Logo
FULL SITE MENU

1997 Mercury 115hp 2+2 how smooth should transition be?

J

jaclark91

Member
I know these engines are supposed to transition from 2 to 4 cylinders at around 1800-2000 rpms. My question is should the engine act like its stuttering/missing for a few seconds during this transition?

I bought a boat with this engine about a year ago and it was poorly adjusted so I did a full link&sync on it according to the manual but I'm having trouble figuring out if the idle jets on the carbs are adjusted right. I took it to the lake and fiddled with the carbs all day today but I couldn't get rid of the stuttering during the transition from 2 to 4 cylinders at around 2000 rpms. I had it running smooth at idle and full throttle but it always wants to stumble for a couple seconds during the transition if I apply the throttle slowly. If I keep it around 2000 rpms it shakes and acts like its missing until I give it more or let off.

This is my first boat and 2 stroke outboard so I don't really have anything to compare it to.
 
I cleaned the carbs and replaced all the gaskets in them before I did the link&sync. I also rebuilt the fuel pump. I thought about rebuilding the accelerator pump but I pushed the button on it down while the boat was idling and it revved up so I figured it was working good.

When I initially got the boat it wouldn't rev over like 3000 rpms or get on plane so that's why I cleaned the carbs and did the fuel pump and stuff but it turned out to be some wires that were burned/shorted between the stator and voltage regulator. I replaced the voltage regulator and repaired the wires and now it will rev all the way up and get on plane so I guess that part is fixed.
 
Reading your post read like I wrote it.........I bought one too last year and I am a bit wiser, not an expert but wiser and happy with my purchase. My first main problem was crud in the fuel lines that wound up in the main filter (cut it open after removal) and limited WOT operation. Not knowing the rig, I went through numerous things, helping but no big change. The filter fixed that.

I have an OEM 2015 revision of the 1996 and newer 75-125 hp 3 and 4 cyl, 2 strokers which is much more detailed than the last one I had when I had a 90 hp of this series. It goes through the setup process pretty well.

I did replace the cyl. 3&4 small filter, located just behind the oil tank on the engine, at the rear of it. Also bought a kit for the accel pump.

My idea of how it works is that if you want 3 and 4 sucking enough fuel to light off, you need to get some rpms built up. Noticing the throttle linkage cam that operates the pump, it "squirts" a volume of fuel contained in it and the adjoining lines as fuel and air are introduced into all the cylinders from the carbs.

If you diddle around in the 1800-2000 rpm range you can play with them coming on line, one hits a time or two then the other, then a little more throttle and they starting hitting more regularly and so on till you get them both (3 and 4) running like 1 and 2). Yes intermittent firing and rougher than normal operation is a characteristic of getting 3 and 4 online in that manner.

On the other hand, if you firewall the throttle, as the cam on the linkage ramps up and presses on the plunger, there is a charge of fuel into 3 and 4 and they come on line rapidly and for the most part smoothly....how smooth depends (on mine) on how much idling around I had done prior to hammering down on it....If I firewall it and leave it, it took (will explain later) a minute or two for 3 and 4 to clean up the plugs to get me all the way up to rated WOT rpms.

My 115 on a 2002 Crestliner 1750 Fish Hawk (you can www that boat and get the sales brochure online for a reference) is more engine than I need so when I am ready to get on plane, I firewall the throttle for a few seconds as the plane out is fast with that hull, getting past the kickin spot rapidly and immediately back off to my preferred cruising speed at 3000-3500 rpms.

I found that 3 and 4 plug fouling is greatly reduced when spending a lot of time at low speeds by simply tilting the engine up in the rear slightly. Not rocket science, just have it level or down in the rear, like for plane out, idling along and it runs a bit rough with visible shaking...not a lot but it's there. Lift the rear and almost immediately it smoothes out like my 6 cyl Tower used to idle.

Other thing I did was to replace the OEM plugs with Iridium and went 2 heat ranges hotter (NGK BPR6HIX NGK stock # 4085) ....reason being ceramics are brown after running rather than grey and I don't have to wait for it to clean up on a WOT run. I have no data on what to expect from that plug if you were spending a Sunday's afternoon skiing or tubing...may be too hot, but for what I do, mostly mid range running, they are perfect.
 
