1986 Merc 115 inline 6 losing power

[mpl006]

Sorry for the long post. I tried to put in all the information I have so that maybe we can come up with something to try easier.

I have a merc 115 inline 6 that I bought with my boat. I have taken the boat out a few times and have had a few issues from just getting it started to keeping it running. Most of that was fixed when I realized the fast idle lever wasn't working correctly and got that fixed. I took off the top carb and replaced all of the gaskets but couldn't get the main jet out because whoever had worked on it before screwed up the jet and I didn't want to mess it up even more. Fast forward a few months and I replaced the gaskets in the other two carbs and everything was running good. Took it out about three weeks ago and ran fine.

Now's where the problems start. This past Tuesday, a friend of mine and I went to the lake and launched the boat, got it started after a little bit of cranking and choking. It was cold, I understood what that was from. When he got in the boat, we idled under a bridge that is right by the launch and when we got clear, I gave it more throttle. When I did, it bogged down. "It's still cold" I thought, so I backed off and let it run a little bit longer. It had done this to me before when I was first getting it to run right and so I would kind of surge the throttle and it finally took it. As it was getting on plane, the power dropped tremendously. There was no noises or anything sounding or feeling unusual. I backed off and started again and it would keep doing it. Then it got to where it would die after getting on plane or if I tried to go much faster than idle. Went on fishing for the rest of the morning and nothing changed. On the way home I remembered that between the last time I had taken the boat out and this time, I had tried to tweak the idle air mixture screws on the carbs and so I thought that was it. When I got home, I went back to the one and a quarter turn from set that is recommended in the service manuel for a starting point.

Today I went back out to see if that fixed it. Well since I'm posting on here, you can probably guess that it didn't. Here is what I have noticed and tried. I have recently changed the tach, gas gauge, and speedo. I noticed that the last time I went out when it ran good, the tach and gas gauge where rock steady. Both on tuesday and today, both gauges where bouncing all over the place. In fact, the gas gauge was jumping to past full and at the same time the tach would drop to 0. I tried to squeeze the bulb to see if it was loosing gas and that kind of worked but not for long. Today I changed four of the six spark plugs on the lake to see if that helped and it didn't. Checked to battery wire connections at the battery and they were snug.

When I got home I got out my DMM and checked the battery removed from the boat and it showed a little over 13 V. Hooked it back up and started the motor and it stayed around the same as when it was disconnected. I put the boat in gear and got it up to about 2000 RPM, it would work fine on the lake up to about 1800 and would start crapping out if I had it hire than that for very long, and the gauges started going crazy and the voltage at the battery was bouncing all over the place. I then disconnected and checked the rectifier and it checked good.

I am at a lose for what to try next. Any help you guys could give would be great.

 

[CharlieB]

Re: 1986 Merc 115 inline 6 losing power

You may have two separate issues, one, the charging or a wiring issue, and two, a fuel issue.

A poor wiring contact, anywhere, may seem fine when tested on the hose and on a solid trailer, then fail once on the water. You need to double check ALL connections, from the battery, thru the motor, thru the wiring harness, to the gauges.

Once all connections are solid, take a meter along the next time the boat goes out and test voltage at the battery, then again at the gauges, they should be very nearly identical.

Your idle mixture is still too lean. 1 and 1/2 turn out from lightly seated it the starting point, this should be enough fuel to get the motor started, warmed, and into gear. From there idle mixture has to be adjusted to 'Best Idle speed' IN Forward gear IN the water. In gear to provide the proper engine load, and IN the water to provide the proper exhaust back-pressure.

Adjust each carb, one at a time, S L O W L Y to give the motor time to respond to each adjustment, and to burn off any excess fuel. Adjust to this 'Best Idle Speed' for each carb, finish one before going on to the next. Once finished return to the first and verify the setting.

Once all carbs are at 'Best Idle' even more fuel is needed for accelerating the motor, two strokes do not have an accelerator pump for this, it has to be adjusted into the idle mixture.

