Mercury 40 HP problem....HELP!!

I think I may have posted about this once before, but it's back, and worse. I have a 2002 Mercury 2 stroke 40 HP with auto oil injection. When I run in low RPM, it dies or runs really sluggish. I had this issue before and was advised to idle it up, which I did, along with changing the plugs and it seemed to work, but now the problem has returned. Only now I have trouble getting it refired when it dies. I have to put the throttle at least half way(in neutral of course) to get it to start sometimes after it dies or after it sits off for a while when I shut it off to fish. Also, late in the day last time out the motor ran extremely sluggish even throttled all the way. Almost like the boat was really loaded down. I took the plugs out today and found the lower plug had quite a bit of oil on it. I cleaned it off and plan to change both along with the oil filter, but I can't figure out why they keep fouling every six months or so. Is it because I need to run a higher grade gas? Could the oil injection be adding too much oil? Not enough gas through carb? Is there a carb cleaner that I can run through it that works at all? Sorry for all the questions. I really don't have the funds or time right now to take it to the shop. If it's something short of tearing the carb apart I can probably do it. Please help.

By the way, I use regular grade gas and Pennzoil Marine XLF oil, if that matters.

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Originally Posted by SpeckWick

I think I may have posted about this once before, but it's back, and worse. I have a 2002 Mercury 2 stroke 40 HP with auto oil injection. When I run in low RPM, it dies or runs really sluggish. I had this issue before and was advised to idle it up, which I did, along with changing the plugs and it seemed to work, but now the problem has returned. Only now I have trouble getting it refired when it dies. I have to put the throttle at least half way(in neutral of course) to get it to start sometimes after it dies or after it sits off for a while when I shut it off to fish. Also, late in the day last time out the motor ran extremely sluggish even throttled all the way. Almost like the boat was really loaded down. I took the plugs out today and found the lower plug had quite a bit of oil on it. I cleaned it off and plan to change both along with the oil filter, but I can't figure out why they keep fouling every six months or so. Is it because I need to run a higher grade gas? Could the oil injection be adding too much oil? Not enough gas through carb? Is there a carb cleaner that I can run through it that works at all? Sorry for all the questions. I really don't have the funds or time right now to take it to the shop. If it's something short of tearing the carb apart I can probably do it. Please help.

By the way, I use regular grade gas and Pennzoil Marine XLF oil, if that matters.

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I am no mechanic, but fuel fowled plugs tells me you may have ignition problems.

Also, I had 40 Mariner with similar prob. Cleaned carbs, replaced jets. Fixed it.

I have never fouled a plug.The problem is a fouled plug if it has oil on it.Take the plug out and hook the plug wire on and turn it over to see if the plug fire's.If it doesnt change the plug and check before installing.If the new plug fire's I would unhook the oil reservoir and plug it.If the new plug dont fire you probally need a new powerpack.While youre in the middle of all this clean the carb's and reset the float.I fixed one a while back and the problem was his gas tank had a vent valve closed.May be something simple.Try to find it because power pack's are not cheap.

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Originally Posted by Slipped Cork

I have never fouled a plug.The problem is a fouled plug if it has oil on it.Take the plug out and hook the plug wire on and turn it over to see if the plug fire's.If it doesnt change the plug and check before installing.If the new plug fire's I would unhook the oil reservoir and plug it.If the new plug dont fire you probally need a new powerpack.While youre in the middle of all this clean the carb's and reset the float.I fixed one a while back and the problem was his gas tank had a vent valve closed.May be something simple.Try to find it because power pack's are not cheap.

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I have never had a carb apart. My fear is if I take it apart and screw something up, I end up spending a bunch of $$ on something that wasn' the problem to begin with. Is this something relatively simple to do? I am pretty mechanically inclined, but new to working on outboards. Also, if the plug fires when I take it out, does this mean I am getting too much oil due to the oil injection system?
One thing I noticed...This evening I had the motor cover off and started it. I noticed some gas laying in the bottom of the motor. Not a bunch, but enough to get my attention, and I'm sure more than what should be there. Again, I am new at troubleshooting and working on motors, so if any of this seems obvious, remember, I'm a rookie.

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Originally Posted by Slipped Cork

I fixed one a while back and the problem was his gas tank had a vent valve closed.May be something simple.Try to find it because power pack's are not cheap.

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Where will I find the vent valve on the gas tank? Again.....rookie.

