Fluctuating RPM on 96 Mariner 115

[pinellas50]

I just replaced the fuel/water seperator filter and changed out the fuel line quick disconnect. This was due to the motor bogging at cruising RPM. I took it out for a test drive today. It seemed to run fine, almost, with none of the bogging it was doing before I worked on it. But..........

I noticed that when I held the RPM at a constant, the motor would for some unknown reason drop down about 200 RPM. If I hit the throttle a little more the motor would get louder and jump about 200 RPM more than where I started. So if I was at 4500, it would drop to 4300, then at the touch of the throttle it would jump straight up to 4700. If I just kept the throttle where it was at that 200 rpm drop would sometimes come back up to the RPM I was originally at. There was always a change in tone from the engine when it did this as if the throttle really did back itself off.

I'm guessing it is the carbs messing around. I don't know if this means I should expect a rebuild in the near future or if it might work itself out. Any ideas?

 

[starcraft1982]

Re: Fluctuating RPM on 96 Mariner 115

Were you able to get up to WOT?guessing around 5500rpms WOT.mAny other acceleration issues?hesitation when throttling up from idle?If it is the carbs it is a very bad idea to wait for it to "work itself out".And for the beep,beep,beep.That is your low oil alarm.It could have a bad connection,could be faulty or could be indicating a problem.Investigate at once.rest is JMO.I would search on unhooking your oil injection and just run premix.Just a better option IMO.

 

[pinellas50]

Re: Fluctuating RPM on 96 Mariner 115

There were absolutely no other issues what so ever. Idle is fine. Slow speed is fine. It transitions to 4cyl fine. It jumps right up on plane and will run fine up at WOT. It's just at cruise around 4200 to 4500 it was doing this slight bog I guess you could call it.

There aren't any warning alarms involved here. Both work but nothing is sounding. I have a water pressure guage and water pressure is fine through the motor.

Eveything I know tells me this is a fuel delivery issue. But I'm new to 2 strokes and multiple carbs..

 

[CharlieB]

Re: Fluctuating RPM on 96 Mariner 115

Some controls no longer have an adjustable throttle tensioner, it may be possible that your control box 'slips' that little bit.

If you keep your hand on the throttle in the one position does the RPM remain steady?

If it was a motor problem your WOT RPM would be affected.

 

[pinellas50]

Re: Fluctuating RPM on 96 Mariner 115

I know my control box will slowly drop the throttle back if you don't keep your hand on it so I was keeping my hand on it so it wouldn't do that. In other words, the throttle position wasn't moving(at least not by me or the throttle control) when this was occuring.

 

[pinellas50]

Re: Fluctuating RPM on 96 Mariner 115

I had the boat out this weekend. The problems still aren't fixed.

At cruise, I noticed the motor would miss from time to time. I turned it off once to fish a channel and had a heck of a time getting it to start back up.

I got it home to flush it out. It was running OK on the muffs but then the idle started to jump up similar to when you pull the fuel line to run the carbs dry. It continued to run between high idle and trying to stall with a couple of back fires out of the exhaust for good measure. At one point it was doing this and I pumped the primer ball and it went back down to its normal idle speed but after a few seconds started repeating the above.

I then noticed that fuel was leaking from the new female quick disconnect fuel fitting I just put on. Any pump of the primer ball would lead to leaking fuel.

So I don't know. A defective fitting out of the box? I haven't read anything saying the motor side or male fitting tends to wear out and cause something like this. But I am of the impression that even this small of a fuel leak will cause problems like I have.

Opinions?

 

[CharlieB]

Re: Fluctuating RPM on 96 Mariner 115

Many prefer to use Mercury quick connect fittings rather than aftermarket.

Some aftermarket have sealing problems which leak, allowing the fuel pump to suck air into the system which can cause the upper carbs to go low on fuel (LEAN). At idle this will raise idle speed, at WOT it can MELT a piston.

Many do away with the quick connect fittings and install a permanent fuel hose with NO disconnect.

