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hanson

Merc 40hp "issue"...

21 posts in this topic

Was reading in a separate topic about a Merc with "bogging" issues and was wondering if I had similar problems.

Got my grandpa's Lund Stinger sitting in the driveway. Its got a Merc 40hp tiller on it. Motor is a bit newer than the boat, probably early 90s. Merc 40hp is 4 cylinder, twin carbs, power tilt & trim, etc.

The boat has 2006 license on it so it hasn't been run in 3 years at a minimum.

When I brought it back to my place, it wouldn't run at all. Quick shot of fuel into the carbs from a spray bottle fired it up but it would kill. After doing this repeatedly, I knew I had to clean the carbs.

So the carbs came off and I ripped them down spraying them out good with carb cleaner. Removed the floats, removed the needles, sprayed the fuel inlet valve out good and sprayed all the jets and passages out good. IMO... a good carb job.

When I got them back on the motor, I could get it to run at only a higher idle. I can throttle the motor up to full throttle and it seams to purrrr like a kitten.

Problem is trying to get the motor to idle down. I can now back the throttle off halfway between the "start" position and the "idle" position on the tiller handle. Any closer to idle than that and it chokes the motor out. IMO, its starving for fuel. I than have to choke it to start it up again.

I've been putzing with this thing for 3 days trying to get it to idle down but it won't do it. Now just today, I tried shifting into and out of gear and throttling it up and it bogs bad in gear when I throttle up.

Reading in this other topic, a gas can vent screw has been mentioned as a possible problem. Does this motor require a can with venting? Grandpa just gave me an extra gas can to use and have thus far assumed it would work.

My family history is with Johnsons and have never had to unscrew a vent screw.

My assumption has been my carb cleaning didn't go as well as I wanted it too. I don't really feel like pulling the carbs again as you have to take the starter, oil tank, and fuel pump off to get to the carb mounting bolts.

I have taken the fuel pump apart which looked fine and put a new fuel filter on.

Like I said, seamingly runs great at high idle but low idle and throttling up are problematic.

Cylinder compression is 100 per cylinder. Don't think I have an issue there.

I'm just flat out stumped now and need this boat running by Thursday night. frown

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so the gas tank you are using isn't vented? i would try to run the motor on a gas tank that is vented... that kind of makes sense to me because when the motor is running on lower idle like you said, it dies out, which makes me think there might be a need for a vented gas tank to keep the right pressure in the gas tank for the gas to easily flow thru. like you said, when you get up to past half throttle it runs fine maybe because as the motor gets more gas, it sucks more gas out of the tank?

i don't know, just a thought. i'm not a motor junky or anything but i'm thinking give it a try with a vented gas can screw

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Motor junky or not, any brainstorming that will help or not is good!

I swear... I've never heard of a 6 gallon outboard can with a vent screw on it. Thats why I'm asking. And wondering if that might be the problem.

Sort of wondering if Grandpa gave me the wrong can as he has about 6-8 six gallon cans sitting around for several boats, a couple with older Johnson tillers.

I can post a photo of this morning in the morning to get a better idea of year.

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How many turns out are the idle mixture screws? they should be about 1 1/2 turns out from lightly seated. Personally, i dont think it would be the tank or vent, you can disconnect a fuel line and it will run as long as there is gas in the float bowls. From the description you gave, it will run at higher rpms but as soon as you try to idle down it dies right away, right?. Doesnt sound like fuel delivery to me. I'd say you either have the idle mix off or there is still some crud in the idle circuit of a carb. Another thing that comes to mind is the pick up timing. Has any one messed with any of the timing adjustments?

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Boatfixer... your echoing the thoughts I've had in my head for a day now. I want to make sure I'm not overlooking something stupid as that is the way things go sometimes.

I'm cleaning the carbs again tomorrow night as I'm convinced that is still the problem.

I'm fairly certain no one has messed with the timing. This boat has been parked for 3 years and as far as I know, it hasn't moved since it was parked.

