Mercury 50 low and idle speed problems

[Dimitri-2000X-Tampa]

This is somewhat of a continuation of the problems that Squidman was having with his Honda 50. He ended the thread being pretty confident that his problems were caused by E-10 gas and less than adequate filtration. Here's my story:

About 4.5 years ago, I got a piece of crud stuck in the high speed jet of one of my carbs that affected the motor's ability to run well at rpm's above about 4000. Did I mention that my 2000 model is the last model year with carbs before Mercury went with EFI on this big foot 50 4 stroke. This problem had to be fixed so I ended up taking apart the carbs, unclogging the jet and then syncing the carbs up. The motor ran like a champ for many years after that, both low speed and high speed. But now, for the last 6-12 months, its been acting up again. The wierd thing is that it is mostly only acting up after the engine has been running for a while, when I first start it up and back out of my slip, it idles fine. But after I come back when the engine is nice and hot, thats when it starts misbehaving. This makes me think its probably not a sync problem. I have three theories so far on what might be causing the problems:

1. Spark plugs. I can't ever recall changing them. I bought some new ones but haven't yet found my silly socket (because these plugs are not normal sized) to replace them yet. I wonder if possibly a very hot plug could start missing. This will be easy to fix when I find my socket and I wanted to do this first before trying anything else. So, its time to clean up my workshop when I can't find something.

2. Auto choke or some other linkage problem. Usually, I am out with my family and by the end of the day, they want to get home so I haven't had the chance to run the engine without the cover and see if maybe the choke is not disengaging properly. I think the way the linkages work, this would only be a low speed problem (choke must disengage at higher speeds) and would make sense that it runs fine when cold since it needs some choke then to enrichen the mixture. This is what I'll try after replacing the plugs.

3. Could my problems be caused by E-10 gas or some other fuel system problem? I read the articles and none did say anything about low speed problems when hot. But I did notice that my gas stations have recently put those stickers on and I may have been running some of this gas lately. Do all stations now have this gas in Florida or can you still find MTBE gas? Yesterday I took a 40 mile ride and thought I would be able to do some sailing, but the wind (and tides) were against me both directions and I ended up miscalulating how much gas I would need so I stopped at a Marina about half way back and filled up another tank. Could this gas (at $4.90 a gallon) be old or E-10 also? I'm still skeptical that my problems could only be caused by poor gas but the engine ran even worse yesterday at low speed up the final canal when I switched to the marina fuel.

For many years, I've used a double inline fuel filtering approach. One of those glass filters with the replacable cartridge on the external fuel line and a standard Mercury paper filter on the internal fuel line. It probably is about time to replace the filters, but seems like if it was poor flow through the filters, then that would also affect high speed operation. Even though I've had them on my previous boats, I've been too lazy to install a big honking water separating filter on the Mac, partially because I don't know where the best place is to put it. But I've had this boat for 5.5 years now and only starting to have these problems recently so I still find it hard to believe I'm getting water in the fuel now unless if it could be that all these Florida stations are just now switching to E-10. And if it is water, why does it run fine at the higher speeds?

So, that is my story and I wonder if any of you have insight into similar issues or advice as I (very slowly) work through the troubleshooting. Slowly because this isnt as critical as a high speed problem and it has also forced me to make absolutely perfect (engine conking out) dock approaches lately.

[Kelly Hanson East]

You almost certainly dont have MTBE gas - no one wants the liability associated with MTBE leaking out.

I think there are a lot fewer cases of ethanol/gas/water separation than people report - its much more likely you have either a gunked up carb or a loose fuel supply o-ring or gasket causing fuel starvation at heavy load than a chemical problem due to gas mixture.

[James V]

In need of an oil change will also cause low engin speed problems.

[Dimitri-2000X-Tampa]

I don't understand how a gunked up carb, fuel starvation or less slippery oil could cause an intermittent problem at low speed. Any more information on this? The engine runs just fine under a high load.

[James V]

I was refering to idle speeds. There is just so much gas flowing into the engin. I have found that change the oil does help if I have gone over the 100 hr limit.

Gunk in carb will give you irregular fuel flow.

Is yours fuel injected?

[ChrisNorton]

Hi,

I have the same motor and had the same exact problem. I was told these models have very, very tiny fuel channels to the carbs (sorry for the lingo...I'm not a mechanic). I guess they clog quite easily. At higher pressures at higher speeds everything runs greats. At low or idle speed at lower pressures the clogs cause it to run rough. So I brought it to an authorized Mercury dealer and they said the small channels in the carbs are nearly impossible to clean out once they get bad. They were able to clean and salvage two of the four but two others had to be replaced. It cost me $2000. Now others on this board told me swapping the carbs out is relatively easy (and much cheaper of course) but I wasn't up to the task. Since this repair, the motor is getting rough again. This time I took it to a non-mercury repair guy but someone I trust and he confirmed these carbs are nearly impossible to clean out. He told me toreplace the two I had cleaned a few years back. I live outside NYC where labor costs are very high so maybe it will cost you much less than what I had to pay since I couldn't do it myself.

Good luck,
Chris

[Dimitri-2000X-Tampa]

James, my 2000 model has carbs. In 2001, Mercury switched to EFI.