Texasmark said:
Reading your post read like I wrote it.........I bought one too last year and I am a bit wiser, not an expert but wiser and happy with my purchase. My first main problem was crud in the fuel lines that wound up in the main filter (cut it open after removal) and limited WOT operation. Not knowing the rig, I went through numerous things, helping but no big change. The filter fixed that.

I have an OEM 2015 revision of the 1996 and newer 75-125 hp 3 and 4 cyl, 2 strokers which is much more detailed than the last one I had when I had a 90 hp of this series. It goes through the setup process pretty well.

I did replace the cyl. 3&4 small filter, located just behind the oil tank on the engine, at the rear of it. Also bought a kit for the accel pump.

My idea of how it works is that if you want 3 and 4 sucking enough fuel to light off, you need to get some rpms built up. Noticing the throttle linkage cam that operates the pump, it "squirts" a volume of fuel contained in it and the adjoining lines as fuel and air are introduced into all the cylinders from the carbs.

If you diddle around in the 1800-2000 rpm range you can play with them coming on line, one hits a time or two then the other, then a little more throttle and they starting hitting more regularly and so on till you get them both (3 and 4) running like 1 and 2). Yes intermittent firing and rougher than normal operation is a characteristic of getting 3 and 4 online in that manner.

On the other hand, if you firewall the throttle, as the cam on the linkage ramps up and presses on the plunger, there is a charge of fuel into 3 and 4 and they come on line rapidly and for the most part smoothly....how smooth depends (on mine) on how much idling around I had done prior to hammering down on it....If I firewall it and leave it, it took (will explain later) a minute or two for 3 and 4 to clean up the plugs to get me all the way up to rated WOT rpms.

My 115 on a 2002 Crestliner 1750 Fish Hawk (you can www that boat and get the sales brochure online for a reference) is more engine than I need so when I am ready to get on plane, I firewall the throttle for a few seconds as the plane out is fast with that hull, getting past the kickin spot rapidly and immediately back off to my preferred cruising speed at 3000-3500 rpms.

I found that 3 and 4 plug fouling is greatly reduced when spending a lot of time at low speeds by simply tilting the engine up in the rear slightly. Not rocket science, just have it level or down in the rear, like for plane out, idling along and it runs a bit rough with visible shaking...not a lot but it's there. Lift the rear and almost immediately it smoothes out like my 6 cyl Tower used to idle.

Other thing I did was to replace the OEM plugs with Iridium and went 2 heat ranges hotter (NGK BPR6HIX NGK stock # 4085) ....reason being ceramics are brown after running rather than grey and I don't have to wait for it to clean up on a WOT run. I have no data on what to expect from that plug if you were spending a Sunday's afternoon skiing or tubing...may be too hot, but for what I do, mostly mid range running, they are perfect.
Click to expand...

Yes the first thing I did with mine when it wouldnt get to WOT was replace that main fuel filte and check the lines. I'm gonna go ahead and replace that small fuel filter behind the oil tank as well. Looking at the service manual there's 2 injectors/check valves in the block after that small filter for #3 and #4 cylinder which is what the other guy is talking about I'm sure. I'll replace that filter and clean those and rebuild the acceleration pump and see if it'll make the transition any smoother. I saw some other people on google saying the transition is kind of rough too though so I kinda doubt it will make the bottom 2 kick on completely smooth without punching the throttle to get past 2000 rpms.

Where'd you get the 2015 revision of the service manual if you don't mind me asking? I have been using the manual on boatinfo but it is for the jet drive models. I looked for the right one when I first got the boat but I didn't have any luck.

Thanks for the help guys I really appreciate it.
 
Last edited:
ebay has several types at a reasonable price for what you get. I prefer spending the bucks for the hard copy as it makes it easy for me when I need to move around the sections when working a problem and my boat is in my shop and my computer is in my house!
 
Yeah I checked on ebay but all the ones I saw on there were digital copies and didn't say if they are the revisions or not. I figured if I bought one of those it'd just end up being someone reselling the same one that is available on boatinfo for free.
 
jaclark91 said:
Yeah I checked on ebay but all the ones I saw on there were digital copies and didn't say if they are the revisions or not. I figured if I bought one of those it'd just end up being someone reselling the same one that is available on boatinfo for free.
Click to expand...

Go back and dig deeper. I just pulled one up a few days ago for another poster and it was a few pages back.
 
Texasmark said:
Go back and dig deeper. I just pulled one up a few days ago for another poster and it was a few pages back.
Click to expand...

