Suddenly lost one cylinder

[goodmariner85]

I have an Evinrude 90hp 1990. I've been working on the boat on land for a few weeks. It has run pretty well for the most part. A pop and stutter now and then at low rpms (on land pretty much all I do) especially when not really warm.

I have started it and adjusted things many times over the past few weeks, including today a few times. I bolted the air silencer cover on and when I started it, it sounded bad, would not keep running at idle speed. I pulled each plug wire. Pulling number 3 wire had no effect. As I played with it over the next hour, sometimes it seemed like there was a very slight bit of power from number 3.

I swapped two coil packs. No difference. The coil pack seems ok.

I swapped two plugs. No difference.

I swapped the wires from the coil pack to the plugs. No difference. Plug wires seem ok.

I manipulated the wires including from power pack while running to see if something might be loose or damaged, but did not notice anything.

I tried an inline spark plug tester. I got a light, but it looked a bit erratic. [EDIT managed to start it again. The light does not look erratic to me, steady flashing as far as I can tell]. I've never used one before, so I'm not sure. I tried to put a spark tester on both the bad cylinder and a good one to compare. But I have been unable to start the motor for some time. I suppose I can try just cranking. I'd really like to compare the sparks.

A mouse had chewed the insulation on one wire from the power pack to the coil pack on the bad cylinder. I repaired it weeks ago, seemed ok. I had to add a small length of wire because there was no slack. The repair is with two shrink tube / solder connectors. Later I found this paint on goop that hardens into insulation. This would have been perfect. Too late. I am contemplating cutting it and using crimp / heat shrink connectors. But if the voltage is already very high in this wire, I wonder if the current is more sensitive to the quality of the connection and if it may burn. I doubt this is causing the problem right now, but I don't know.

Compression was around 125 on all four cylinders last summer. I remeasured when I started this spring on land. I got 110, 114, 112, 110. Checked again a couple of days later and got 115, 120, 110, 119. I don't have an explanation for this (both were measured with a cold engine at 60 degrees air temp) The bad cylinder is number three, with 110 psi.

An important point: Even when the engine was running "well" I noticed that pulling the plug on number three showed that it had significantly less power than the others, but still had some. This has been the case since I started working on it a few weeks ago.

I did a link and sync a couple of days ago, including the Joe Reeves method. High speed timing was off by about 2 degrees. I corrected it. The low speed timing was not off by much. It was hard to correct because I found it difficult to set the throttle to when the carbs just begin to open and measure the timing at the same time. But I got close I think. The engine seemed to run fine after the link and sync. A few hitches during the first few minutes of running.

The question is what to look at next?

 

[goodmariner85]

I just measured the compression again.

102, 110, 10, 110

So, I guess i can stop looking for electrical problems.

 

[racerone]

Take that head off and have a look.

 

[saltchuckmatt]

You might have been running lean and cooked it.

Retest with some oil in the cylinders....something is wonky here.

 

[racerone]

A bit of oil won't help.-----Likely suffered from broken piston rings.----Everyone of those motors will get that cancer !!

 

[goodmariner85]

When I first started it a few weeks ago, I forgot to add oil to the gas. I'm pretty sure that is what destroyed it. Took a lot of work to start. Unfortunately, I did manage to start it and ran it for one minute or so. When I finally realized what I did, I drained the carbs and pumped gas-oil through till I saw green gas come out. It then started up pretty easily.

I thought I must have destroyed it. I measured compression. This is when I saw about 110 to 120, so worse than last summer when I got 4x125 psi. Although I didn't measure at the end of the season in the fall, so I'm not 100 percent sure.

After this it ran more or less ok on muffs while I worked on it over a few weeks: New control cables, fixed leaky carbs. Link and sync. Fixed loose bolts on remote control. Really got it in decent shape.

I called my friend to borrow the trailer, took the tarp-tent off. Bolted on air silencer and put cowling on. Started motor more time before I went to get the trailer. It ran really badly. I kept running it to check the coils, spark, etc. (better if I had stopped running it immediately, but I didnt think of cylinders) I finally re-checked the compression. I saw that number 3 was at zero! I think the damage from no oil caught up with me.

I had a ton of stuff to do and had already spent way too much time on the boat. So it has been sitting there since I posted last week. My plan has been, as @racerone says, to pull the head off and see if I luck out and it can be saved. I expect the bolts will come out without too much trouble, so it should be quick.

I know people post here about the lemons they bought accidentally. I bought it last summer without inspecting and measured 125 psi on all cylinders. It ran bad, but it was just a coil pack. Got lucky!

I'll report here what I find after taking off the head (or both).