Please, no offense but if U don't know where gas vent is/looks like please don't take apart carbs. just make sure gas vent is open & bulb should be fairly hard when motor is running, if thats OK take to mech as it can be pretty easy to mess up carbs & U won't find out if this was problem #1

Is it an inboard tank or a portable one?

grubby, it's an inboard tank.

jaxsprat, no offense taken, believe me. If I knew what I was doing or even thought I did, the carb would already be cleaned if not but to eliminate that as a possible cause of the problem. So rest assured I don't plan on taking it apart. I am trying to cover all the simple things it could be before taking it to the shop. As for the fuel primer bulb, when the motor is running, or right after it dies, the bulb is not hard. I can pump it and make it firm again, but as it runs it softens. Man, it's a good thing everyone here knows we are talking about fuel bulbs.....LOL. Sorry. Anyway, what could that mean if the bulb isn't firm while running? Would it have any bearing on the fouled plug or the lack of power, dying. Obviously if the motor is getting air instead of gas it will die or not start. Could it be as simple as replacing the bulb?

If you never had a carb apart dont do this one.I guess I got a little techinal about this.I should have explained the float's have to be set to spec's.Most all onboard vent's cant be closed.If you have a portable tank like the one I fixed was a portable.They have a vent valve and if it's not open it wont run long.I would say if the plug fire's the oil may be the problem.As far as the gas in the bottom of the motor goe's the overflow will cause that along with numerous other thing's.I'll give some good advice now,take it to a mechanic.

I just had a pretty much identical problem with my 25 HP Merc. It would cut off on me at low speeds, would get bogged down after running a while, and finally got to where it would not run for more than a couple of minutes. Turned out that the motor was not cycling water and was not cooling properly and was causing the motor to seize up, which was an impeller problem. Get a repair manual for your motor ($20-$30) and it will guide you through how to get to the impeller. Check to see if it is worn or damaged at all. However, you really may want to consider saving up a little bit and have a mechanic take a look at the carb as well because they probably need a cleaning beyond what a spray carb cleaner can do. Also, if you take out the plugs to see if they generate a spark, which you should do, MAKE SURE you have the plug grounded against a metal part on the motor. If you don't it'll shock the daylights out of you (speaking from experience). Good luck.

Black Crow, there is a strong stream of water coming out of the motor. If the impeller was the issue, wouldn't that be noticable by a lack of water spitting out? I've only had one occassion where that happened and it was when the air temp was below freezing and I quickly turned the motor off. When I restarted it the stream was fine, and has been since.
Thanks for the help and advice everyone. I'm sure a mechanic is in my not too distant future. I will try the spark thing, replace the plugs and the bulb and hose. Can't hurt after 6 years to replace the bulb and hose anyway. I'll do the things I know I can do without breaking it, and if that doesn't work I'll give in and let someone who knows what they are doing look at it. With $4.00 a gallon gas and all it is tough to have to drop money on boat motor repair right now, but you gotta have your priorities.....right?

My 50hp did the same thing and it was the power pack.

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Originally Posted by SpeckWick

I think I may have posted about this once before, but it's back, and worse. I have a 2002 Mercury 2 stroke 40 HP with auto oil injection. When I run in low RPM, it dies or runs really sluggish. I had this issue before and was advised to idle it up, which I did, along with changing the plugs and it seemed to work, but now the problem has returned. Only now I have trouble getting it refired when it dies. I have to put the throttle at least half way(in neutral of course) to get it to start sometimes after it dies or after it sits off for a while when I shut it off to fish. Also, late in the day last time out the motor ran extremely sluggish even throttled all the way. Almost like the boat was really loaded down. I took the plugs out today and found the lower plug had quite a bit of oil on it. I cleaned it off and plan to change both along with the oil filter, but I can't figure out why they keep fouling every six months or so. Is it because I need to run a higher grade gas? Could the oil injection be adding too much oil? Not enough gas through carb? Is there a carb cleaner that I can run through it that works at all? Sorry for all the questions. I really don't have the funds or time right now to take it to the shop. If it's something short of tearing the carb apart I can probably do it. Please help.

By the way, I use regular grade gas and Pennzoil Marine XLF oil, if that matters.

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Speck Wick,
You ask about changing the oil filter, I have the same motor but have not seen any oil filter on mine, ...:eek: where is it ?.

crappie cowboy

Speck, you need to verify what your problem is first. Electrical or fuel? Make sure you have a strong spark at all spugs. You can do this with a cheap spark tester available from your local auto parts store. If spark is good try pumping the primer bulb when it starts to die. If it acts like it wants to pick up speed then you have pin holes in the fuel pump diaphram. With the description you give it could be either problem.

