90HP Mariner not firing on all cylinders?

[jerkbait]

Hi all,

Hi have a 1995 90hp Mariner 2 stroke 3 cylinder. On the 4th it was running fine earlier in the day. We trolled for 3-4 hours and when we went to head in i could not get any top end power. Seemed like a fowled plug, so i changed all 3 plugs. The top plug was wet and the other 2 were dry. New plugs no difference. The top plug does not look like it has burned since i put it in and the other 2 show some signs of burning. All 3 plugs have spark, i thought maybe the top one had weaker spark, but now yesterday it seemed just as strong as the other 2. When we are trolling it seems to be missing and every once in awhile the motor will rev up like it is firing on all 3 again and then back to running ruff. Any thoughts? Mechanic in town thinks maybe switch box. Seems like electrical to me but now i am wondering. Since the plug i thought had no or little spark zapped the heck out of me yesterday.

Thanks.

[boatfixer]

Intermittent spark issues are no fun! Its always easier to trouble shoot when no spark is present while testing.

The clean plug you mentioned suggests no spark. Intermittent like that usually suggests a bad switchbox.

[jerkbait]

Thank You!

[leech~~]

Do a comp, check. Those 3 cyc units some times had cooling issues on one of the cyc. Check for a stuck ring. I have had 3-3 cyc Force and Merc eng's and two of them ended with this issue. I also do a lot of low speed trolling and they can carbon up and the rings stick. Hopefully not! Good Luck!

[wastewaterguru]

Sounds to me like the top cylinder is simply flooding. Always rule out the fuel/air issues before chasing spark issues. Much easier to deal with.

[boatfixer]

Then that plug would show signs of flooding.

[jerkbait]

Thanks for the help all. I am goin to bring it in and have him test it out.

[jerkbait]

Update on the issue. I brought the boat into the shop last Friday so he could test the switch box. Of course nothing showed for sure signs. So he decided to give me a coil to try. If that didn't work then order a switch box. I out the coil in last night and low and behold it solved the problem. Yeah!!! Hopefully it stays that way now.

[Wanderer]

Thought I'd read through some posts to see if a similar problem to mine had been hashed out and here it is...

Glad the coil fixed your problem. I have a similar issue with my 1990 Yamaha Pro 50. Missing on the bottom cylinder at low rpm. I can throttle up and she sputters to life and runs fine top end but once I get her below half throttle she dumps that bottom cylinder again.

I changed the plugs - no change in performance. The top two are burning normally but the bottom is washed (I can post pics if needed). I figured it was the coil so I changed that as well - no change in performance.

Any thing come to mind?

Thanks!

[leech~~]

Do a compression check. Sounds like my 90hp Force ran when the top rings locked up!

[Wanderer]

Thanks Steve,

I'll give that a try over the weekend. The motor was running perfectly to start the season but started missing at trolling speeds and idle. It's been gradually getting worse. I haven't used it much at all since the problem started - mostly just testing the attempted fixes.