Topic: 1971 Evinrude Ski-Twin 40hp spark plug gap closed

This topic has 7 replies, 6 voices, and was last updated 2 years, 2 months ago by fleetwin.

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August 20, 2023 at 11:02 am #279455
MarkD
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Yesterday at a chapter meet I was running my 1971 Evinrude 40hp Ski-Twin. After about 5 minutes it started to act like it wasn’t getting fuel. With help from others, it got worse and worse and it was suggested one cylinder wasn’t running at that point. I took the spark plugs out and one electrode had no gap, completely closed down. They are Champion J6Cs and have been in the motor for about 3 years with not much use overall. This year the outboard has been run about 1 hour and hasn’t sounded as good as prior years. The correct gap was created in the spark plug and they were reinstalled, and the engine sounded like both cylinders were running again and had pretty good power, but still not as good as previous years. After about 10 minutes of running we took the boat out, and when I got home the gaps in both spark plugs were fine, so no rapid change.

There were a few ideas on how this might have happened or will happen again, but I wanted to see what others might think.
August 20, 2023 at 12:22 pm #279456
frankr
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Something got squished between the piston and spark plug. Likely a piece of a broken piston ring. Remove the bypass cover from the side of the powerhead and peer in to see what you can see,
August 20, 2023 at 2:12 pm #279460
jeff-register
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alao with covers off use a very small flat screwdriver to push on the ring thru the port to see if it still has spring to it. The ring might be broke 180 degrees from exhaust port. Maybe.
August 20, 2023 at 3:20 pm #279465
MarkD
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Good idea (broken piston ring), and one that didn’t come up yesterday. I will take a look as soon as I get a chance.
August 21, 2023 at 3:27 pm #279502
Mumbles
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These motors used the tapered pressure back rings in the top groove and the piston metal was quite thin above the groove. This is where these pistons like to break and if you can see the top of the piston thru the spark plug hole, any damage might be visible.

Here’s where a borescope for your phone would come in handy. It would save pulling the head to check for damage.
August 21, 2023 at 6:05 pm #279506
MarkD
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I was able to check the compression today in each cylinder and both were 90 psi, which seems pretty good. That there is no difference between the two probably rules out a broken ring. Could a piece break off the piston top as described and not affect the compression? I did look in in the spark plug hole but didn’t see any damage.
August 22, 2023 at 8:52 am #279517
dave-bernard
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A piece of carbon broke off and bounced around in there.
August 22, 2023 at 10:47 am #279524
fleetwin
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The only other thing that comes to mind is a worn crankpin or wrist pin bearing creating extra slop allowing the piston to rise higher and crush the gap. It is easy enough to check for piston/rod slop though without pulling the head/covers. Usually you can hear the knocking noise at idle, sometimes goes away when the throttle is raised though.
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