Chris, thanks for posting your story. WOW! 2000 bucks is a lot to spend on any engine repair. I don't think the motor is even worth that much anymore. When I cleaned out the carbs a few years ago, I broke a couple of those small feeder tubes and had to replace them with some regular fuel hose that I found at an auto parts store. Its never leaked since then. But I have been skeptical that it is a flow problem because I still think it would affect high speed operation too. Now if you are talking about the low speed jets, thats certainly a possibility but I still can't understand why it would be intermittent. The last time I had a carb jet problem, it was a very consistent problem. But pulling the boat out of the water and disassembling the carbs will be my action of last resort if I can't find anything else.

I went out last weekend and ran the engine without the cover for a while. I messed with the choke linkages and that didn't have much effect. I'm starting to think I may have multiple problems here and I will have to rule them out one by one. In the mean time, I raised the idle speed somewhat so that it wouldn't conk too easily but of course, then it runs too fast when it is running well and makes a big bang when you shift gears. I think I may have an occasional missing condition at lower rpm's, so I first need to change out the plugs to rule out a flaky spark plug (which are several years old). Then, I was also hearing a valve sticking (slight valve knock) occasionally so I should take James advice and change the oil soon, but first, I'll run some stuff in the oil for a while to clean up the valves, maybe seafoam. Unfortunately, I don't have an hour meter but it has been 1.5 yrs since the last oil change so its due. Maybe dirty valves are the whole problem (and occasional sticking may also be the cause of an intermittent problem - this is the downside of a 4 stroke). Then I will rebuild the fuel line and both the fuel filters and if all that still doesn't fix it, I'll have to clean out the carbs, which is a pita.

[cuisto]

Have been experiencing the same problem and the mechanic at my marina has identified the problem. Seems the automatic choke (99 bigfoot merc 50) has failes and was dumping too much fuel in once warmed up..fine at throtle but not so good at idle. It would stall when returning to the slip or worse, when thrown into revers from slow forward. Unfortunatly the part was back ordered and has been so for 3 months. I know this is dreadful but having an inflagtable dingy in the front of my slip has been a godsend this summer. The part is now in and am told it will be repaired this week. Will post the results.

[ChrisNorton]

That is interesting. Now I am wondering if I have the same problem and it isn't my carbs. It does sound just like my behavior. let me know how it goes. How much is the part? Is it easy to install?

[Dimitri-2000X-Tampa]

Well, this problem gets stranger and stranger. I'm still running on a 2-3 week old tank of marina gas and yesterday I went out and the engine ran fine the whole darned day. When I tilted up the motor while sailing, I noticed that a little gas came out where the hose meets the fitting which goes on the engine. I pushed it in a bit because it still has a silly clamp (ie. a cheapo non hose clamp on it). All my other clamps are proper hose clamps and shouldn't leak. I wonder if it has been sucking a tiny bit of air, but still, why never runs bad at high speed? This is what I would expect with an air leak.

I'm still not ruling out that this is a bad fuel issue with ethanol. I've been hearing a lot of stories lately around the neighborhood. I'm gonna put a hose clamp on the last fuel line leaky and then phone around to some gas stations to see who doesn't have ethanol in their gas other than marinas. I've never experienced such a wacky problem like this with any engine. I suppose I will re-lower my idle speed a bit since now it is idling too high, try to run with non ethanol gas and put in another update after a few more days out. If the problem returns, then it could still be a flaky ignition issue that hits when the plugs get hot and the engine is running slow.

[Dimitri-2000X-Tampa]

Just to close out this old post and my long running 2 season problem with my motor, it was still intermittent last November when I went to Cayo Costa but after a long cold winter, when I kicked it over in March, it just wouldn't idle at all anymore. So I finally removed the carbs and cracked them open to find crud in 3 out of the 4 bowls. The last time I had the carbs open was about 6 years ago and at that time, it was an obvious problem with a piece of crud in one of the high speed jets. This time was less clear since it was a low speed problem. Since the motor was running well at high speed under load, I procrastinated longer about fixing it and suspected a bunch of other stuff. But once again, dirty carbs was the problem.

With a low speed problem, I had to remove the pilot jets (which are above the main jets in the same tube). Although the main jets have a clear and obvious passage to the throttle body, the low speed jets have some alternate tiny passage ways. But once I pulled the jets out, I was able to put the straw of a can of carb cleaner in there and blow the passageways out. It idles again now although slightly rougher than it should so probably the carbs need re-syncing again and I've misplaced the little tubes for the gauge. Its a pain in the butt but at least it is pretty predictable, unlike some folks with computer issues on more modern motors.

Anyway, I really don't want to have a third incident so I bought the big honkin water separating filter (10 micron with drainable see-thru bowl) now since obviously, my two dinky inline filters were just not cutting it. Guess I will use more fuel stabilizers too...and dump old fuel more often. I got the filter all hooked up and was about to mount it (and a 3-way valve attached too) under the stern captain seat but right before I drilled, I realized that where I was putting it would interfere with putting the companionway hatch under the stern seat to block some of the sound of the outboard (and block the hole under the stern seat so small people and things can't fall out the open transom). So, I decided to pause for a day and see where others have mounted it. I guess I could still put it on that surface if I moved the valve somewhere else, but the obvious place to mount the valve (on the lip of the fuel area) will make it stick out and get snagged. Or, I could move the filter 90 degrees around the rear face of that stern pillar but then it will be harder to get to and may get splashed with sea water more often (I hear they rust out after a while). Not too much of these big filter installs in the mods section. If you've had a particularly good or bad experience with the installation of these big huge filters, please visit the mods forum where I've started a new topic on this.