Merc 125

[cocodrieboy]

My engine runs well at anything half throttle to full. However, anything below half throttle, there is alot of vibration and it doesn't seem to run as well.<br /><br />Compression tests are fine.<br /><br />Any suggestions?

 

[KCLOST]

Re: Merc 125

There are more than one reasons why this could happen....<br /><br />First we need year and serial number of your motor...<br />Second, good compression is good but do you have good spark to all cylinders....?<br />If you have a tach, what is the WOT rpm that you are getting, also what is the minimum rpm that the engine will run at?<br />Based on your description, I assume the vibration in caused by one or more of the cylinders not firing at lower rpms.....(not to say that it's also happening at higher speeds)<br /><br />Can you better describe what is happening? Does the vibration seam to be ignition related or could it also be a prop issue..(never know)

 

[cocodrieboy]

Re: Merc 125

Thanks, KCLOST.<br /><br />My Merc is a 1995. At WOT, the RPM is at around 5500 (I don't know how accurate the guage is, however).<br /><br />I guess my big concern is that since I know very little about outboards and thus cannot do much (or any) mechanical work myself, what would be the worst case for a engine that is exhibiting these symptons.<br /><br />For example, I have a Yamaha 40 that I could put a glass of water on the engine and it would fall off--not so with this Merc, especially at low idle.<br /><br />Is rough idling common with Mercs?

 

[hondon]

Re: Merc 125

This engine is a loop charged four cylinder.Loopers generally have a little shake and rattle to them but this engine is unique in that Merc found that this engine idled better running on only two cylinders.Your bottom cyls do not kick in to about 1800 rpms and you also have an accelerator pump.Technically refered to as 2 + 4 and has been around awhile.

 

[quantumleap]

Re: Merc 125

Actually referred to as a 2+2. The engine was designed to run on 2 cylinders at low rpm's to make it more versatile. By running on 2 cyl's, it become an effective troller and eliminates the need for a kicker. It would idle smoother if it ran on all four all the time but it is designed as a do-all motor. It gets awesome milage when trolling. Do you run a SS prop, because these can increase noise and vibration on this motor. Also, check to see if your upper exhaust port is above the waterline at idle. if not raise trim. Usually when coming off plane, the upper exhaust port gets a good dose of water at the same time the engine cuts back to 2 cyls and the carbs load up a bit. After coming off plane, put it in neutral and give it a little gas for a few seconds and the idle usually smooths right out. There are a few quirks like these with the 2+2's, but when you figure out how to make them happy, they run like a champ and get great gas mileage thanks to the design but have plenty of power when you get on it. Also, the recommended rpm range is 4850-5250 rpms so you could probably increase your prop pitch one size. Enjoy!!

 

[quantumleap]

Re: Merc 125

There's something else I have been thinking, I wonder if this particular motor design could be more prone to carbon fouling due to the off/on of two cylinders all the time. There is always oil fuel mix present to lubricate, but may foul more when initially kicking on bottom 2 cyls. I don't know? Might try to decarb your motor and see if that helps things.<br /><br />Also, while on this subject, I would like to state that I personally feel this is the most un-advertised, misrepresented motor in the history of outboards! Look back in the forums and there are so many owners of these 2+2's that had no idea how their motors functioned. Many of the boat dealers aren't even aware of the 2+2 function. I'm sure the mechanics in back know, but many of the sales people pitch it as a run of the mill inline 4. I feel that Mercury should have done a better job in marketing and sales training on this motor to better inform buyers of the features this motor has to offer. Many people buy a boat with this motor and complain of rough low speed operation and how the motor lunges at mid RPM's(when the last 2 kick in). In some circles this outboard has gotten a bad rap, but when properly setup, it is a very good performing and reliable motor! I love mine and wouldn't trade it for anything! (o.k. maybe an 04 yami 150 4-stroke!)

 

[cocodrieboy]

Re: Merc 125

So, is rough idling at these low RPMs nothing to be concerned about?

 

[LubeDude]

Re: Merc 125

It may run a little rough, but it shouldnt be jumping around on the back of your boat either, still sounds like there might be something else going on to me. Has this engine always done this?

 

[quantumleap]

Re: Merc 125

Well, if you're sure it's not carbon fouled and you've ruled out the exhaust port problem then that may be as good as you can get. It will not idle as well as a traditional 4 or 6 cyl if that's what you mean. Mine really idles quite well except coming off plane. Double check the location of your upper exhaust port at idle in the water. I assume no stainless prop?? Let me know.

 

[cocodrieboy]

Re: Merc 125

Yes, it's a ss prop. I compare the idling of the Merc 125 to my Yamaha 40hp, which really idles smooth. Never having been around larger outboards, I don't have much to compare it to.<br /><br />The bottom line is that running WOT, it sounds fine; anything less than 1/4 throttle, it sounds rough. <br /><br />I'm going to run a Mercury fuel additive Sunday and see if that doesn't help.

 

[jeffestes]

Re: Merc 125

Did the additive do anything? I have a Merc 115 that acts very similar. The rattle or Knock goes away about 1/4 trottle. The boat will get up and go though. I had a mechanic that doesn't normally work on outboards tell me that I needed to do an overhaul. But I want a second opinion. Can anyone answer whether or not my 1996 Merc 115 is a 2+2 engine?<br /><br />Thanks,