125 merc missing at higher rpms

[timmytes]

i ahve a 1999 125 merc. when runnung above 2500-3000 rpm's it seems to miss occasionally on one cylinder. it seems to do it when the boat jolts, as when we hit a wave. does this sound like something a carb rebuild would fix, or maybe something different?<br /><br />thanks

 

[WillyBWright]

Re: 125 merc missing at higher rpms

Drain the carbs and see what's in there. I suspect water in the gas. If there is green stuff on any of the drain plugs you'll need to rebuild them.

 

[timmytes]

Re: 125 merc missing at higher rpms

i drained the carb bowls, and also pulled off the fuel/water seperator, and it was loaded with water and dirt. almost all water inside of it. i replaced it and replaced theinline fuel filter and i seemed to run right. is there any damege that could have been done to the motor with all of that water in the lines???<br /><br />thanks

 

[quantumleap]

Re: 125 merc missing at higher rpms

If your filter had that much water in it you'll need to check you tank for dirt/water as that's where it came from. I would clean out the tank well before running it anymore.

 

[timmytes]

Re: 125 merc missing at higher rpms

well i thought i had fixed the problem, but took it out today, and it did it again. what would make it run bad during days when the water is rougher? when you hit a large wave it kind makes a sputtering sound and looses power, then it will pick up again. weird.

 

[WillyBWright]

Re: 125 merc missing at higher rpms

What Quantum leap said. Water settles to the bottom, gas floats. Rough water stirs things up and what was benignly sitting on the bottom got thrown into the mix. I'd advise investing in a water separating fuel filter setup. It looks like a spin-on oil filter, but it's specifically for fuel. Water collects in the bottom while gas floats out the top. Eventually it will fill with water and you'll have to dump it. No biggie. Just screw it back on and reuse it a few times. Eventually the water will work its way out. I'd tell you how to do it quicker, but dealing with gas tanks scares me. All it takes is one static spark and next thing you see is the Pearly Gates.

 

[dilligafda]

Re: 125 merc missing at higher rpms

If the problem happens again, check the main wiring harness plug. Hitting waves and the engine cutting out could be a loose connection there.

 

[SlowBoat5]

Re: 125 merc missing at higher rpms

Cadet, I have the same motor with the same problem except mine will do it on flat water. This is what I have tried, "on Mercury rep's recommendation, replaced the ignition system, $460.00 in parts, no help, but he didn't charge me. Rebuild the carb's and fuel pump. Disconnected the oil injection. Replaced all fuel lines, bulb and drained/cleaned fuel tank. Tried it with a small remote tank. The only known improvement was 6 cans of Deep Creek. Problem is still there but not as bad. If you find the problem please let me know. Thanks.

 

[SingleShot]

Re: 125 merc missing at higher rpms

SlowBoat,<br /><br />When you replaced the ignition parts did you replace the stator as well?? Is is possible to get another fly wheel or have the magnets checked. Does this mis happen right away or after some time. With electronics, temp and humidity can have an effect. But seeing as all of the ignition compenents have been replaced I'm wondering if you may have a weak magnet.<br /><br />On the fuel side, this is the midrange, have you tried to run slightly rich just to see if you may be running slightly lean and observing a lean cough. That is setting the idle mix to a richer setting. At high rpm's the main jet takes effect. Not sure where the transition is from the Idle jet to main jet is.<br /><br />Something to try. I have the same motor and haven't had this problem yet.<br /><br />Good luck<br /><br />JK

 

[timmytes]

Re: 125 merc missing at higher rpms

last weekend, i completely rebuilt the 4 carbs, changed out the plugs, and it is still doing it. the real difference is that mine only does it on real choppy days. in flat water or on small waves it runs fine. i have not yet tried to decarbonize it yet, that is next week. I'm no mechanic, so if this sounds rediculous, don't shoot me, bit is it possible that there could be a short of some kind in the throttle that a severe jolt to the boat would cause it to momentarily loose spark,(the RPM guage also quit working), just a thought.<br /><br />Later

 

[SlowBoat5]

Re: 125 merc missing at higher rpms

Singleshot, Thanks for the reply. The stator was replaced but not sure about the magnets, a local mechanic did the work. The mechanic has a dyno and engine analyzer, it did not show any ignition problem. The miss happens right from the get go. It is more prolific at colder temp and higher altitude. I use the boat at elevations from 3,000' to 9,000' so I have to adjust the idle mixture each time. I will try set a rich setting.<br />There is a later post from a guy with a 1997 115hp with the same problem, maybe we can get this fixedfor all of us.

 

[99tango]

Re: 125 merc missing at higher rpms

You said that it misses sometimes on one cylinder...Have you tried getting it to act up and then pull off a spark plug wire to see if you can determine which one is missing? Try using a timing light and test each spark plug wire one by one when it acts up and see if the light stops flashing on one certain cylinder. If this is the case and it's only one cylinder then check the resistance of the ignition coil and see if it's within specs. I'm sure you'll need a service manual to start with. You can test the ground side of the coil that feeds that bad cylinder and see if it loses ground when it loses spark. Most ignition coils have a hot and a ground wire that feed it and when the boat is running the hot always stays hot and the ignition system will make and break the ground circuit. If you use a test light and plug the aligator clip side on the positive battery terminal then use the probe side of the test light and touch the ground side of the ignition coil. When the boat is running the test light should practically stay on because the ground is being turned off and on so fast that it will look like it is steady on. If it does lose the ground then the light will turn off and the ignition coil is probably ok and you might have a bad switchbox or stator. If the light stays on and it still missed from that cylinder then I would say the ignition coil is bad.

 

[timmytes]

Re: 125 merc missing at higher rpms

finally took the boat to a mercury mechanic. he looked it over, test drove it, and told me that one of the carburators was periodically running rich, which was causing the bog down or miss. they wanted to charge about $600 to rebuild the carbs. after i told them that they were just rebuilt about a month ago, he changed his mind. Is it possible for one of those carbs to run rich intermitenly like he said, or do you guys think that was just a stab in the dark??? the carbs seemed real simple to me. the only thing that you really had to adjust on the rebuild was the float height. <br /><br />tim