I will check the idle mixture screws as well. If I'm not mistaken, there is only 1 screw that you can adjust on these carbs. Correct? If not, which one am I looking for?

You are right, motor seams to run great. But I keep trying to idle it down lower and lower with the the throttle, the idle will get lumpy sounding and then kill. No fuel delivery on the low end.

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I've got the same outboard and its a darn good one.

Not all tanks have a vent screw and use a valve in the cap.

Just to be sure there isn't a vacuum, crack the cap open then shut it. It almost sounds like your winding the outboard up without a load on it. You should keep the RPMs down.

First verify the hose is on right with gas flowing through the bulb in the correct direction

Pull and inspect the plugs. Are they fouled, wet, or dry.

So you have a loss of power and can't idle. Squeeze the buld up and see if the engine picks up power. If so you have a air leak at a connection(s) restriction in hose, bad or broken dip tube in the tank, fuel pump, filter,

If you pump the bulb while running and there is no difference and you know for sure there is gas in the bulb then it eliminates the above and leads to but not limited to one or both carbs. When cleaning a carb its the tiny passages and circuits the usually need the cleaning most. What do the plugs look like? To be sure, replace them. Have you checked for spark on all 4 wires?

Adding.

Have you had this on the water. If so how was the upper end power?

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Adding what happened to me after I cleaned the carbs on a Evinrude 25hp... same symptoms after I got the bowl on right side up wink

but what fixed it for me was I unscrewed the needle and took a air compressor and gave a shot of air, and then all was well with the world. It then ran like a Mercury wink

I thought I had cleaned everything good with the carb rebuild, but I must have got a grain of sand or something in the jet, as I put it together on the dock... doh!

Good luck Hanson! It is frustrating, huh?... Especially when I am not a mechanic but thought I did everything right - but didn't, ha! smile

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I swear... I've never heard of a 6 gallon outboard can with a vent screw on it.

All of them that I have seen/used have a vent screw on the cap.

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I've seen them both ways with respect to a vent.

Chris - when you cleaned them, did you use air as well, or just carb cleaner?

marine_man

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Well... I think I got it. grin

Running over to the local lake very first thing tomorrow to give it the water test but I'm happy with what I see in the driveway.

I tried a few of your suggestions before ripping into the carbs again. Fuel can with cap on & cap off made no difference at all. Also couldn't tell a noticeable difference when pumping the primer bulb and when on low idle, pumping the primer bulb made no difference as it eventually sputtered out.

Then I began the carb project again. Start to finish, this has gotta be about a 2-1/2 to 3 hour job. I'm sure the mechanics in the house can bang it out but there is no way to quickly loosen or tighten some of those nuts, then keeping track of hoses, making sure everything is secure, mixing a new drink, etc.

Ripped both carbs apart again, sprayed every single little passage out with both carb cleaner and air, and put it back together.

Guess what? It runs this time.

I still can't leave the motor alone on the lowest of low idles but an 1/8" turn of the tiller handle above the lowest idle and it runs just fine, a very low lumpy putt, putt, putt idle that I'm happy with. Shift it into gear, give it slight throttle and no bog at all. Happy with that too. And... I can restart the engine without choking it now as well. Was unable to do that previously.

So I think I got it, water test in the morning will be the major hurdle but I think it will be fine now.

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Good deal Chris! Sounds like deja vu of my carb job earlier this spring smile Second time is the charm, ha!

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Was hoping it was cool but its not cool. Just got back from the local lake and I couldn't even get the boat out of the boat ramp. frown

Seams to start and idle just fine. However, put it in gear and she dies on me. Every single time, forward and reverse, for the half hour I was there putzing around with it.

I about give up. frown Its not my boat, I'm just trying to get it running for my favorite tournament this weekend as my boat motor isn't running either. Thing that sucks is we're registered and paid ($400) for the tourny and its non-refundable if you drop out. We were registered back in Feb or March, before my motor started making noise on me at the Sturgeon Excursion.

