulled off spark plug boots one at a time. Issue seems to be with #4 coil. When Pulling off that boot, no change.

Swapped coil #2 & 4 around and issue moved to #2. Replaced the plug wire on the coil again with no change. Have replacement coil on the way. Question is could this be something other than just a coil?

 

[jimmbo]

Could be something as Simple as a Bad Coil. Since the Miss moved when you moved the Coil, and remained when you swapped wires, it kinda points to the Coil.
Replace the Coil, if you are still having Issues, then more Diagnostics is in order

 

[Crosbyman]

chat gpt opinion ...

1. Ignition System (most common on old OMCs)​

• Stator & charge coils – These can develop heat-related failures. They may work when cold but lose output when warm, causing a weak or missing spark.
• Power pack (CDI unit) – Classic for intermittent misfire or total loss on one or more cylinders, especially after running 15–30 minutes.
• Timer base/pickup coils – Cracked wires or corrosion cause inconsistent triggering.
• Coil packs – A single coil breaking down intermittently will cause power loss on its cylinder.
• Loose/dirty grounds – Check all ground connections on the power pack and coil mounting.

A quick spark test when the problem happens is key. If spark is lost on one or more cylinders, it’s ignition.