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1996 Mercury 115 surging at higher RPM's

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Mygreenihc

New member
Hello guys,

I typically do not post a question until I have done everything that I can think of to fix a problem. I have worked with this issue for quite some time and do not know what else to look for. I hope someone here has seen this before or has a suggestion that will help resolve my issue. First off, this is a 1996 Mercury 115 ELPTO. It is an inline four cylinder and has the two carbs with no idle jets and two with idle jets. Here is what I am seeing and what I have done so far to resolve it.

What I am seeing:
At idle, the engine runs fine. Nice and smooth for as long as I want to idle. No idle problems that I can see. When taking off, this thing comes out of the hole great, plenty of power, and shoots off like a rocket. I can consistently out run my Dad's 150 Yamaha and brother 250 Pro XS for the first 200 yards. (you have to brag about whatever you can when you run a 115) My problem shows up around 4200 rpm's or so and up. If I hold the throttle at about 3/4 open, it will pick up and drop off randomly by 3 to 400 rpm's. At times, I can slowly work it up to 4800, but at other times I cannot get it above 4600 and in all cases, it will not stay there more than a few seconds. I have, on very few occasions, be able to get it close to 5k, but then it drops right back down. Sometimes it eases back up slow and others it picks the nose of the boat up and shoots off, just for a few seconds. I have noticed too that if I back off of the throttle a little, it will start working its way back up, but if I push it to WOT, it goes flat and will not gain any more speed, but quits surging.

Here is what I have done to resolve the issue: (Note, all of the parts replaced were with new OEM parts except for Stator, trigger, and switchbox, which were new CDI replacements.)

Compression check shows all four cylinders between 126 and 128.
Tested spark using spark gauge (connects to end of wire and grounds to block)
Pulled my fuel tank and clean/dried it before reinstalling with fresh 91 octane fuel.
Replaced tank vent with new vent and hose.
Replaced fuel line from tank to engine with new bulb.
Replaced all fuel lines and filters on engine with new modern fuel line.
Rebuilt fuel pump with OEM kit.
Rebuilt all four carbs with new OEM kits.
Replaced Stator, trigger, wiring assembly, switchbox, coils, plugs and wires.
Tediously checked throttle linkage and checked that all butterflies are opening completely and together. There is no play in any of the butterflies.
Not related, but this engine has a new control box and cable from the cockpit, and a new solenoid and starter.

There are only two things that I can think of that I did not do, one is when I rebuilt the carbs, I did not run a drill bit through the jets. I did not see any buildup anywhere in the carbs or in the jet passageway and I did not have an .062 and .064 drill bit on hand. The second thing is that I did not remove the pickup tube from the tank when I cleaned it. Another thing to note is that this engine has never had any ethanol in it that I know of and I have owned it since it was new, always pure gasoline.
If anyone can help, I will be very appreciative.

Thanks,
Brad
 
Try running it on a separate tank and see if it still happens. Also have someone pump bulb when this happens to see if iits a pressure problem. It sounds like it has a carb/carbs running lean from: 1)debris restricting inlet needle 2) float level off or hanging 3) restricted fuel flow not keeping carbs supplied at WOT.
 
Agree w/faztbullet...you may/should revisit the carb rebuild. Soaking the carb bodies in lacquer thinner several hours or overnight will dissolve any hardened fuel varnish in the circuits. Use a clean paint can you can get from a paint store to safely seal the vapors. Did you inspect the reeds? I found a chunk of fiberglass holding one open last year.
 
Thanks guys for the response.

I actually plan to test the external tank thing tomorrow. I will post up what results I see.

My first thought was that the carbs were not being supplied with enough fuel at higher RPM's, and it still could be the case, but it was doing the same thing before and after the carb rebuilds. I replaced the floats as well when I rebuilt them and I measured the float height with a dial caliper and I believe them all to be good. I also bought a new gallon can of Berryman's to soak them in. I soaked each one after dis-assembly for about ten minutes. Then I washed them out with carb cleaner and let them dry. It was my first rebuild, so I could have made a mistake, but it was fairly simple and straight forward and I was thorough. Also, I rebuilt them going scrictly by the Mercury manual.

