Mercury outboard won't stay idling

nwpete

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My Mercury 40hp 2 stroke (2001) won't stay idling and I really need an advice as to what could be wrong with it. Let me provide you with all the facts. I have also attached a video of it dying while it idles for about a minute (sometimes shorter).

- The outboard keeps idling when I push the "choke/enrichment" button before it's about to die by pressing the key in. Yes, it's an electric start. When I press the key in, the engine starts revving higher in rpm for a bit and returns to the normal rpm and then the engine dies.

- I have cleaned the carburetor (WME 2XK) very thoroughly. I used a carb cleaner, air blow gun and a piece of wire to clean out all the jets and holes. They didn't seem blocked but I made sure anyway. Before I cleaned the carb, I noticed that it was actually very clean without any varnish or gum. I properly set the float per my Mercury Repair Manual. Replaced all the gaskets and the needle.

- I doubt it's the fuel pump and here is why. The engine keeps running so long as I push the choke button. Also the primer bulb stays firm and the fuel filter stays full of fuel while running the engine. I can see the fuel level in the semi-transparent fuel filter canister. Also, keeping pressing the primer bulb, which is already almost firm does not prevent the engine to die.

- After starting, when I push the throttle lever up in neutral position, it still dies. So probably no IAC issue?

- It dies even if I use fresh fuel. Whatever is in my old fuel tank is only about one or two months old.

- There is fuel in the carb bowl when I unscrew the nut.

- Possible fuel hose blockage? When I press the primer bulb in the beginning, it fills the fuel filter right up so I don't feel there is any blockage from the fuel tank to fuel filter. After the engine dies, when I unscrew the carb bowl, enough fuel spills out so I don't think the hose from the filter into the carb is blocked either.

It sure sounds like a classic blocked carb jet issue, but I swear the carb is clean and EVERY hole and jet is unblocked. So at this point, I really have NO idea what's wrong with this thing.
 

jimmbo

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You haven't mentioned if you tried adjusting the idle mixture?
 

nwpete

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Yup I tried adjusting the idle mixture screw but it doesn't help. It's currently set at the number of rotation based on the manual. I turned half turn counterclockwise each time and tried until the screw was getting loose and it still died.
 

jimmbo

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The setting in the manual is an initial setting and the final ideal setting is usually leaner
 

nwpete

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I just adjusted the idle mixture at leaner settings clockwise 1/4 turn each until I couldn't start anymore but each time it still died.
 

nwpete

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It keeps running fine at wot on the water. As soon as I reduce the speed to near idle, it sputters and dies again.
 

racerone

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Need to post results of a compression test.-----Followed by a spark check.------Does spark jump a gap of 3./8" or more, yes or no ?
 

QBhoy

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You don't mention that you changed the spark plugs ? Or did you ?
 

nwpete

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I'm getting a spark tester this afternoon. Will update. As far as the compression, 110 and 115 psi for both cylinders. Spark plugs are about two weeks old.
 

nwpete

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Here is an update to what I found out today.

I get good sparks as they jump the gap of at least 3/8" or more.

While idling before it dies, when I pump the throttle lever up and down, I am able to keep the engine going.

Also, I have a Mercury kicker that uses the same fuel line connector as my big Mercury. So I swapped the hose and primer bulb and tried. Same result. It dies during idle. The kicker motor never had ANY issues and the bulb and hoses are pretty new. So I believe I eliminated the primer bulb / hose issues too at this point.

Still very puzzled about this...
 

SuperEnzo

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Dec 11, 2014
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Have you checked the timing at idle? I had an Evinrude outboard that would run fine at WOT but would die at idle and only survived with constant choke bumps and the carbs were spotless. Do you own a timing light?
 

nwpete

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No I haven't checked timing at all. I don't have a timing light... Your evinrude's issue sounds like mine. Was it bad timing that caused your engine to die at idle?
 

SuperEnzo

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it was a 1984 evinrude 70hp and the idle timing was supposed to be 4 degrees TDC (Top Dead Center) but somebody had messed with the linkages and it was now -3 degrees TDC, meaning that combustion was starting after TDC. His engine ran great at WOT because the idle timing and WOT timing are 2 different settings. People often mess with the timing linkages because they're trying to adjust the idle rpm. Does your boat have a tachometer? What is engine rpm at idle in neutral? It should be around 750 rpm in gear, so around 900rpm in neutral. I think it would be worth your while to purchase a timing light. Very useful. Have you ever timed an engine? Do you have the service manual?
 

nwpete

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Thanks for the idea about the idle timing. I do own a cheap Kedsum tachometer and last time I was idling at around 850-900 rpm. When I adjust the idle speed using the idle speed screw, it still stalls at idle. I don't own a timing light and have never timed an engine before so I'll study this and get a timing light if it's something I can tackle on my own. I guess I understand the concept that it could stall during idle if the combustion timing is off but it still goes on for about a minute before it dies. I'm not a mechanic so excuse my lack of knowledge. Just trying to understand. It's been a long discouraging process.
 