I really hate having to pay to fix someone elses (even my grandpa's) motor so was hoping I could get it done on my own. Beyond carbs, I'm lost. If I was made of money, I'd have everything into a shop and fixed but I'm not, and its getting frustrating.

I've pretty much run out of time to work on this thing as well. I've got maybe 2-3 hours tonight after work.

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It shouldn't have a "very low lumpy putt, putt, putt idle"

Its a 4 cyl and should idle smooth. Could be your not banging on all 4. Is there a sneeze mixed in there or do you hear a cyl come alive once in a while? Did you test the gas for water?

Did you pull the spark plugs and what do they look like. Really need to know that. Depending on what you find there it could be as simply as new plugs, or low speed adjustment. Would also help to know if you tested spark on all 4.

Really need to know what the plugs look like.

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I think you are probably right Frank on one cylinder not firing. Its a bit tough to tell on a 4 cylinder to the untrained ear. It was "sneezing" a bit.

The gas shouldn't be the problem. I just put 5 gallons of fresh gas into an empty can and have been using that.

I did put 4 new plugs in as well.

I'll probably have to do some investigating to find out which cylinder isn't firing tonight.

I do know this, about 8 years ago or so, I was up fishing Mille Lacs with grandpa in this boat and we had motor problems. Sort of the same deal if I recall correctly and the mechanic that helped us replaced a coil. I wouldn't be a bit surprised if its the same one.

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Hanson,

I had that very same motor and understand well the possible carb issues. From your descriptions above you have a problem with foreign material plugging up the low speed jet circuits on one or both carbs. Since the last carb cleanout cured the problem, but it came back once you ran it for a while, I bet you have old dirty gas in the tank, or in the fuel delivery circuit somewhere prior to the carbs. You initial comment that it had not been run for three years is the clue here. You may have decomposed fuel anywhere in the system that is working it's way to the carbs and plugging those low speed jets.

I would do the following;

1. Dump the gas tank and clean it thoroughly to make sure there is no gummy residue in it. (BTW the tank may have a vent screw on the cap. That needs to be open when running, but is not what is causing this problem) Fill it with clean gas and put some Seafoam in the mix to help clean things out as it runs.

2. Purge the fuel line from the tank to the connector.

3. Look closely at the fuel lines on the motor. There is a fuel filter screen that you can clean out. As I remember, it is on the fuel pump. You may want to take that apart and make sure there is nothing loose in there.

4. Hook the fuel line back up, take the lines off at the carbs and pump gas through to purge the system again.

5. Last, unscrew the low speed needles all the way and pull them out. Purge the system again to clean out the carb circuit. Put the needles back in, lightly screw them in and back out to adjust. Test run and if the problem remains, try screwing the needles in and out a couple of times to move any obstruction. Test run again. If the problem persists you will need to pull the carbs again and clean them out.

This motor should idle with a smooth purr when set up right so keep at it!

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check your float level one might be to high which the engine would be flooding it self but would run better going wide open i have played around with a simlar motor the best way is to take the carbs off again and hook up the gas line directly to one of the carbs just remove the t connector then hold the carb in your hand and squeeze the bulb if it leaks real bad either your seat is letting gas by or your float is set to high it is very easy to bump the tab when you tore it down i has a simlar thing happen last year also replace your hose with a merk one or at least get your fitting from them the after market one are not that good

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Good Advice Guys...

Chris - on the coil... get a set of insulated pliers, and while the motor is running (on the hose or in the lake) pull one cap wire off at a time. If you find one that doesn't affect how well the engine idles / runs you've found the problem cylinder.

You can try and swap coil packs around to isolate if it's the coil pack or something else.

marine_man

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Temporary solution... good buddy is lending me a boat for the tournament this weekend.

Grandpa's motor is going to wait until I get back.

I must say, there has been some fantastic help so far in this thread, both with ideas and methodology. Thanks again so far, we'll pick the discussion up again next week. smile

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toasted head gasket?? what did the plugs look like after running??

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