Mercury 2stroke Engine Issue/Question

[ericbres]

Question for you boat guys - figured/hoped this would be something a more experienced boater might know or have happened off the top of their head.

We've got a 20' center console w/ a 125 Mercury 2 stroke that's been acting weird and am hoping to spare a last trip in for service (we've already lost 7 weeks this summer with it in for other various work).
If I hit the throttle full - it'll go to, say, 6k rpm and 6mph. Which isn't max speed for the boat. It doesn't "sound" like anything is wrong and will run like that for a for good few minutes. Occasionally, eventually, it'll slowly creep up in RPMs to ~8or9k and it's typical max speed of 12mph.
(note, speedo broken. All speeds estimated from the fish finder)
Sometimes (rarely) after we've been using the boat for a while when we hit the throttle full it WILL go straight up to it's max RPMs and standard max speeds.

Now - for the "rest of the story", as it were - it recently had the throttle replaced. Throttle cable replaced. Carbs completely redone. Sending unit replaced.
But, due to owning it for a couple years with a bad sending unit, thus no idea how much fuel was in it - it's never actually had the fuel cleaned out. Probably got some real crap in the bottom of that tank.

Thoughts?

[Bowmandan]

A 20 foot boat with a 125  should go well beyond 12 mph.   Something is terribly wrong unless your boat is extremely heavy

[ericbres]

12mph is just a reference point to what is displayed on the fish finder ... it's not an accurate representation of speed. The speedometer has never worked on the boat as long as we've owned it.

[trophytaker1]

Probably has a bad coil so its not running on all cylinders.

[Mac Attack]

12mph is just a reference point to what is displayed on the fish finder ... it's not an accurate representation of speed. The speedometer has never worked on the boat as long as we've owned it.

Take a gps out and get a true speed reading please
I don't know of any boat that will plane out at 12 mph
Also, what type of hull is it
Post a picture of it please
Thanks

Mac

[ericbres]

It's just a number ... I'll edit my first post to say 30mph if it helps? There is no way the boat is max speeding at 12mph ... no way. Again, I was just using that number as a reference because that's what I had as a "fixed" point of reference thanks to the fish finder. I had a fixed number of 6 and a fixed number of 12. They are in no way representative to any accurate speeds.

[ericbres]

Take a gps out and get a true speed reading please
I don't know of any boat that will plane out at 12 mph
Also, what type of hull is it
Post a picture of it please
Thanks

Mac

Hi Mac - as far as the hull,
If you do a Yahoo! Image search for a ProSprots 1950 center console, that's my boat (only we installed a hard top this year, for what it's worth). Thanks.

[Mac Attack]

Hi Mac - as far as the hull,
If you do a Yahoo! Image search for a ProSprots 1950 center console, that's my boat (only we installed a hard top this year, for what it's worth). Thanks.

That boat, without the hardtop should do 40+ with that 115
Use a GPS
And without hearing or testing it I cannot tell you if a cylinder isn't firing
But I would start looking at the fuel, carb, and filters with your earlier comment on the fuel.

[JerryofWNY]

I had an old Merc 40 horse that behaved much like yours. Did many of the same things at the local marina. They could not solve the problem and charged me only their parts price which I insisted on (they weren't going to charge me anything). Found a guy who diagnosed the problem over the phone. I took it to him and he was spot on. The one question he asked me was if the motor was used for trolling very much. It was and he told me he would guarantee that the problem was a plugged exhaust port. So I ask you, does the motor have a long history of being used to troll? If so have this looked in to.

[boondox]

Just stuff to try that's cheap to do..

