Merc 125 Rough Idle/Smooth Cruise

[ragnad]

My parents have a 2002 Merc 125 on their 23' Hurricane Deck Boat.

It always starts easily, but runs really rough (like it's missing) at idle and when running at low speeds. When we take it up on plane, it runs like a dream.

Any ideas what would cause this behavior, what I can do to test it, or how it can be repaired ?

Thanks !

[Bob H.]

I'm not familiar with that year engine but I'll take a shot. I'm assuming that engine runs on half the cylinders during idle and low speeds. I'd try some carb cleaner in the fuel tank. Rationale: considering the type of boat I bet it spends a fair amount of time at low speed. The carbs seeing the most use probably need a good cleaning. I suspect that a low speed passage may be partially blocked. Run some fuel system cleaner through the system or open and clean the carbs (my preference since I'm anal with maintenance). Also suggest a filter/water seperator system such as a Racor spin-on filter. Layup storage habits may be a contributor to the problem if the carbs aren't drained prior to layup.

[ragnad]

Thanks for the reply. I'll clean the carbs this weekend. I replaced the water separator last fall. It's probably time to invest in the Racor with the see-through bowl.

[HawgHvn]

I just had a similar experience - hopefully not the exact same, though.

Started readily enough, idled very rough, jam it up on plane and ran like a dream. Throttled back to drift through the CBBT pilings and the whole back of the boat shook like the rump of a Boston Terrier about to get fed a piece of prime rib. Finally died and just before I called SeaTow managed to get it running again - cruised back at top throttle no problem - idled back at the pier and never got it started again.

Bubba fixed it for $950.00. Let's see. He said things like: the last person that worked on it set the timing up too high; one of the cylinders was burned up; $950.00 if the head isn't cracked - if it is the cost goes up but I might have a spare head laying around here somewheres; at idle/low speed there isn't enough compression for the cylinder to operate properly but at speed the rest of the cylinders make up for it only with a slight loss of power.

Ever seen a valve that looks like a mushroom?

Runs like new, now!

Do a compression check. Good luck! I hope it's just a loose plug or somethin'!

[Bob H.]

I thought this was a 2-stroke. If there's a mushroomed valve there's a piston problem. A lot of low performance engines use sodium filled valves and when something goes wrong you don't find much left - they disintegrate. Unless the head was magnafluxed and a crack is found the head can usually be reused (low performance engines) if the seat area hasn't been damaged. Sounds like a keeper came loose and dropped the valve.

[mercurytech]

Your 125 merc is working on what is called a 2+2 system. At idle up until about 1800-2000 rpm it only runs on #1 & #2 cyl. Over 2000 and the bottom 2 cyl come into play. You could also have an ignition problem. I worked on one before and it was acting like a carb problem. Turned out to be a bad CDM, and on this engine when one CDM goes bad it effects 2 cylenders. Hope this helps, and if you take your engine somewhere and they say the head is cracked pack up and go somewhere else. All inline merc products (except Verado) are what is called blind bore or "intergrated head engines" meaning there is no head. If you need to know more about this let me know.

[threesea's]

This is funny, I just noticed this on my Mercury 115 Offshore. Salesman explained the 2+2 system to me but the transom still shakes her a$$ like a two dollar hooker when at very low speeds and sounds like it wants to quit. One other thing my motor does that you did not mention is sometimes when i am about 1/3 of the way into the throttle and running for a little while at that speed the motor/throttle will sometimes "power up" without me touching the throttle at all, it will just speed up, not much but its noticable. I am not even trimming the motor up and it just jumps in speed on its own. And, if I have been running wide open and throttle all the way down you will hear a clicking noise at the motor as the boat settles down to idle. I will post what I find out if I find the problem.

[rrr]

Bob H-you should start writing your own how-to books on motors.

[erikrs301]

Yea, if it is a 2+2 setup the seemingly rough idle is normal because it is only on 2 cylinders. I have an '05 125 Merc Salty with this setup, only 30 hrs and it acts like this. It should jump to all 4 cylinders at about 2K rpms. If you try running just below that, the rpms may jump around as it is in between the 2/4 cylinder rpms range. I would think it is best to back it down a little to stay at 2 cylinders or bump up the throttle to run all 4 instead of letting it jump around.

I was having some issues w/ ethanol fuel earlier in the season. Went through 3 fuel filters in a weekend. Clogged filter led to not enough fuel and only oil being injected. This fouled up the plugs. New filter, cleaned plugs, started and ran fine...although I did lose a screw driver in the drink. Now I carry spare plugs and filters at all time and buy only MTBE gas while I can up here in PA.