'93 115HP Mercury - Sputtering at mid RPM

[convergent]

The last two times we've gone out our motor has been cutting out and sputtering quite a bit at mid RPM. Its a '93 Mercury 115 HP, 2+2 2 stroke. It does it for about 20-30 minutes and is then fine for the rest of the day.

It starts from cold on the first crank now every time we go out, and this problem just popped up. It is the worst when we try to start off pulling a skier. It makes it to about 3000 RPMs fine, and then stalls... then cuts back in then stalls again. after about 3 or 4 times it picks up and runs smooth for the rest of the pull. This is obviously a problem because its right when the skier is getting up and often the fall when it cuts out. Never cuts out at all once running up at 4000 RPMs. It might do it for 2 to 3 pulls and then we are good for the rest of the time we are out on the lake.

I'm not sure if it is related at all, but I replaced the starter last week and its done this each time we've gone out since then. The first time out after the new starter, I noticed the tach wasn't working as we were heading out of the no wake zone, but it shortly started working and hasn't been a problem sense. I'm not sure if I disrupted something when I was working on the starter.

I was thinking it may be fuel related, but why would anything fuel related change once we'd been running for a while? Any ideas on the best way to troubleshoot this, or is there something from my description that points to a specific problem.

As always, thanks for the help.

 

[Coloradolakeboy]

Re: '93 115HP Mercury - Sputtering at mid RPM

I have the same problem, rough running until warmed up. From what I have read here, I would guess one or more cylinders have carbon sticking the rings until the motor warms up. Power Tune or Seafoam has been recommended in those threads to remove the carbon. I bought the Power Tune as recommended by Fazt and aChris but haven't used it yet. When is the last time the carbs were rebuilt? I guess it could be several things.

 

[convergent]

Re: '93 115HP Mercury - Sputtering at mid RPM

Interesting suggestion. I have no idea about the carbs since I've only had the boat for a couple of months. The PO seemed to maintain it well and it's been running smooth until this. I've been taking it out several times a week for several hours of watersports so carbon deposits would seem odd to show up now vs when we first got it. The PO didn't use it a lot.

On the Power Tune, the instructions on their website say to spray the whole can in while running and then let it set. How would this work with a 2+2? If I spray it in while idling, will it work in all 4 cylinders since it's only running on 2 at that point? Also, where is the intake that I should spray it in?

Any other thoughts in the problem from others?

 

[convergent]

Re: '93 115HP Mercury - Sputtering at mid RPM

bump... any more thoughts on this?

 

[convergent]

Re: '93 115HP Mercury - Sputtering at mid RPM

Update on the problem... went out yesterday and pulled skiers and wakeboarders for about 4 hours and it never sputtered once. Odd because the last two outings it did it a lot for the first half hour. I will see what happens next time out.

 

[Dave1027]

Re: '93 115HP Mercury - Sputtering at mid RPM

When I powertuned my 2+2 motor I used the choke/enrichner lines to feed the can directly into the intake manifolds. Very simple that way. Don't have to remove the carb cover. It feeds all 4 cylinders evenly and completely.

 

[drum23]

Re: '93 115HP Mercury - Sputtering at mid RPM

You might want to check all your grounds. My engine had a similar problem last month where it would sputter at mid to high rpms randomly. My mechanic assured me the problem was the carb and rebuilt it, but I still had the same problem after he supposedly fixed it. So I took it back to him and he "discovered" the engine sounded like it was arcing. Turned out it was just a loose wire on the #1 cylinder.