Had some similar problems w/ a 90 Merc. Ended up being the fuel line from the fuel filter to carbs had broken down and small pieces of rubber had entered the carbs. Mech. said merc engines were bad about the inside of these lines deteriorating because of gasoline, especially in older motors. Mine is a 98. They have since started using a different type of line that does not break down inside.

It would not hurt a thing to dump a can of sea foam in your tank and see if that helps....If you have an onboard fuel tank chances are you vent will probally located in your gas cap on the boat....

some of the vents on the inboard tanks are just a hose that goes to the side of the boat to a screened vent. You can disconect it and see if its pluged up. If you have indoor outdoor carpet in your boat the little pieces of that stuff can get in your vent and plug it up. If some of the pieces got down in your tank through the vent it could have your screen in the tank thats right below your gas line coming out of the tank If it has a b-nut and elbow where you gas line come out of your tank take it off and see if its open and clean. had this happen on my 85 hp and it drove me crazy until I found it. i replaced the bulb and fuel lines and still had a problem. but found it and haven't had the joy of it comeing back cause I removed all the carpet and put TUFF STUFF in it instead. Sounds like you got a gas line plugged or its drawing some air from some where. GOOD LUCK.

I have a 2003 Merc 40 hp 2 stroke and had the same problem. In my case the problem was the valve in the hose line bulb had gone bad. Replaced the bulb and everything was just fine. I would try this first before spending a lot of money on other things.

I'm no mechanic, but if you can borrow a portable tank and run a new line an bulb to the motor then try it, you can either eliminate or narrow down the problem to the fuel delivery system. If it's starving for fuel, it could be a small crack in the line, or the bulb or a vent in the tank as was suggested. However, since you're fouling plugs, you may have some other issue as well.

Also, you mentioned a fuel cleaner. I use Seafoam in every tank and go through the decarb process as well. Once you eliminate the fouling issue, I would recommend you decarb it with Seafoam or something similar since the oil has not been getting completely burnt due to the problem.

Oh, and no one mentioned the fuel filter. It could need changing.

Wannabe...

Sounds like to me that the low speed jet on the carb is about half stopped up or the float is sticking open and flooding out. How does it run at WOT?


They say that a can of SEA FOAM in the gas tank is called a "tuneup in a can". Bout $8.00 a bottle and does the gas tank and motor well. Cleans the carb's as the motor is running.

dont know if this will help, but you could check to see if you have a knob inside the motor that will change the gas /air mixture. if your fouling plugs, it could be running to rich and just needs a little leaning out. the knob inside of mine is bright red

If oil on the plug that mean's that plug wasnt firing.Do the check test and let me know along with the other's the result's.I never use sea foam but it might be a good thing,I dont know anything about that stuff.If your gas and oil mixture is correct you should have no problem with gumming issue's.A worn or broken ring on the piston can cause this issue also as it will not allow enough pressure to push the gas into the chamber.Do the test with the plug first and let us know.

Got home late tonight and haven't had a chance to check things yet. Had to get the boys to a ball game. If I get a chance before dark I'll check the plugs and the vents and whatnot and get back. Thanks for hanging with me.

OK, just checked for spark. I have spark at both plugs. I started the motor. It started on the first crank. Ran for a minute in idle and died. When I tried to restart it, it wouldn't at first. I put it in neutral and opened it up. It started and sputtered and spit blue smoke, then ran okay, but not at low idle. When I was trying to restart it, I walked back and noticed gas coming from the front of the carb. Flooded, obviously. I have a neighbor who is an auto mechanic and finally caught him home and he seems to think it's the carb diaphram or float allowing it to flood. He says this will cause fouled plugs and all the other symptoms I have. The vent is clear, I have spark. I am inclined to believe he may be right. It all makes sense to me, but again, I'm no expert. Is he wrong? Any other things I should check first? I am hoping to limp through a tournament this Sunday and next Sunday then I have the whole month of July and August to get it rebuilt. Am I taking too big a chance to run it this way?

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Originally Posted by SpeckWick

OK, just checked for spark. I have spark at both plugs. I started the motor. It started on the first crank. Ran for a minute in idle and died. When I tried to restart it, it wouldn't at first. I put it in neutral and opened it up. It started and sputtered and spit blue smoke, then ran okay, but not at low idle. When I was trying to restart it, I walked back and noticed gas coming from the front of the carb. Flooded, obviously. I have a neighbor who is an auto mechanic and finally caught him home and he seems to think it's the carb diaphram or float allowing it to flood. He says this will cause fouled plugs and all the other symptoms I have. The vent is clear, I have spark. I am inclined to believe he may be right. It all makes sense to me, but again, I'm no expert. Is he wrong? Any other things I should check first? I am hoping to limp through a tournament this Sunday and next Sunday then I have the whole month of July and August to get it rebuilt. Am I taking too big a chance to run it this way?