You may end up almost 1/8 turn richer so that is an easy place to start, turn each idle screw out 1/8 turn. Now test acceleration from idle in gear rapidly open the throttle Wide Open. ANY hesitation, lean cough, bog, or miss, is an indication the motor wants MORE fuel. turn each screw out just the width of the screwdriver blade slot and repeat acceleration test.

Repeat to perfection.

If the first 1/8 turn out worked fine, you could be too rich, turn each screw IN just the width of the screwdriver blade slot and test acceleration until you 'just' find performance begins to fall off, then open each screw back to the last 'perfect' setting, that blade slot width can make a surprising difference.

 

[mpl006]

Re: 1986 Merc 115 inline 6 losing power

Thanks for the reply. I will give that a shot. The part I don't understand is that the last time I took the boat out, before this started messing up, I had the carbs in the same spot that I have them now and when I went out Tuesday, they were actually richer. :noidea:

I have another question about testing the carbs. I understand what you are saying about being in the water as opposed to the ear muffs so my question is if I have a tank that I can put the motor in, would that be sufficient? I understand about the trailer being solid so if it were a wiring issue it may not present itself, but for the rest of the tests would it be passible? I just hate driving 30-45 mins to get to the lake to test something.

I don't know if I mentioned this in my first post but this is my first boat and I am VERY green when it comes to outboards but do enjoy learning. So if I say something that sounds off, let me know. I can take constructive criticism quite well.

 

[Georgesalmon]

Re: 1986 Merc 115 inline 6 losing power

The info from Charlie is absolutely correct for how to set the carbs for idle and acceleration. For what its worth I tried to set my '79 115 in a 55 gal drum. It didn't work so good, when I got to the lake I had to do all the setting over again to get it right. Don't know why, just didn't work well doing in the barrel. Also sounds to me like a loose or corroded wire somewhere causing your other problems. Both the tach and the fuel gauge are sensitive to ground connections so pay special attention to them. JMHO

 

[CharlieB]

Re: 1986 Merc 115 inline 6 losing power

The only problem using a barrel is exhaust gas recircing, causing the fuel mixture NOT to be correct.

Remember that water level needs to be the SAME as with the boat in the water.

If the water level is correct and maybe a fan located so to blow the exhaust gases away, the barrel may work fine.

 

[mpl006]

Re: 1986 Merc 115 inline 6 losing power

I figured there was a reason, I just didn't know what it was.

Update on the boat. I guess I'm getting selective memory at the ripe ole age of 28. For some reason I remembered reading in the manuel that it said to start at 1 and 1/4 out from lightly seated. When I took it out the last time I am sure that I had taken it out the one and 1/2 turns and left it there. When I got home, I started over and remembered the 1 and 1/4 so I really don't know where I ended up. After taking it out and it not working, I again reset it to 1 and 1/4. Well yesterday I redid it to 1 and 1/2 and my wife and I brought it out to the closest lake and gave it a go. Again, it had the same problems. I had high hopes because it was idling rough after taking it 1 and 1/2 out and when it was running good, it idled rough. Well after it not working, I had my wife steer and just put it in forward and started adjusting the screws and got the top carb to start missing and brought it back to running smooth then moved to the middle carb and started adjusting it and it started to die so I turned it back but it died anyway after a few attempts to start it back, it wouldn't stay running in just forward at a low speed. The creek we were in is full of stumps and we had not been on it since the water is up as high as it is now so we turned around and went back to the landing.

As much as I really wanted to try to fix this on my own, but I know when I'm in over my head. I already had visions of dumping the motor in the lake and telling my wife it fell off. :lol: So in order to keep my marriage alive and my sanity, I brought it to a local shop with who is supposed to be the best guy for about 500 miles to work on these motors. That recommendation actually came from another shop so I tend to believe it. Anyway, they are only charging me $75 to go through it and troubleshoot it. Depending on what they say, I'll be back on here to see if you guys agree and to see if it may be something I can do to save some money.

