1995 115HP Mariner sputters at 2400 to 2800 RPM

[Old Ironmaker]

I bought the Star Craft Superfisherman 190 with the 95' 115HP Mariner 7 years ago. The engine runs very well other than if for some reason it will sputter between 2400 RPM and around 2800 RPM. Sputter is the best way I can describe it. When there is occasion to run at between 2400 and 2800 RPM, which isn't often, she just doesn't like it. It has been this way since I bought it new to me. Through the hole shot it will on occasion, not always, do the same thing when I reach that RPM then once past 2800 RPM or so she runs just fine. I have been meaning to address this but when the season comes it just isn't a real issue. No one has been able to answer as to why including a few seasoned Merc mechanics. I have the who, what, when and where? I need the why. I service this boat and engine well. I thank you for your advice.

Johnny D

 

[Texasmark]

How about a SWAG (Scientific Wild Arsed Guess): Pull the plugs and see that they are all about the same color/ state of wear and check the piston crowns while you are there looking for uniformity and checking on carbon buildup. Leave them out for the following testing:

Get a timing light and check the timing (top piston for a signal....#1) from the idle position through the complete throttle advance range (while running the starter), paying particular attention that the timing increases steadily through that range and winds up at 21 BTDC at WOT (or thereabouts....don't know your exact WOT number but will be close to 21).

Then go back through the idle to WOT throttle movement but this time watch the linkage and carb butterflies noting that there is a smooth transition on all 3 with uniform opening across the throttle range.

Get back with the results including the carbon buildup on the piston crowns.

What is the attitude of your boat when in that rpm range? Are you coming onto plane?

 

[racerone]

This 4 cylinder motor only runs on the top 2 carburetors / cylinders at idle.--The bottom 2 carburetors kick in at around 1800 RPM.-----Is there an accelerator pump on it ?---And if so has it been checked out ?---Any other trouble shooting done ?

 

[Old Ironmaker]

How about a SWAG (Scientific Wild Arsed Guess): Pull the plugs and see that they are all about the same color/ state of wear and check the piston crowns while you are there looking for uniformity and checking on carbon buildup. Leave them out for the following testing:

Get a timing light and check the timing (top piston for a signal....#1) from the idle position through the complete throttle advance range (while running the starter), paying particular attention that the timing increases steadily through that range and winds up at 21 BTDC at WOT (or thereabouts....don't know your exact WOT number but will be close to 21).

Then go back through the idle to WOT throttle movement but this time watch the linkage and carb butterflies noting that there is a smooth transition on all 3 with uniform opening across the throttle range.
Get back with the results including the carbon buildup on the piston crowns.

What is the attitude of your boat when in that rpm range? Are you coming onto plane?

We are about 5 weeks from getting the shrink-wrap off the boat here, Lake Erie, and I will get it inside the shop and look at what you suggest, thanks. I have a good friend that is a retired marine mechanic (is there such a thing as a retired Mechanic?) and will show him your suggestions. It is beyond my scope of mechanical skills. I always surround myself with smarter people than I am in certain skill sets. It's nice to have smart pals. I change the plugs once a season on the 115 and they always come out very clean, reusable clean but I toss them or give them away. It basically only gets us out onto the fish and then I use the kicker or electric. Attitude at that RPM, a good question that I can't answer. It will be fully on plane at around 3500 RPM I think, but will check closely. I seldom go full throttle which is around 4800 RPM.

Racerone, another question I can't answer. Like I said earlier I haven't troubleshot this issue yet I only asked a few mechanics about it and in all fairness they would have to look at it for a real answer. It will be a while before I can get her on the muffs. Snow storm coming tomorrow AM so it's a tad early to put water to the engine and there are a few boats being worked on before it's my turn.

 

[ondarvr]

Full throttle should not be 4800, you should be seeing at least another 4-500 RPMs on a heathy motor with the correct prop in place.

 

[Texasmark]

First off I goofed. I forgot the year model making it a 2+2. Racerone brought that to our attention. Sorry.

Second, 4800 is wayyyyy low for that engine at WOT, and ondarvr caught that one.

Even though you don't run up there, you need to setup your gear train anyway. Your prop is part of your gear train. You are lugging that engine at 4800 WOT setup. You ought to be up around 5200-5500, forget which for the 115.....my 90 triple of that series was 5500 and I think the 4 cyl 125 is also 5500. I think the rating on that engine is 5250 since it is essentially the same as the 125, just dropped 10 ponies and rpms could do that...smaller jets in the carbs or something like that.....service manual lists jets and that would be a quick look up.....I had the service manual for my engine which covered your year model and engine sizes from 75 triples thru 125 2+2s.

No doubt in my mind that your problem is related to the second set of cylinders getting spark and running quantity of fuel to kick in and do their part of sharing the load. I never had one and never worked on one but it seems to me that if you are lugging the engine, especially since you are right in the rpm range in question while in the hole coming onto plane, the hardest load on the engine since you are trying to get the hull out of the water and on top where drag is much much less....lightening the load on the engine. So there could easily be some interaction with the engine responding to the throttle as it was designed to do.

