Outboard stutters in mid rev range

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labougie

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I have a single pot 2-stroke 4hp Mariner (Mercury). The idle, slow running and WOT are perfect but in the middle of the rev range with a constant throttle setting, it stutters, slows down by (I'd guess) 100rpm, then gradually speeds up to the original revs, only then to stutter and repeat the symptom ad infinitum. My first thought was spark plug, but putting a new one in (BH7S-10 as per manual) has made it only marginally better. The spark plug that came out was pretty clean. The motor (although mid 90s) has had almost no use.

Can anyone please suggest the next thing to look at? Thanks in advance.
 
Correctly gapped the plug?. The one you mentioned is supplied gapped at 1mm ( that's what the -10 suffix to the number means) Not checked but would expect 0.030" or 0.7 to 0.8 mm.

It would be nice to check the spark will jump 10mm.

I 'd be checking the fuel system I think. In line filter, fuel pump, carb even....... gently choking make any difference?

You dont say if you are using an internal or remote tank. If remote does pumping the fuel line bulb help at all?

presumably using fresh fuel? 50:1 ????

A pro would do a compression test most liikely
 
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OK, fuel tank is internal ATM, fuel is fresh and 50:1. The manual says 10mm gap,but I'll check next time that there's a good spark at 10mm - I'll also try gentle choking which I haven't yet and get a compression figure. Thanks
 
dirt in the mid range jets.

The new E5 petrol has 5% alcohol which is very aggressive on older components, along with altering the fueling requirements. corrodes aluminum alloys, and affects some brasses. .



Strip and clean carb in ultra sonic bath.

I would also look at the reed valves behind the carb, if fitted on that model, as these might be sticking, but not until the carb is completely clean.
 
labougie said:
OK, fuel tank is internal ATM, fuel is fresh and 50:1. The manual says 10mm gap,but I'll check next time that there's a good spark at 10mm - I'll also try gentle choking which I haven't yet and get a compression figure. Thanks
Click to expand...

You are right about plug gap ... 0.040" or 1.0mm
 
Dont have a diagram, for your model but usually there are a number of jets , some being behind or below a larger one, along with the small holes in the emulsion tube all affect mid range running.
 
I have had 2 strokes that stuttered a wee bit at constant throttle, I found ignoring it to be a reasonable approach as the engines kept going! A trick that used to work to clear jets on cars was to rev the engine and put your hand over the intake briefly a few times, theory is the engine sucks the jets clean.
 
VicS said:
Do you have a link to a diagram that shows what you mean by mid range jets
Click to expand...
Diagram here. Just pilot, main and emulsion tube. http://www.marineengine.com/parts/mariner-outboard-parts/4-2stroke/0p401000-up-bel/carburetor

Maybe unlikely, but I think that the fuel level may have a bearing on this. If the needle valve is not releasing until the float level has dropped enough this could lead to the surging. A slightly sticky needle (whether brass or rubber) can be cleaned by twizzling it in some very fine (600 or finer) wet'n'dry.
 
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Lakesailor said:
Diagram here. Just pilot, main and emulsion tube. http://www.marineengine.com/parts/mariner-outboard-parts/4-2stroke/0p401000-up-bel/carburetor

Maybe unlikely, but I think that the fuel level may have a bearing on this. If the needle valve is not releasing until the float level has dropped enough this could lead to the surging. A slightly sticky needle (whether brass or rubber) can be cleaned by twizzling it in some very fine (600 or finer) wet'n'dry.
Click to expand...

And check the little clip #7 is in place if it is a soft tip needle.


But was wondering if it is a Mariner badged Mercury as the Op implies if it has a Tillotson or Walbro carb ?????

http://www.marineengine.com/parts/m...8281-and-up-bel/carburetor-assembly-tillotson

http://www.marineengine.com/parts/m...9258281-and-up-bel/carburetor-assembly-walbro
 
labougie said:
Thanks for all the input. My carb is here:

http://www.boats.net/parts/search/Merc2/Mariner/4-5/9589154%20THRU%200P016999/CARBURETOR/parts.html

I'll probably be down at the boat in the next few days to see if light choking makes any difference and I'll report back.
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That looks like the same carb as the one Lakey found ... I was put off by the serial number on his link.

Just wondering if when you say stuttering it's 4 stroking. A two stroke can do that if its running slightly rich and they tend to do it when lightly loaded rather than fully loaded in other words at part open throttle but not at WOT

My Seagull does it. I've leaned to mixture out to minimise it but I can't prevent it totally without making the mixture too weak at slow speed. It's a regular and steady da,da,da,da,da instead of a continuous brrrrrrrrrrrrrrrrrrrrrrr.

You might check the float level.

There is no high speed mixture adjustment but you might try tweeking the slow running adjustment. With it idling and slowed right down screw the idle mixture screw in until it just starts to falter then back out until it just runs sweetly again. Readjust the idle speed.
 
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Right, giving it a little choke improves matters slightly, but it's still not running smoothly. The in-line filter is dirty so I'll replace that and take the carb off to give it a cleanup. Compression is 135psi - healthy enough?
 
labougie said:
Right, giving it a little choke improves matters slightly, but it's still not running smoothly. The in-line filter is dirty so I'll replace that and take the carb off to give it a cleanup. Compression is 135psi - healthy enough?
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135 psi is very good. Maybe your gauge reads high!
 
In-line filter is unlikely to affect only the mid-range. Wants changing though. Giving a bit of choke and slightly improving it still makes me think your fuel level is varying (the extra suck when you choke the inlet increases the suck in the emulsion tube).
Pay particulr attention to the needle valve and seat. Use a magnifier to inspect the needle tip and if it has those flutes on the side of the needle body give them a good clean.

See the notches on this one?



oldneedle.jpg
 
VicS said:
135 psi is very good. Maybe your gauge reads high!
Click to expand...

Funny - that crossed my mind when I got 150psi from a mid 80s 4hp 6E0 Mariner. It's just an ordinary automotive Gunson gauge - who knows these things?

I'll probably take the carb off and have a look/clean over the weekend.
 
Lakesailor said:
In-line filter is unlikely to affect only the mid-range. Wants changing though. Giving a bit of choke and slightly improving it still makes me think your fuel level is varying (the extra suck when you choke the inlet increases the suck in the emulsion tube).
Pay particulr attention to the needle valve and seat. Use a magnifier to inspect the needle tip and if it has those flutes on the side of the needle body give them a good clean.

See the notches on this one?



oldneedle.jpg
Click to expand...
Are those notches on the flange nearest the camera normal or the result of wear?
 
It all seems to be very clean in there. I was expecting some gunge in the bowl - none. Main and slow jets are clear and needle valve looks fine:

DSCN2039.jpgDSCN2037.jpg

I couldn't get at the seat but I'll look again. All in all, it seems to confirm what I was told when I bought it - it's had a couple of hours use from new.

One thing puzzled me - when I took the plug out (new, run for 15 mins) to check the compression half the plug was still clean, while the other half was oily. I'll try and get a pic up if it sounds like it might be relevant. I'll probably try it with the new fuel filter tomorrow.
 
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