I did not inspect the reed valves at all. That is something that I should have done, but didn't. I will though.

I will let you guys know what I find.

Thanks again,
Brad
 
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I soaked each one after dis-assembly for about ten minutes.
Click to expand...
All that did was remove the surface dirt. An overnight soak would remove any questions to cleanliness. A blow dry w/100 psi compressed air would dislodge any crud that had softened from the soak. Good luck.
 
I think the same way, but I was under the impression that outboard carbs had some sort of coating inside that the carb soak would remove if it was left for more than about 15 minutes. Oh well, I guess that is how we learn. Thanks for the advice.
 
i would AGREE ON THE FUEL PROBLEM ISSUE but i would def check the reeds they can damage a good engine if ur int in selling that switch box off ur motor u replaced i need one im in north fla jax area my home num is 352 278 0955 if no answer leave message thanks darrell im fixing a ranger bass boat for my daughter and husband to starttournament fishing with me
la
 
OK, so I tear back into the carbs and do it again. I have two quick questions:

1: Can I soak these thing in carb dip overnight? That is the way we always did it on Holley's and Quadrajets. We'd soak them over night and rebuild them the next day. I was told that I could not do that with outboard carbs. Anybody got any input on this?

2. I rebuilt these two months ago with new OEM kits, do you think I need to buy new kits or will the gaskets be reuseable in a case like this?

Thanks guys for the input.
Brad
 
It's ok to soak hard metal parts...won't hurt them. Follow the directions for the carb cleaning solution. Blow dry w/100 psi comp. air. I like to give each idle circuit a final spray of canned carb cleaner and blow it dry. The final spray allows you to see how fuel will pass through the circuits. Reuse the gaskets; if they leak get new ones.
 
guyjg said:
I like to give each idle circuit a final spray of canned carb cleaner and blow it dry. The final spray allows you to see how fuel will pass through the circuits.
Click to expand...

I will tear them back down again and see what I can find. The quote above is a good idea. I have searched long and hard trying to find a diagram or something similar showing the passages that a carb contains. I do not know what hole the spray should shoot out of when squirted into an different hole. That sort of information would be nice, but is apparently not available.

Thanks for the help guyjg,
Brad
 
b.hobbs45 said:
Did you ever find out what was wrong? I'm having the exact same problem. Same motor
Click to expand...

b.hobbs45,

After going though the carbs again, there was not much difference. I took it to a very respected Mercury mechanic and he rebuilt everything except the carbs and it changed, but actually made the problem more pronounced. It made me realize that the issue is actually fuel related. He has the boat now going through the carbs again and I should know more in about a week from now.

Brad
 
Does your boat have a water separator/ strainer BEFORE the fuel pump? If so, that can cause all kinds of trouble unless the fuel is always clean and the filter in perfect shape. The vacuum fuel pumps on OBs are very weak, so any restriction before them can cause trouble like what you are seeing.

Jeff
 
Mygreenihc said:
b.hobbs45,

After going though the carbs again, there was not much difference. I took it to a very respected Mercury mechanic and he rebuilt everything except the carbs and it changed, but actually made the problem more pronounced. It made me realize that the issue is actually fuel related. He has the boat now going through the carbs again and I should know more in about a week from now.

Brad
Click to expand...

Sounds like we have the exact same problem you described my motors simptoms exactly. I lose and gain rpm's very rapidly and intermittent.
I did run it last week and at WOT sprayed some fuel in each carb and it just bogged the motor so I'm still exploring the electrical end of it though I have already replaced the whole ignition system except for the trigger and wiring harness.
Right after this all started I found my voltage regulator was fried but I'm not sure if its just a quincidence or not.
Keep in touch Brad I'll let you know if I find out anything and you do the same.
Brandon
 
fastjeff said:
Does your boat have a water separator/ strainer BEFORE the fuel pump? If so, that can cause all kinds of trouble unless the fuel is always clean and the filter in perfect shape. The vacuum fuel pumps on OBs are very weak, so any restriction before them can cause trouble like what you are seeing.

Jeff
Click to expand...

I don't have a water separator filter system on mine
 
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