1993 Johnson 90 has started doing strange things to spark plugs

My 90 Johnson suddenly lost power to the point that we could only crawl back to the boat ramp but as soon as I cut the revs it stalled. The only way to keep it running was to have it on full throttle, but it was only running at less than trawling speed.
I checked the spark plugs, 3 were good and one had a melted center electrode with pitting around the lip of the metal case.
I replaced all the spark plugs with Champion QL77JC4 plugs(the same as I had run for a number of years). I put the ear muffs on it and it seemed to run well and started well. I then took it for a run and it just didn't seem quite right. It was a little slower to start than normal and it had a slight intermittent miss. I was running at about one third throttle when it lost power and again I had to crawl back to the ramp. Checked the plugs, three were good and one had a flattened side electrode with pitting on top of the side electrode and again around the rim of the metal case. A different cylinder to the first time.
I'm not sure what's going on. My thoughts are tending towards the timing but I haven't encountered this before. Fuel is pre-mixed.
Any input would be greatly appreciated. Thanks.

Re: 1993 Johnson 90 has started doing strange things to spark plugs

Do a compression test. My rough guess is that you have a cylinder and piston going bad. Keep on running, and you end up with a total break down.
Remove the by-pass cover and head and have a closer look.

Re: 1993 Johnson 90 has started doing strange things to spark plugs

Haffiman said:

Do a compression test. My rough guess is that you have a cylinder and piston going bad. Keep on running, and you end up with a total break down.
Remove the by-pass cover and head and have a closer look.

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Thanks Haffiman

Re: 1993 Johnson 90 has started doing strange things to spark plugs

Compression test will tell you a lot. If plug is getting damaged that quickly you may have a broken ring flying around in head. take head off and examine pisron and cyl walls for scoring and debris.

Re: 1993 Johnson 90 has started doing strange things to spark plugs

Thanks.I will check compressions.What makes me curious is that it has happened on two cylinders at different times.The cylinder which originally melted the electrode didnt have a problem after replacing the plugs.