So if everything is in order, aka engine doesn't need any kind of fixing, or servicing, and boat hull is clean and boat is in good order, you know how to and do use your trim system correctly for max speed at WOT (ensuring 4800 is all you can get out of your boat as is) then I'd look at a lower pitched prop for starters. If you are skeptical you can go to the top of this page, get into the propeller section and fill in the blanks to find the right prop for you.

I'd look at 5200-4800 = 400 rpms/200 rpm per inch of pitch change = a 2" reduction minimum in pitch from your prop's pitch preferably from the same mfgr. and type to reduce other variables in the equation. You can get an alum 3 blade for about 90 bucks on here....or could not too long ago.

That's cheaper than any mechanic you'd have to pay to investigate your engine. Besides it would be good for your engine regardless and you could use your current for a spare. Then come back on here and let's talk about where you are and go from there before you spend any repair money on it.

 

[flyingscott]

All the 4 cylinders operating range is 4750-5250. The triples are rated at a higher rpm. Your motor should be transitioning to the 4 cyl at 1800 rpm. I had the 125 version and had a similar problem. My problem was in how I was using the throttle they do not like it when you role the throttle up slowly. I found out if I was quicker on the throttle it would accelerate better and I would avoid the sputter. Does it sputter when you back down off the throttle or just when accelerating. It sounds like you are getting caught in the transition to 4 cylinders. If it's not how you are throttling up check the accelerator pumps and linkages I believe these have them in the bottom 2 carbs maybe not.I don't remember mine

 

[Faztbullet]

Check the accelerator pup and check the 2 injectors on cylinders 3&4

 

[Old Ironmaker]

Thanks for all the great suggestions all. I was working in the shop today and asked about this to our Guru. He said he already told me about the transition and the carbs kicking in. He said we will look at the carbs this spring and clean them up etc. And yes now that I think about it I just throttle through that RPM and I won't even know it is happening. He also said my wide out throttle should be around 5400 RPM depending on what prop I have on. I think it's 17 or 19 degree, I will check when the wrap is off. I never see WOT because I do not to run the 20 year old engine wide out, just like I wouldn't red line any car or truck. He did remind me to once and a while to go all out to keep everything nice and clean.

So we will take a look at the carbs this spring. I will get back with findings when we do that.

Thanks so much everyone. I love this site.

 

[ondarvr]

Outboards aren't like car and truck motors, while not necessarily great for the motor to run at peak RPMs for long periods of time, they are designed to be run at those RPMs. They have rev limiters to keep them from going boom. The Peak RPMs under load are where prop needs are determined.

 

[flyingscott]

Why not run the engine wide open babying them is probably worse and mercury recommends 4750-5250 for wide open operation. There is no reason to take it higher because that is not a motor that benefits from running more RPM.

 

[ondarvr]

The top of the RPM range is where it should be. This will also make the transition from 2 to 4 cylinders less noticeable.

 

[Old Ironmaker]

I run 4800 RPM when wide out, on calm seas with the boat loaded with 400 plus pounds of humanity and tackle plus the 15 horse kicker and 4 batteries etc. I will get to around 33 MPH, solo faster. With a few Lake Erie Walleye each that would be about another 60 pounds!! This 115 is a 95' so if my math is right it's 21 years old. Other than this issue I have had zippo problems with it. If I look at the ignition switch it starts. The previous owner ran 4800 RPM and I follow suit.

 

[flyingscott]

your motor is a youngster. My youngest is 1988 the oldest 1948 they all run good and they are used like boat motors.

 

[quicktach]

Is your advance sticking at a certain point in travel?

 

[Old Ironmaker]

Is your advance sticking at a certain point in travel?

Not being mechanically inclined please elaborate.

As far as running balls out I just don't do It, no one I fish with does either. A short run here is 5 miles, it doesn't make any sense to me to run at full RPM especially in fuel costs. We are paying $7.00 a gallon at times. It's about $5.75 CDN here right now for premium. At a Marina add $1.50.

 

[flyingscott]

More than likely your timing advance isn't sticking. Wasn't criticizing sometimes they just need to be run out.

 

[ondarvr]

Nobody said you were supposed to run wide open all the time, few people do, just that you aren't going to hurt the motor by running it wide open to find the correct prop. For longest motor life and best performance you should be at or near the upper end of the RPM range when running full throttle, running lower in the range at full throttle can increase carbon deposits. The motor will be lugging and under more stress at your cruise RPM if you don't get it propped correctly.

 

[Old Ironmaker]

I run full out at 4800 RPM as I said before. And will run full throttle once and a while to clean carbon deposits. I think 4800 is adequate, it has been. I never run at say 3000 RPM unless I have to if I got caught in very rough water and my back won't take pounding. That's about the only time I would do that.

Thanks again. Off to Florida in the morning.

 

[Texasmark]

I guess this is a dead head answer as Old iron is probably off to Florida, but iterating or reiterating the point which ever, you have to set up your gear train (prop included) at the WOT rpms then it really doesn't matter where you run it unless you sit for extended periods where you are half on and half off plane. That's a lot of work for the engine. Need to get on the water or back off. My definition of getting on the water (on plane) is when the fall back water, coming across the transom, inrushing from the sides, full of bubbles and turbulence, smoothes out and ceases to touch it. That's also the time that the bow usually starts coming back down.