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Yup, you need to rebuild the carbs using new needle and seats. They are really very simple to rebuild once you get them off. As long as its flooding the cylinder instead of running dry it won't hurt to run it. You might try adding some Seafoam to the gas. You might get lucky (and thats a big might).

I agree with IBNFSN and your neighbor.As long as the cylinder is getting fuel,oil mixture you will be ok.I would take an extra plug or 2 to limp through those tournament's but you should get the carb rebuilt soon.Be glad it wasnt a power pack lol.

Thanks for the help. My neighbor has offered to help me/show me how to get the carb off and rebuild it so hopefully next time I won't be totally lost when a problem arrises. I'll try the Seafoam stuff, too. I'll add some now, then once we get the carb rebuilt I'll start using it regularly. Question is how long has this problem been going on? I've been thinking I had the weakest 40 HP motor on the water and it may just be a carb issue all along. Thanks again for all the help. This site is the best.

I bought a new 25hp mercury a few year's ago and it didnt run right.I pulled the carb off and the float was close but not right.With a little tweaking it ran perfect.Not even factory made mean's it's right.Let us know if the rebuild help's.

Glad you got a good neighbor after you see him do it and check the float level to see if it is letting too much fuel in. you will see just how simple the carbs are on an outboard are to work on and you said you have some mechanical ability so the next time you will have the confidence to do it yourself.

GOOD LUCK AND GOOD FISHING.

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Originally Posted by SpeckWick

Thanks for the help. My neighbor has offered to help me/show me how to get the carb off and rebuild it so hopefully next time I won't be totally lost when a problem arrises. I'll try the Seafoam stuff, too. I'll add some now, then once we get the carb rebuilt I'll start using it regularly. Question is how long has this problem been going on? I've been thinking I had the weakest 40 HP motor on the water and it may just be a carb issue all along. Thanks again for all the help. This site is the best.

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Them 40 merc when tuned and tweaked right are not weak motors at all.

When you put the starter back on, don't over torque the bolt. Aluminum block threads are easy to strip. Experience here. Replace the jets if you can. One speck of anything, it want run right. Experience here too. I could not see the trash in mine. Soaking in carb cleaner did not fix the problem. New jet did the trick. Be sure to get the right jet. Been here too.

I have a 48 hp evinrude that acts the same way as you describe the 40 hp merc acts.

We found that when I shut down the motor and didn't run out the fuel in the carb... fuel would gunk up in the carb....looks like little balls...these would block fuel flow thru jets... this would occur some times in one or two weeks of sitting up.

If you have left fuel in carb between trips...you might want to open up carb and take a look for residue such as loose gunk in bowl. If its' there clean it out and re-assemble before spending $$$.

Regards

Smackover

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Originally Posted by Cane Pole

When you put the starter back on, don't over torque the bolt. Aluminum block threads are easy to strip. Experience here. Replace the jets if you can. One speck of anything, it want run right. Experience here too. I could not see the trash in mine. Soaking in carb cleaner did not fix the problem. New jet did the trick. Be sure to get the right jet. Been here too.

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Cane Pole, You make me laught, your so funny, I think everything has happened to you. That's not funny I know, but the way you talk about it is what's funny...your a hoot.

crappie cowboy

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Originally Posted by crappie cowboy

Cane Pole, You make me laught, your so funny, I think everything has happened to you. That's not funny I know, but the way you talk about it is what's funny...your a hoot.

crappie cowboy

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Over a 1/2 century of trial and error and Boy Scout training. Mostly error. Never got a badge in motor fixin'.:(

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Originally Posted by smackover

I have a 48 hp evinrude that acts the same way as you describe the 40 hp merc acts.

We found that when I shut down the motor and didn't run out the fuel in the carb... fuel would gunk up in the carb....looks like little balls...these would block fuel flow thru jets... this would occur some times in one or two weeks of sitting up.

If you have left fuel in carb between trips...you might want to open up carb and take a look for residue such as loose gunk in bowl. If its' there clean it out and re-assemble before spending $$$.

Regards

Smackover

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Thing is, would that cause the fuel to flood? I guess it would since the fuel coming in couldn't get out through the jets. I'll definitely check it out. When I do get the thing apart, I am going to replace as much of the carb components as I can. No sense going through all this then putting it back together only to find that other parts are bad and have to start over. If I'm gonna do it, might as well do it all. Even if it is just gunked up, a rebuild now will head off any carb problems for a while.