Thanks, I ended up getting one of the digital ones for like $10 and it is a bit different from the free one I was using as I realized the one on boatinfo was only for models up to 1993.

One more quick question, does yours beep when you turn the key on? From reading on google it seems some people were saying there is an oil warning alarm that does a test beep when you turn the key on and I think mine is disconnected as mine doesn't beep and I don't see any speaker where it would even beep from on my boat.
 
Alright. By injectors you mean like the jets in the carbs right?
Click to expand...
No...follow the fuel line from accelerator pump around to electrical plate and you will find 2 small injectors that go to #3 and #4 cylinders. These will screw apart and can be cleaned as long as needle tip is not damaged or swollen.
 
jaclark91 said:
One more quick question, does yours beep when you turn the key on?
Click to expand...

No. Looking at the wiring diagram and my engine, I don't have the modulation module. Without the module, there is nothing to cause the beep to occur. Ground is what makes the horn honk (after the ignition key is moved off the OFF position) and unless oil is low or the OT sensor are shorted....closed, there is no path to ground to make the beep.
 
There is no oil module as the oil pump is not monitored only the oil level..only time it should sound is when overheated or oil low.
 
Thanks guys, I unplugged the oil tank sensor and turned the key on and grounded the blue wire with my power probe and didn't hear any sound then I looked under the dash and found the little buzzer thing I think is supposed to sound and hooked it up to a 12v atv battery with some test leads and it didn't make any sound so I guess I need to replace that as well.

I have a 1/4 acre pond in my front yard do yall reckon if I backed my boat into that and tuned the carbs on the trailer after I rebuild the accelerator pump it would be the same as doing it on the lake? My local lake is a no wake during the week and on the weekends theres always busy with a lot of jet skiiers and tubers and its pretty small so awkward to mess with it there on the water
 
Ground the tan/lt blue stripe wire, pin 3 in the 8 pin connector......that's the alarm wire that goes to the low side of the horn. The high side is 12v switched from the ignition switch.

I wouldn't do the pond thing as it is a 2 stroke and burns oil and you will have an oil glaze across your entire pond. Oil settles out at the molecule level. I saw a test where 1 teaspoon of light oil completely covered a pond your size.
 
Thanks for the help guys, I messed with that warning horn some more and got it working. The sensor at the bottom of the oil tank was bad and the warning horn stays on if you turn the key on with it plugged in correctly so I guess someone unplugged some of the wires. I unplugged the oil sensor for now until my new one comes in the mail. My boat actually does do the test beep when the key is turned on.

I happened to put my hand on the starter while I was messing with the boat and noticed it was extremely hot. I took my perko switch off to replace my battery terminal wires and it was actually melted on the back around where one of the battery wires connect to it. I bet that is why my yellow stator wires were melted when I first got the boat as they rest against the starter. I think the old starter had a short inside of it, I replaced the starter and the new one spins much faster than the old one. I will be keeping an eye on the battery wires though to make sure they don't get hot anymore.

I also rebuilt the accelerator pump and cleaned the injectors. The old accelerator pump diaphragm looked fine but the plunger that the cam presses down on on the new one was like 1/8 of an inch longer, not sure if they updated it to be longer or the old one just wore down that much.

I took the boat to the lake today to try to dial in the idle adjustment screws again and never could get rid of that stutter fully when the bottom cylinders kick in, if that's how they all are though then I'll just live with it. I just wanted to make sure it was running the way it's supposed to.
 
Last edited:
I really see no reason to dilly dally around the transition area. My engine is a 115 and I could probably operate this boat to my satisfaction with a 75. The difference would be the 75 putting out all it has and this one loafing.

When I get up, I advance the throttle to probably the ¾ position and the boat immediately comes up, quickly passes through the transition region and in probably 3 seconds, I am cutting the throttle back to my sweet spot....3000-3500 depending on how I feel, RPM. If I want to slow down to a barely on plane attitude, I do the same thing initially, but continue to cut the throttle back to where I want it....but only do it to see what happens....seldom.

Possibly the fact that I run a ported prop (Square hole, original Merc. Laser II) helps me to pass through the transition region faster. Like I posted somewhere recently, my prop slip is 28% in that region (running 20-25 MPH) yet at WOT it's only 8.......47 MPH at 5250 RPM.