I really want to thank you guys for the suggestions. I really hurts the ole pride to not be able to figure it out and be able to fix it. Of course it's still not over yet, so maybe I can still be half satisfied.

 

[mercurymang]

Re: 1986 Merc 115 inline 6 losing power

Good luck. I think $75 is a pretty good deal for you. Be sure to let us know what you find out.

 

[CharlieB]

Re: 1986 Merc 115 inline 6 losing power

It is always possible to have an air leak, sucking into the fuel system, displacing fuel and the carbs start running low, until the motor dies.

The easiest test for this is to replace the hose between the fuel pump and carbs with a clear hose.

Run the motor and see if any air bubbles are passing thru, indicating air sucking in somewhere before the fuel pump.

Usual culprits are poor O-ring seal at one of the quick disconnect fittings, loose hose clamp, old cracking brittle hoses.

Just remember to remove the clear hose once finished testing as clear hose is NOT rated or approved for fuel use, only a short time for testing.

 

[mpl006]

Re: 1986 Merc 115 inline 6 losing power

Time for a little update. I talked to the shop on Thursday and they said that they had just finished diagnosing the problem and had it fixed. Alright I thought. Asked what it was and he told me that their tech and checked the ignition system and that checked out ok, then he checked the carbs and they checked ok. Did a link and sync and took it out on the river and it lost power on him just like it had me. He brought it back to the shop and found that it was only firing on 5 cylinders and that the number six plug was fouled. Figures I thought, the one I couldn't get to to change and that is the one that was bad. So around $100 later, I go to pick it up on Friday at around 3. Drove 45-50 mins to the nearest lake that I knew a little bit about, launched the boat, parked the truck, got to open water, hammered on it, got on plane, alright I'm home..... lost power again. Tried it a few more times and it keep doing it. Put it back on the trailer and went back to the shop and let them know it wasn't fixed. Talked to the tech and let him know a little bit more than he was apparently told about what was happening with it and he said he was able to get it on plane but backed off after he did because he didn't realize that it was taking a while to lose power just thought that it couldn't get up to begin with. He told me he would bring it back in the shop and start looking at it right then. They were open for another hour and a half and were open until 12 on Saturday. He said to call him Saturday and he would let me know what he found.

He called me a few minutes before 10, when I was going to call and asked me if I had rebuilt the carbs and I told him yes and he told me that I didn't epoxy a screw back on that was supposed to and I was losing fuel from the carbs because of it and that the top carb had a lot of trash in it and that it was losing fuel and that was why it was dying. He said that I needed to redo the carbs and make sure to epoxy the screws in and change the fuel lines in the engine and clean out the fuel tank. He had changed the fuel filter for me and changed the boat side quick disconnect because it was leaking air at the connection. He told me that it would be about $300 for him to do it but if I had already done the carbs then it wouldn't be anything that I couldn't do. Not wanting to pay that much for something that I can do, I told him I would come get it and he was fine with that. When I picked it up, he walked me through what all he found and what needed to be changed. He had cut open the old fuel filter and found some dirt in it and showed me the screws that should be epoxied. These screws had not been sealed when I took the carbs down and I'm assuming was the cause of them leaking in the first place.

So I started working on it today. I pumped the gas out of the tank and passed it through a filter back to my gas cans and then the last bit was put in a 5 gallon bucket, I had more gas than I realized in there. I was able to get the tank out and washed the outside and pressure washed the inside. I did find dirt and a few rocks even in the tank when I took it out. I plan on replacing all the fuel lines as they are original to the boat and I will just feel better doing it. I noticed while I was taking the tank out that there was a little bit of gas in the vent line and that the gasket behind the vent was broken so I am going to replace the vent as well.

Questions:
1. Does this sound like it could actually be my problem?
2. Should the vent line have had fuel in it and if not, should I replace it too?

Sorry again for the long post and thanks again for taking the time to read and offer support. Hopefully one day I will be able to repay the favor to the forum.

