Read my plugs? - 99 Merc 90hp 2-stroke

[Eric Steinberg]

Greetings helpful strangers! I'm hoping some experienced folks might be able to look at the attached plugs and tell me what you think is going on. These are in a 1999 90hp 2-stroke Mercury. The engine suffers occasional misfires, which at idle or low rpms often stall the engine. This has been a nuisance problem for a long time, which has slowly escalated to severe nuisance status (considering ditching for a 4 stroke if I don't make headway soon).

I/my family have owned this engine since new. The block and head have never been touched. I have done the following "recently" to try to fix the misfire and plug fouling. This has been over the course of a few years, but only tens of engine hours... sadly I don't get to run this boat much.

1. New plugs
2. Rebuilt carbs (new gaskets, o-rings, etc... old stuff still looked pristine anyway)
3. Followed complete procedure in mercury service manual to sync and adjust carbs, linkages, and ignition timing
4. Measured oil injection pump output per service manual procedure and adjusted
5. New plugs (again)

Every trip starts and ends with 5-10 minutes of no-wake speeds. I have NOT done a comp test. The plug on the left is the top cylinder.

Thanks in advance!

 

[ondarvr]

The first plug looks like it was running rich, or cold.

Have you adjusted the low speed mixture screws on each carb? They can have a big effect on how it runs at low RPMs.

 

[Eric Steinberg]

The first plug looks like it was running rich, or cold.

Have you adjusted the low speed mixture screws on each carb? They can have a big effect on how it runs at low RPMs.

I set all mixture screws to the "default" position per the service manual. Have not adjusted them individually, and wouldn't really know how to except by examining plugs, adjusting, and re-checking plugs. A less iterative process would be welcomed.

 

[racerone]

On this type of problem ( stalling ) you start with a compression test.-----Just post your numbers here.

 

[ondarvr]

After checking the compression you can give the screws a slight turn each way and see if it gets better. You can't hurt anything and can always put them back to the default setting.

 

[robert graham]

How old is the fuel in your tank?....maybe degraded or contaminated?....

 

[ondarvr]

A less iterative process would be welcomed.

A little more info, you don't check anything after you turn the screw, just turn it and see if it runs better, adjust until you get the best results, nothing is done after that, pretty simple.

If it doesn't help then look deeper.

 

[Eric Steinberg]

How old is the fuel in your tank?....maybe degraded or contaminated?....

The misfire/stall has been going on through MANY tanks of fuel (and even two different fuel TANKS, as the engine is on its second hull.

Compression is 110psi on all cylinders.

I'll try to get a video of the misfire later... it's like a big "cough" with smoke, and maybe happens once or twice a minute at idle.

 

[Eric Steinberg]

And it looks like the last time I tinkered with it, I had already leaned out the top carb a bit. Mixture screw on top carb/first plug was 3/4 turn out, the rest were 1+1/4 turns out.

 

[Eric Steinberg]

Here is a video showing the cough/hiccup (occurs once at the beginning of the video and again at the end)
https://youtu.be/TCjreWHkWjo

 

[Johnson110]

Why not try spark plugs that you gap, NGKs and see how it runs.

 

[ondarvr]

If it?s a lean sneeze, turning it to make it leaner will make the sneeze worse.

There is another situation that can cause a misfire at low RPM's on these motors, a bad rectifier and/or melted wires going to it. I have the same motor, only a couple of years newer, and it had a bad sneeze on the center cylinder when I got it, after doing a google search I found other people with a miss that was solved by replacing the rectifier, also people with melted wires going to it. On mine the issue was running lean, I was able to adjust it and solve the problem

 

[Eric Steinberg]

If it’s a lean sneeze, turning it to make it leaner will make the sneeze worse.

There is another situation that can cause a misfire at low RPM's on these motors, a bad rectifier and/or melted wires going to it. I have the same motor, only a couple of years newer, and it had a bad sneeze on the center cylinder when I got it,

How did you identify that it was the center cylinder? Do you think my problem is more than likely the top cylinder because of that plug looking like it does?

 

[ondarvr]

Several ways to tell. Adjust the cylinders (carbs) one at a time, if it changes the symptoms, for better or worse on one cylinder, that could be the one.

Put your hand over each carb one by one, again, if it gets better or worse on on one cylinder it could be the culprit

Pull the plug wires off one at a time, it should run OK on any two, if it doesn't the one with that it can't run on could have issues.

A thermostat that's not working correctly can cause it to run cold and have a carboned up plug

 

[merc850]

Are you adjusting the low speed needles in gear and underway with motor running @ 900-1000 rpms?

 

[DavidMoore]

Use the proper NGK plugs - NGK-BUHW-2

Each screw should be 1 1/2 turns out from lightly seated to start, your top carb is probably too lean, if in doubt always error on the rich side to avoid engine damage.

Then put the boat in water, get engine warm, with engine in fwd gear at around idle speed screw the top carb in till the engine complains, then back it out 1/2 turn or a fraction more.
Repeat for the next 2 carbs.
Set idle speed in fwd gear to around 700 rpm, you can go higher if you want so long as you can still smoothly shift.

Job done. You can make minor tweeks if you want. I don't but I'm lazy.

 

[Eric Steinberg]

Use the proper NGK plugs - NGK-BUHW-2

Each screw should be 1 1/2 turns out from lightly seated to start, your top carb is probably too lean, if in doubt always error on the rich side to avoid engine damage.

Then put the boat in water, get engine warm, with engine in fwd gear at around idle speed screw the top carb in till the engine complains, then back it out 1/2 turn or a fraction more.
Repeat for the next 2 carbs.
Set idle speed in fwd gear to around 700 rpm, you can go higher if you want so long as you can still smoothly shift.

Job done. You can make minor tweeks if you want. I don't but I'm lazy.

Thanks, this is definitely the kind of procedure I was looking for. Out of curiosity, what low speed ignition timing would you recommend when doing this? Mercury gives a really wide range of timing for idle, and uses timing to set idle speed, which i don't quite understand.

 

[ondarvr]

The idle timing is not important, just get the RPM's correct.

 

[DavidMoore]

+ 1
You said it yourself, the idle timing sets the idle speed.
If the engine idles then the timings close to where it needs to be. Once you set the idle speed to where you want it, then the timing is also where you want it. The value is unimportant.
Do the procedure at idle speed or a little over, most important part is boat in the water in fwd gear.

 

[Eric Steinberg]

The engine doesn't "complain" when adjusting one carb at a time until the mixture screw is very nearly bottomed out. Is that what you would expect?