Did or do you use that motor for trolling?? Like low rpm and low speed like 1.25 mph to like 3.0 mph ??  Sounds like the carb is gumed or gumming up  could be debris in the fuel to can you empty the tank our or is it part of the haul??  Try picking up a smaller tank like a 12 or 15 gallon lastly..   try finishing out the season then rip tank out after words...  betting debris in the tank or the carb jets are fouling or plugs are fouling or even coil pack....  that make the spark plugs fire.. first things I would do is check the spark plugs to a fouling chart like like coloration of plugs see if oil is leaking to them or non complete combustion has happened.. etc etc.. spark plugs tell a lot also check gaps..  see if any cylinders are wet with fuel if they are that probllay a bad coi.. check spark with a spark gap tester found at most auto parts stores..  see if you got a nice blue wite ish spark not orange or yellow.. you can do the color test with a screw driver in plug boot.. touch to ground and hold handle not touching metal zap you feel.. if every thing else check out and suspect debris in fuel  pick up a large-small tank so you can finish off season..   that would be last item to do cause you can't return fuel tanks once used.. another thing to try is Lucas fuel treat ment it cleans the crap out of the carb and cylinders.. just add to fuel and fill up as it runs it should run better.. also what kind of prop is it running like did some one replace it so they could troll off that motor???  cause the mph seems really low.. I will get a spark plug chart off Web or a picture you can view pulling spark plugs is cheap same with checking gap of them etc.. 

[boondox]

Check for oil fouling or oil covered spark plugs.. reason I wondered this is my brother had a mercury black max optimal 150 horse that would have trouble starting...  and would have a little hesitation.. but we noticed it cause the rpm were low and some thing would unclog as we drove it at high rpm. Wich you said same symptoms...   Wich we found our 2 cycle engines are notorious for thats why people are buying kicker motors for there boats.  But any way check the oil jet it maybe to large of opening cause to much oil flooding in and fouling the engine..  or the coil packs are bad on a cylinder and not lighting all the fuel oil..that's why I say investigate before a full rebuild is needed like a exhaust port.. thd motor will have to be torn down and block dipped to clear exhaust port.. hope you catch problem be fore gets to far.  hope this info help you out best of luck..

[hunts2long]

Keep in mind that a motor also needs air to run. I had a hornets nest in my air in-take. Motor would run at low speeds fine but would shut down at higher speeds. Just a thought....h2l

[ericbres]

Thanks for all the replies guys.
I'm off to camp in a few hours for the next few days and will be able to spend plenty of time taking a look at just about everything that was posted.

A few things to follow up on / answer a couple questions:
- No, we haven't trolled with it to speak of. Just got the down-riggers this year and really only trolled Winni for the derby weekend. Other than that we've only used it for striper fishing and taking the kids tubing and/or going to camp and hanging around.
- It JUST had a complete carb rebuilt (due to poor idle / running as low speeds loading/unloading). As in, literally the week before this issue began we picked it up at the marina from it being done. So, I'd have to assume(?) it shouldn't be a fouled/gummed up carb, right?
- As far as the fuel, yes, there is a strong possibility the fuel is bad. The gauge has never worked until this season. Just got it fixed. Until this season, we've always just topped it off before taking it out. So, in theory it's always had at least 1/2 tank or more of "old gas" sitting in it (for at least 3 years).

In any case, again, off to camp and will spend some time trying to take a look at some of the stuff listed here. There is another tank at camp for the pontoon boat, that may be my first step - hooking that up to see how it runs (on a clean tank of gas with clean lines). I'm also curious about the air - the boat was full of wasps/nests when it was finally opened this season.

Thanks again.

[Keith Walters]

If i hear you right, motor is turning high rpms but boat is going slow.
It that is the case, your prop might have a spun hub. Google "spun hub". Try a different prop.
Best of luck,
Keith

[ericbres]

UPDATE -
Just got back from camp and went through a NUMBER of things on the boat (all for the better I suppose because it got a great cleaning, if nothing else).
The fuel lines and air intake are all in pretty good shape. So those were ruled out.

Just as I was about to pull plugs, a couple guys pulled up and started a conversation with me "boy, she sounds like she's running a bit rough".
Long story, and a lot of pulling stuff apart and dockside trouble-shooting, short ... the tank is full of water.

Somewhat of a double-edged sword - our poly tank is problematic enough this time of year from what the guy said (he gets a run of boats in this time of year with similar issues) but also thanks to the broken gas gauge, we've just been "topping off" a tank likely part full of water anyway for the past couple of years.

Now ... of to pump out the darn thing ...

Thanks for all the input guys. Hope my update can help if anyone comes across similar issues in the future.