 

[starcraft1982]

Re: Fluctuating RPM on 96 Mariner 115

One,dont continually run your motor until you fix the problem.If its not getting fuel,its not getting oil(boom!).Also you mentioned that its similar to pulling the lines to run the carbs dry.DO NOT DO THIS!Same reason as running lean!By the sounds of things youre doing,really should do a compression test to see if any damage has been done.

 

[pinellas50]

Re: Fluctuating RPM on 96 Mariner 115

I still haven't figured this bug out. Since my last post I have done a compression test, replaced the spark plugs, replaced all fuel filters and fuel/water seperator filter, replaed both sides of the fuel line disconnect fittings at the motor with Mercury parts, rebuilt the fuel pump, and rebuilt the carbs. I had a mechanic do the link and sync after the carb rebuild and he looked things over while he was under the cowl.

When I was telling him these symptoms he immediately said it sounded like something electrical. That was my very first initial thought as well.

The one thing the mechanic did recommend was when the fluctuation occurrs to hit the enricher{choke} to hit the motor with more fuel to see if it cleared up. If it did than I might still be chasing something fuel related. Well I did this and all it did was cause the motor to go rich and start missing for a split second.

So at this point I am concentrating on the electric side of the house. I talked with the mechanic about him putting my boat in his test tank and doing the diagnostics on it. Well, my hull style prevents that from happening so I would have to pay him to trailer the boat to a lake and then do diagnostics as he runs up and down the lake. I think I can throw parts at it cheaper than that option would cost me.

Anyway, my next queston refers to the switchbox.

I can clearly see the timing advance linkage set up on the motor and how it advances as the throttle increases. The timing is at full advance relatively early in the full swing of the throttle, in my opinion. Does the switchbox have any ability to effect timing advance/retard independently from what the mechanical linkage is doing? In other words, it the linkage puts the timing to full at say 2K rpm, does the switch box continue to add timing as the rpms increase?

 

[pinellas50]

Re: Fluctuating RPM on 96 Mariner 115

No one knows?

 

[CharlieB]

Re: Fluctuating RPM on 96 Mariner 115

Generally, the timing is fixed by the timing lever advancing the trigger.

HOWEVER, switchboxes with a BIAS can advance timing by grounding the Bias. This was used with the now discontinued Idle stabiliser which could assist a low idle speed by grounding/advancing timing thereby increasing idle speed. Failure of this module led to high RPM distruction of motors, hence, the idle stabilizer was discontinued.

Some of the switchboxes are said to increas timing once RPM exceeds 6K

Otherwise, timing is not normally set by a switchbox.

Have you tested the fuel system for air leaks, sucking into the system?

Temporarily install a clear hose between the fuel pump and carbs, observe for 'bubbles' indicating air sucking into the line anywhere between the tank and pump.

 

[fyrdriver]

Re: Fluctuating RPM on 96 Mariner 115

Dont know if you remember but you tried to help with my initial motor diagnosis. I have a 96 115 with what sounds like the same problem. Low speed, transition, wot all good. Just 3-4000 miss/bog/rpm drop of 100 rpm. Just got off the phone with my mech who said nothing wrong with the fuel system, originally thought it was a leaking accelerator valve. They sea trialed it, too. His recommendation was to run it and when something breaks then fix it. Not exactly inspiring! He sounds like your mechanic in the sense that it would be cheaper to fix if and when it breaks than to spend tons of labor hours and throwing parts at it. His only guess was a carb might be starting to leak a little fuel into the cyl but found no obvious leak when testing. Its not really related but they found a bad regulator. Supposedly the electrical and charging systems were good. Dont know if this helps at all but your not alone...