 

[CharlieB]

Re: 1986 Merc 115 inline 6 losing power

Check my post #8

Next time out, take along any kind of inductive tool, tach, timing light spark tester. As Soon as you begin to lose power, cut back to an idle and using an inductive tester check EACH spark plug wire, each should register.

It is possible that one coil (suspect #6) may be heating and failing under large load.

PROVE THE FAULT.

It may be possible to heat the coil with a blow drier and do a spark test on the trailer. I much prefer a running high throttle load.

 

[mpl006]

Re: 1986 Merc 115 inline 6 losing power

IT'S ALIVE!! Well mostly. I just got back from taking it out. I took the carbs out and cleaned them out again and replaced all of the fuel lines in the engine with the exception of the line leading from the top carb to the enrichener valve, the line leading from the valve to the tee and the two lines from the tee to the block. I would have changed those as well but couldn't get the lines off of the plastic elbows in the block and didn't want to pry and take a chance at breaking them, plus I haven't had an issue with the enrichener. I also took out the gas tank and cleaned it out and filtered all of the gas out of the tank and back in. All of this was what the repair tech said needed to be done. I went above that and rebuilt the fuel pump just cause it was $11 for the kit and I already had everything else apart so might as well.

Got all of my pieces in on thursday and friday and got it back together friday night. The last time I rebuilt the carbs, I didn't change the little washer that goes above the needle valve seat for two reasons, they wouldn't come out easy so again, I didn't want to force something out that wasn't supposed to come out and the ones that came with the rebuild kits were a whole lot smaller than what appeared to be in the carb. This time they needle valves had the correct size washers so I changed them out. In doing this, I noticed that the number one carb had two of these in it. Not sure if that would cause a problem or not but I just put the new one in.

Yesterday was the moment of truth, starting it on muffs at the house. I pumped the primer bulb and got it just short of being firm, the motor seems to like to flood easy and started cranking, and cranking, and cranking, letting the starter cool down between cranks. Tried fast idle, no change, tried choking it, no change. Took a break and thought, maybe the battery is low so I charged it for a few hours. Went back out and did the same and got the same results. Then I took the fuel line loose from the top of the fuel pump and pumped the bulb and what I can only describe as something resembling mud came out for a few pumps and then clear gas. Ok that's gross. Hooked it back up and pumped and tried again, still nothing. Took the line off of the output of the fuel filter and had my wife pump the bulb so I could catch the gas. Took it a few pumps but had gas coming out of it and it was clean. Hooked it back up and had her crank it and it almost started so I went and pulled up the fast idle and got it to crank. I ran for a few seconds and died. I could get it to start back up, but I would have to pump the bulb again. It would run for a few seconds and die. I thought, the fuel pump isn't working. I went and took it apart and noticed that there wasn't any pressure when I loosened a screw like there had been before and looked inside and that's when I remembered what I thought was odd on friday. The two check valves. In the fuel pump when I took it apart was a rubber gasket and a plastic washer maybe 1/4 the size of the gasket. With the kit, the two were the same size. Not knowing at the time what they were, I just changed them with the new ones. Later I looked up how those pumps worked and figured out they were the check valves. Well with the plastic as big as the gasket, it wouldn't be able to open the correct way and let any gas into the middle chamber and out to the carbs. So I changed them back to the old plastic pieces and noticed that there was some more of this "dirt" that was in the old fuel filter and that I had cleaned out of the pump when I took it down the first time. I call it "dirt" because it kind of looks like it but has a little bit more of a gold or copper color to it. Anyway, put it back on the boat, pumped the bulb and she fired up. I let it run for a little while on the hose and it ran the whole time.