Just for grins back when I first bought the boat and was getting it the way I wanted it, I deliberately inched the throttle forward finding the transition RPM band and paying attention to 3 and 4 coming online and the smoothness or lack thereof of that process. Just doesn't make sense to dilly dally there. I never did it with any other boat so why should I do it with this one?
 
Let me ask you this, if the idle adjustment screws were out of wack could that cause the boat to not reach WOT and get up on plane? Reason I ask is because I had the boat running pretty good yesterday and I took it out again today and the engine was stuttering and stuff again and it wouldn't reach WOT or get on plane. I'm thinking there's some sort of electrical issue somewhere but it looks like I need to buy a $100 wiring harness adapter to go between the CDM and stator to effectively test them.

I have another one of these motors that is 125hp that I got from an auction several years ago that is supposedly good so I'm thinking about just swapping the electronics from it on my current motor to try to figure out what is bad by trial and error.
 
Actually, you know what I just took the fuel pump off my other motor and put it on this one and it seems to have a lot more power and much smoother transition in the test tank I made. I will have to test it out at the lake when I get a chance and see how it is. I haven't ever been able to tell my true rpm or if I'm at WOT because my tach doesn't read correctly, I think the screw on the back to set the pulse/number of cylinders is in the wrong position but I can't put it to the correct position because the sticker on the back that had the markings is torn off.
 
The idle screw sets the lower limit of the spark advance control arm. WOT uses the upper limit.

The Mercury service manual pretty much shows commonality between 100 thru 125 engines from 1998 up verified by the manual I have which is a 2015 revision of the same manual published in the mid 90's. Manual covers engines starting in 1994 and between that year and '98 there were differences which seemed to cease with the '98 year model. Carburetors would be the difference if anything (assumption) between HP ratings. Therefore no reason why your 125 can't supply you with parts if the year models are close enough for the parts to be similar, like using individual comprehensive, all inclusive CD modules per cylinder in later models vs a switchbox.
 
Last edited:
Texasmark said:
The idle screw sets the lower limit of the spark advance control arm. WOT uses the upper limit.

The Mercury service manual pretty much shows commonality between 100 thru 125 engines from 1998 up verified by the manual I have which is a 2015 revision of the same manual published in the mid 90's. Manual covers engines starting in 1994 and between that year and '98 there were differences which seemed to cease with the '98 year model. Carburetors would be the difference if anything (assumption) between HP ratings. Therefore no reason why your 125 can't supply you with parts if the year models are close enough for the parts to be similar, like using individual comprehensive, all inclusive CD modules per cylinder in later models vs a switchbox.
Click to expand...

I actually meant to say the idle mixture screws on the top two carbs, not the idle timing screw.

And yes both of my engines have the CDMs rather than the ignition coils and a voltage regulator/rectifier where the switchbox is on the earlier motors. I was pretty confused when I was using the manual I got from boatinfo since it kept telling me to go to the switchbox to test stuff. One of the reasons I bought this boat is because I would have a backup motor if I blew this one up or I needed a new lower unit or something. I thought about taking the carbs off the 125hp and putting them on the 115hp but I decided not to until I got this one running good.
 
jaclark91 said:
I actually meant to say the idle mixture screws on the top two carbs, not the idle timing screw.

And yes both of my engines have the CDMs rather than the ignition coils and a voltage regulator/rectifier where the switchbox is on the earlier motors. I was pretty confused when I was using the manual I got from boatinfo since it kept telling me to go to the switchbox to test stuff. One of the reasons I bought this boat is because I would have a backup motor if I blew this one up or I needed a new lower unit or something. I thought about taking the carbs off the 125hp and putting them on the 115hp but I decided not to until I got this one running good.
Click to expand...

Good idea.

On the carb change, Page 1A-7 and 8 has your carb data. You can always put the old ones back on. Don't know other differences between the engines. Both are still rated at 5350 rpm, probably since they both use the same Over rev limiter module. In checking specks, all 4 cylinder engines are grouped except for carbs. 115 has 6 optional carbs and each cylinder of a specific carb type selected uses a different jet size. 125 engines have 4 options, each with different jets. Seems to get complicated. and carbs could get mixed up easily if you don't mark their position. The 4 mechanics that post on here regularly could probably answer the question as to whether or not you are opening a can of worms!!!!!
 
faztbullet said:
No...follow the fuel line from accelerator pump around to electrical plate and you will find 2 small injectors that go to #3 and #4 cylinders. These will screw apart and can be cleaned as long as needle tip is not damaged or swollen.
Click to expand...
Sweet info. I think my 115 2+2 is also a little week in this transition area also. I think i found the main cause of my week transistion speed problem but cleaning theses injectors seem like a something to do just for peace of mind and rule them out as a problem. My question do these injectors is do they need parts/seals replaced when i take them apart or is it just a take apart clean a reassemble thing?