 

[pinellas50]

Re: Fluctuating RPM on 96 Mariner 115

Charlie, thanks for the education on switch boxes. You know, I replaced everything on the fuel system above the deck and the water/fuel seperator filter below. Deductive reasoning told me that rules out fuel. But now that you mention it I realize deductive reasoning can also be called assumption. So I'll give the clear line trick a go when I have the chance to mess with it again.

fyr, I had to go back and reread what you were dealing with. I remember now. I just reread your post and noticed mention of the voltage regulator.

The voltage regulator did cross my mind with mine. I replaced it when I first purchased the boat as it was fried. Since then the amp guage always shows charging while underway and the tach has always been working so I didn't think about it any further. My understanding was a bad regulator could effect the ignition system but that would show as an ignition miss, not what I have. Am I wrong on that?

fyr, my thought process has drifted towards the exact same conclusion your mechanic told you. As long as I can ensure the fuel system is up to snuff and the power head won't melt down, sooner or later the ghost will show itself if you keep running it. That's not really the way I want to go but I'm running out of ideas at this point.

Mabey I'll try to find the one thread I found a long time ago about how to make your own DBA meter.

Thanks for the responses!

 

[fyrdriver]

Re: Fluctuating RPM on 96 Mariner 115

Yeah, I just mentioned the regulator, it did not fix my problem, it was just affecting my tach and volts(not charging).
I would feel a little better if I were in your shoes because you went thru your whole fuel system. That was kinda my concern, running it lean and blowing it up. But my guy assured me the fuel system was OK. Haha

I'll make a deal with you, the first one to break lets the other know what the problem was...

 

[pinellas50]

Re: Fluctuating RPM on 96 Mariner 115

I'll make a deal with you, the first one to break lets the other know what the problem was...

Deal!

 

[CharlieB]

Re: Fluctuating RPM on 96 Mariner 115

Another thought, at the problem speed/RPM is your boat 'porpoising', raising and falling in the water, possibly causing the load on the engine to vary just enough to see the 200 change because of the change in load?

 

[pinellas50]

Re: Fluctuating RPM on 96 Mariner 115

No. It's running smooth and flat. If only it were that easy.

 

[pinellas50]

Re: Fluctuating RPM on 96 Mariner 115

Update:

I had the boat out this weekend. I had a full load(6 on board), coolers, and everything else involved with a ride and letting the kids run around on one of the local sand bars. It was about a 1/2 hour ride between slow speed and normal running each way. At one point I ran it up to 5K RPM, which wasn't quite WOT, to make sure things seemed good up top.

The boat ran almost flawlessly. It had a slight bog at first when I hit the throttle. I'm going to richen the idle circuit a tad as the link and sync I had done did not include a water test. So I was expecting a slight adjustment there. Though it didn't bog during my last test and tune run and that bog cleared up after the motor ran a little bit this time. Everything else about the motor's performance was great.

That RPM fluctuation thing I have been talking about was not present during this trip. My only guess on that is the fact I had a full load so the motor had more resistance to push. I only noticed the fluctuation thing when I had the boat out by myself.

This is all probably useless information but mabey it will help someone else out in the future.

 

[fyrdriver]

Re: Fluctuating RPM on 96 Mariner 115

Hopefully you worked the kinks out of it... Mines still running, still intermittantly dropping rpms, but I too have had trips where it runs fine then all of a sudden it comes back the next time I use it. (Sorry-not trying to discourage) Did you do anything in the meantime that you think may have been the cause/remedy? BTW mine only does it in the range of around 3-4000 rpms. At WOT it never does it, also if I'm anywhere outside that range the same applies.

 

[pinellas50]

Re: Fluctuating RPM on 96 Mariner 115

Yeah, I hoping everything is good too. Time will tell.

My motor also only did the fluctuation thing between 4200 and about 4500. Not outside that range just like you are experiencing.

The only thing I did since taking it for a test run after the mechanic did the link and sync was park it on the side of the house and let it sit. My time has been getting eaten up lately and the boat isn't used nearly as much as I would like.

For now I think I am just going to take your mechanics advice and run it. I'll still keep up with our deal and let you know if anything lets go that would explain what we are dealing with.