Today, after it stopped raining, the wife and I got in the boat and took the 45 min drive to the nearest lake that isn't infested with stumps and lucky for me, we were the only ones at the launch. I backed the boat most of the way down and climbed in and had her bring me down the rest of the way and pumped the bulb and tried and tried to get it to crank and it wouldn't do it for anything. Dang, I flooded her. Pulled it up away from the launch and cleaned off the five spark plugs I could get to and let it sit for a few mins and backed down to try again. It took a few times but it finally kicked off. Alright, here goes nothing. I backed the boat off the trailer and the wife went and parked the truck and I picked her up and here we go. We get out of the no wake zone and I let her have it. She got up on plane, ok, I've been here before, and she stayed up. Woohoo!!!! She was running. The lake was kind of choppy and I don't have a whole lot of experience so I didn't go too crazy with it but she was going. So I go fish for a little while and we are heading back and she is still running but this time, it took a little while to get up on plane but she stayed up afterwards.

Now for my next questions. When I was heading back in, it kind of felt like it was loosing a little bit of gas every now and then and was running around 4k at WOT then it would act like it was getting the gas back and jump to like 4500 and then go back down to 4k. I was reading another thread earlier about the fuel line from the tank to the engine and I can't remember who it was was saying that the inside of the hose was supposed to be clear and when it starts to deteriorate, it will turn a yellow color. Mine is all kinds of yellow so because of this and the fact that I changed just about every other hose, I want to change these. My question is, what size should I go with, 5/16" or 3/8"?

Also, I didn't take the time to setup the mixture screws this time out but will this week before I do any more major running. I know that it needs to be in the water and in gear. I'm assuming that it just needs to be barely in forward and not anything more than that so is this something I could do from the trailer if I am lucky enough to be there by myself again or should I be out of the water? I really want to do it right the first time and so I'm trying to make it as stupid proof as possible and if I can do it on the trailer, then I don't have to try to convince the wife to come out with me to steer.

I also read somewhere else that I guy was telling people that had these engines to remove the bowl vent. He claimed leaving them in would lean out the mixture at mid range RPM. Is this guy right or should I leave them in?

Thanks again for the help and for reading my novels. LOL

 

[CharlieB]

Re: 1986 Merc 115 inline 6 losing power

If your engine is stock there so not be any need to modify the carbs, just keep them clean.

The 'mud' that came out of your fuel line, I hope none of it got into the carbs again, that would require another dis-assembly and rinsing clean again.

A water separating fuel filter is a great addition to prevent future muck from getting into the carbs.

That yellowed lining in the fuel hose is a good indicator that it is time to change that hose.

 

[sutor623]

Re: 1986 Merc 115 inline 6 losing power

I just worked on this exact same motor. I did the carbs and fuel pump and it didnt really change much. What you should do is download or print out the CDI ignition manual. Since you have a DVA meter you can check for your voltage on all of the required points. The two switchboxes, stator, trigger and rectifier. The issue we had was the motor was running great for 5-6 mins, then it would start acting funny. Give it gas and it would fall on its face. We ended up having a rectifier and stator issue, that only showed itself when the electronics got warm. I also pulled out the idle stabilizer assembly, as it is not needed and causes many issues with this motor.

Now I didn't read your entire thread so I'm not sure that this will help you, but if you have any questions I'd be glad to help.

 

[mpl006]

Re: 1986 Merc 115 inline 6 losing power

Well after changing everything but the tank and cleaning the carbs a few times, I took the boat out today with no problems, except the fish didn't show up. Thanks for all the help guys.

I do have one more question. I went ahead and picked up a water separator filter and need a little help with installing it. I'm assuming the fittings need some sort of sealant on the threads like water pipes do but I'm sure the white teflon tape isn't going to cut it with 2 stroke gas and oil so what is the recommended sealant?

Thanks again.

 

[CharlieB]

Re: 1986 Merc 115 inline 6 losing power

Tape can lose fragments that can cause problems if/when they get into the carb.

Av Perm sealant (liquid) or liquid teflon both work well, it doesn't take a lot, I just use about as much as three drops of water on my finger then roll the male thread in it so there is some down in the bottom between each thread yet not so much as to only fill the gaps about half way, as it is screwed in it will climb the threads.