Jaclark, I think most of my transition speed problem was due to a bad coil which fired intermittently so it was hard to pinpoint weather it was really a problem or not. It wasn't until last time out it became obvious that i had a misfiring cylinder. At cruising speed it started to jump abruptly a couple of hundred RPMS. I knew it was 3 or 4 because it only happen above the 1800rpm kick-in threshold. A quick ohms test on the coils showed #3 was off, and after taking it out i could see the crack. i'm waiting for a replacement coil, we will see.
 
Roland J said:
Sweet info. I think my 115 2+2 is also a little week in this transition area also. I think i found the main cause of my week transistion speed problem but cleaning theses injectors seem like a something to do just for peace of mind and rule them out as a problem. My question do these injectors is do they need parts/seals replaced when i take them apart or is it just a take apart clean a reassemble thing?

Jaclark, I think most of my transition speed problem was due to a bad coil which fired intermittently so it was hard to pinpoint weather it was really a problem or not. It wasn't until last time out it became obvious that i had a misfiring cylinder. At cruising speed it started to jump abruptly a couple of hundred RPMS. I knew it was 3 or 4 because it only happen above the 1800rpm kick-in threshold. A quick ohms test on the coils showed #3 was off, and after taking it out i could see the crack. i'm waiting for a replacement coil, we will see.
Click to expand...

The injectors are very simple. Mine was just a barb fitting that screws into another fitting that screws into the block. When you unscrew them from each other there is a spring inside and a needle like from inside a carb that keeps fuel from entering unless there is enough pressure to push the spring back. I didn't need any parts to clean mine.

I think all my rough transitioning/stuttering problems were stemming from a fuel pump diagphram that lost a lot of its elasticity due to age or maybe ethanol gas. I bought another fuel pump on ebay last year and rebuilt it and put it on there but it didn't seem to help much and then I noticed it leaking after a few trips out, then I read a guy online saying that if you can blow through the fuel pump one way and not the other and not through the pulse hose then it's fine so I thought it was fine and put my original one back on.

I took the pump I rebuilt back apart a few days ago and sprayed all the gaskets with some permatex gasket sealant and put it back together and my boat has a lot more power and much smoother transition in the tank I made so I'm hoping it's fixed. I will take it to the lake in the next couple days when I get a chance and verify.
 
Yeah, i can see a weak fuel pump doing that.

So i took a look under the oil reservoir and i see #3 injector but #4 looks to be buried deep, probable requiring the lower cowling removal. to find it pretty tight area.

also I don't see anywhere the rebuild kit /diaphragm for the accelerator pump. Anybody know the part number for it?
 
Roland J said:
Yeah, i can see a weak fuel pump doing that.

So i took a look under the oil reservoir and i see #3 injector but #4 looks to be buried deep, probable requiring the lower cowling removal. to find it pretty tight area.

also I don't see anywhere the rebuild kit /diaphragm for the accelerator pump. Anybody know the part number for it?
Click to expand...

I got a set of wobble socket extensions from harbor freight for like $10 and I was able to get the #4 injector off with that without removing the cowling. Here's a link to the accelerator pump kit I bought:
https://www.ebay.com/itm/Repair-Kit-Accelerator-Pump-Mercury-80-125HP-1988-2004-811287/283449889732
 
jaclark91 said:
I got a set of wobble socket extensions from harbor freight for like $10 and I was able to get the #4 injector off with that without removing the cowling. Here's a link to the accelerator pump kit I bought:
https://www.ebay.com/itm/Repair-Kit-Accelerator-Pump-Mercury-80-125HP-1988-2004-811287/283449889732
Click to expand...

Excellent, ordered. Thanks for the link and part number. I'll just do the pump (along with the coil) and leave the injectors out of it for now.

I guess different serial number motors may have better access than others. I cant even see #4 from any angle
on my 1995 115. There's a plate that covers that section of the motor that has a lot of electrical attached to it on my model.

Mine also has the crappy oil reservoir with the square lid that wants to leak every time you tilt up the motor but that another subject.
 
Back
Top