Need help with 89 Mercury 2 stroke 4 cyl bogs down at WOT

[Birdman84]

Guys, I am new to this forum and I really need some help! Recently picked up a 14' McKee tri-hull with an '89 Mercury 40hp on it. I have taken it out about 4 times now since purchasing 2 months ago, and it seems to start fine, runs fine, doesn't shut off, but when I try to get over 50% throttle it begins to bog down like it just doesn't want to open up, and the boat won't plane (max speed it about 7 mph)...feels like a ton of drag! When I put the boat back up after each use, I have noticed a sludgy very dark oil dripping from the seals behind the prop (unburnt oil I presume?)

Things I have tried - pulled the carbs off (2 of them) cleaned them out really well, tried to "decarb" the motor, installed a fuel/water separator, bought a new gas tank and fittings, and eventually brought it to a merc dealer. The dealer had it for 2 weeks, and said they determined the issue to be the seal behind the fuel pump that as bad and letting out pressure at WOT. Compression is good on all cylinders. Well, paid the mechanic, took it out this weekend, and it is still doing the very same thing I cannot figure out what else it could be. I have scoured the forums here for answers but not seeing many others with quite the same problem.

Again, it runs perfectly fine EXCEPT over 50% throttle you can hear a slight vibration inside the motor (maybe a cylinder misfiring?) and it bogs until I bring it back below 50% throttle. The guy who sold it to me replaced the stator on it himself before selling it to me. Is there timing on this motor? Is it possible the timing is off? Bad coil? Any help would be GREATLY appreciated, I can't afford to keep bringing this thing to mechanics and dishing out hundreds with no solution! Thanks in advance!

 

[scamp12]

Hi did you try running with out the hood . not seeing a motor run is difficult to diagnos. Lets assume. Both plugs have a good blue spark . I would bring up to were it bogs and hold it there for twenty -30 seconds . pull a plug see what it looks like if its lean its a fuel issue. If its wet then it spark . This motor has mechanical. Advance unless someone forgotten. Some piece and it wont go all the waylinkage wise . Also check fuel pressure . maybe someone messed with the advance plate under the flywheel . something could of losen up .

 

[Birdman84]

Thanks for the reply. I must say I don't know anything about the flywheel assembly or advance plate you speak of but I will try to get the flywheel off in the next few days and take a look, see if anything looks unusual. I did try running without the hood and noticed only one significant thing. The mechanic mentioned to me there was a "pinhole" leak in the exhaust manifold cover but didn't think it was significant enough to fix right now. When I took it out this past weekend after him telling me this, I took off the hood while running 50% throttle and yea, pinhole leak was probably an understatement. It was leaking enough that it completely filled up the reservoir underneath the motor and I could visually see it pissing out from the bottom of the cover. Small hole, but water is pissing out of there pretty good whether under a load or not. Even on the muffs in the driveway. Could this cause a bogging at WOT? The mechanic lead me to believe this shouldn't be related.

Getting pretty discouraged now, I called the last mechanic back who most recently worked on it to see what else it could be. His answer was that "maybe the motor is just falling apart on the inside" which wasn't a reassuring feeling to say the least! He seems to think it could be leaking water into one or several cylinders and maybe the motor is on the way out. I have a hard time believing that because it has good compression and runs great otherwise. He said to check the plugs to see if any were clean. Just checked, all had a slight bit of dark residue/oil sludge on them. Asked him if he checked the timing and if that could be an issue as I've seen some others have similar causes from the timing being off, but he said it was a real small chance that would be causing this problem, but could check it of course if I reach deeper into my pockets!! Praying this motor isn't destined for the dumpster.

 

[frantically relaxing]

My advice- search for a mechanic who actually acts interested in giving you some decent answers and maybe even making it run right. As for the water pissing, if it's pissing outside, it may be pissing or steaming inside. Are you seeing water droplets on the plugs?

 

[Birdman84]

Didn't notice any water droplets/moisture around the plugs when I checked them. Is it possible the oil/fuel mix is incorrect? Would that create such symptoms? It is oil injected. I know with my 2 stroke weed eater (forgive my ignorance of boat motors) if I mix the gas/oil incorrectly sometimes it will just bog down when at full throttle similar to what I am seeing with this motor. And as far as the mechanic goes, tell me about it. I recently moved here to Wilmington, NC and have only been able to rely on several recommendations on where to take it. Seems they were not very good ones. Have another guy telling me about a 3rd that may help but I am leery to spend more $ only to get more let downs.

 

[Birdman84]

Also, the last mechanic "cooked" the carbs and rebuilt them with new seals, and tested spark - said all was good there. According to him, in a bucket of water at the shop it will "scream" but I'm not seeing that on the water under a real load. Compression test from the 1st shop I brought it to read: (#1) 135 (#2) 135 (#3) 125 (#4) 130.

 

[scamp12]

Wrong mix will foul plugs out . the hole you have might cause a lean condition. On the water . my motor will bog down in the barrel and if i take the hood off it screams in gear . on the water its fine . the barrel will make these motors run rich . so yours screams in barrel . Any holes in a two stroke exhaust. Will. Make it run like crap. Good spark / compression. Sounds good . can you hear an exhuast leak ?

 

[Barnacle_Bill]

If you have the square fuel pump (and I believe you do) rebuild it. Be sure to use an OEM kit as it comes with new designed check valves. It has been found that the old designed valves curl up on the edges and not seal properly at WOT. Mercury/Mariner Service Info Advisory # 2001-17 explains how to test the pump for this problem but it would be simpler to just go ahead a rebuild it.

 

[Birdman84]

No Title

Thanks to all for the input. I cannot hear any exhaust leak, everything sounds normal. The fuel pump is square yes, and I will go ahead and order a rebuild kit to be on the safe side...seems like a fairly easy job taking it apart. A few updates - I discovered that one of the spark plug wires was damaged and fixed that (actually shocked me when I touched the wire!), also checked the timing adjustment so that checks out. I also noted while running, the top 2 spark plugs were hot, while the bottom 2 were only warm. Also, while running, if I put my hand over the throat of the carbs I had varying results. It has 2 carbs, the top one would almost choke the engine out and die, while covering the lower carb only made it bog down a tiny bit. Seems the top 2 cylinders are doing most of the work. While running it in the driveway, I noticed it smelled different - like bad fuel. Took the water separator filter off (just installed it on Friday) and it was about 3/4 of the way full with water. Included a pic of what I collected. Now I am thinking this is quite possibly my issue. I was shocked though because I just emptied out my tank about 3 weeks ago and filled it with ethanol free gas...not sure where that much water came from. It has been sitting for a bit at shops around town though so I suspect water somehow is seeping into the tank. Considering replacing the tank. Disposed of the gas in the tank today. Letting it dry out and will refill with fresh gas tomorrow. Haven't been able to take it out on the water yet but will try sometime this week and post results.

 

[Birdman84]

Took it out this evening with fresh e-free gas, still same thing, although the rpm's increased about 20% . This time I had a mechanic I found through reference with me. He adjusted the carbs on the water, but still wouldn't open up. He took it with him, said he thinks the motor is fouled from sitting for long periods and a lot of water being run through it and needs to be run for a while with frequent plug cleanings/changing to clean the "trash" out of the motor. Very skeptical at this point. Will post my outcome if I ever figure this one out..

 

[fishin98]

Post a picture of your motor and electronic module on the side. I had a 1983 Merc 50HP 4cyl.....did the same thing...BIGGEST POS ever made. The electronic module would short out every year or two, causing 1 or more dead cyl.....Merc FINALLY got it right on the next GEN of electronic modules.