SuperEnzo

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Are you the original owner? When did it start doing this?

Checking the timing is super easy, google mercury 40hp 2 stroke timing and watch youtube videos on it. On the air box cover it will say the settings for idle and WOT (Wide Open Throttle) timing in degrees. Check that and well go from there, I have other suggestions if its not that.

I know how you feel. Keep at it, be scientific and you'll get there. Think about this... engine runs fine at full speed but it won't idle. What is the variable that is changing between idle and full speed? Those are the things you need to check. Like idle timing, intermittent spark at idle but not WOT(intermittent spark can be checked with a timing light so just another reason to buy one), clogged low speed jet and a clear high speed jet. See what I'm getting at?
 

nwpete

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Thanks for your help. I'm gonna get to the bottom of this. I'm not the original owner. I recently got it back in July. I believe the previous owner is not the original owner either. I was a total newbie when I first got it I didn't even know I was supposed to push the key in for choke for cold starting. It worked fine the first time on the lake after I somehow got it started and I boated all day on the lake that day in various speed. I turned it off and back on many times and no problem. The second time I took it out to a river, it died while I was running. Around the third and fourth time on the water, same thing except it died sooner as I was getting away from the dock. When I put it on the muff at home, it ran fine. When I started it in a container full of water, it ran fine so long as I adjusted idle. Until that point, I thought it was just the idle speed / mixture adjustment problem which was weird because it would do fine on land either on a muff or a trash can but not on real water. So I did some research and figured maybe I needed some carb cleaning. I was inexperienced I decided to go with seafoam/gas cleaning method. So sprayed some seafoam in both carb and cylinders and I ran a gallon of fresh gas with a can of seafoam ran it for a while. Smoked like crazy. It died towards the end. I restarted it but it would only last about 30 sec or min in idle unless I push the key in to choke. This was the first time it started happening on land. I changed spark plugs but same thing. So I studied how to take the carb out and did a thorough cleaning. It was pretty clean. I sprayed carb cleaner through every jets and holes and blew it with compressed air and tested it with a thin guitar string. I also realized that there were some debris (tiny pieces of leaves) in the main gas tank on the bottom. So I siphoned everything out and filtered it and cleaned the tank. So now everything is clean. Cleaned the fuel filter. After putting the carb back on, same thing. It would die in idle like in the video. Before it dies, the rpm goes down and it runs rough and shakes. I will get a timing light but in the mean time last night I decided I'll take the carburetor out again and clean it one more time in case some existing debris in fuel lines in cowling before I cleaned out the fuel tank was clogging any jets. So I took it out and cleaned it last night. I did find some tiny particles although I didn't feel like it was blocking anything. I'm gonna put it back in today and test it and also spray carb cleaner while it's running around the connections and hoses to see if there's any vacuum leak. Will update. I'm gonna order a timing light next.

BTW, on the air box, it says, "8BTDC/22BTDC". So 8 degrees before top dead center for idle and 22 for wot?
 

ondarvr

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The actual timing # at idle isn't important, you just set it so the RPMs are correct, it is what it is.

Running lean tends to increase the RPMs right before it dies, running rich tends cause it to run rough before it dies.
 

nwpete

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Well I just put the carburetor back on after my second cleaning. I really wasn't feeling optimistic at all but when I started.... she kept going and going and going without stalling!!! I am so thrilled right now it's ridiculous! I think it's fixed now. The lesson I learned from this long process is how perfectly clean my carburetor has to be. It's a one-carburetor engine and that's really it. I mentioned how I found a couple of tiny little debris in my carb last night during my second cleaning. It was so little I didn't think it was a problem. I remember some weird fibrous thing was partially clogging the top hole of the main gas jet tower. Another partial clog was in one of the progression holes.. super tiny holes with super tiny debris. Cleaned the hell out of everything all over again including all the orifices with an autozone carb cleaning spray. I thought I did a perfect job the first time I cleaned it but either I didn't or the carb jets got clogged up immediately again since there may have been still some debris inside the existing fuel in the fuel lines in the cowling. Now that all the debris are completely gone from the entire system with an immaculate carburetor, it's running again. Wow I feel like that was one heck of a puzzle though the real solution wasn't all that complicated in the end. I gotta take it to the river and do the ultimate test at some point but I'm optimistic. Thank you for all the help especially SuperEnzo.
 
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