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Old 26-05-2022, 07:53   #1
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Outboard, too rich, too lean???

Mercury 2-stroke, 4-hp, six years old, Model: 1B680388
On the way to the dinghy dock, the engine seemed sluggish and as I slowed, it died. It started up but putting it into gear immediately killed it.
I re-adjusted the mixture screw, turning it out the required 1½ turns, then backing to and fro until it sounded good. Still died in gear.
The local guy at the dock (mechanic?) fiddled with the mixture screw and lowered the idle rpm slightly and got it running, perfectly it seemed.
The next day, wanting to see what he had done, I re-seated the mixture screw and was surprised to find he had backed it out 1/6th of a turn. Hardly anything.
The service manual says it's better to run a little rich rather than lean, but like I said, the engine is running pretty good right now.
So anyone have an opinion? Don't mess with it? Or try turning the screw out to enrich the mix?
Thanks everyone,
Bry
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Old 26-05-2022, 08:02   #2
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Re: Outboard, too rich, too lean???

If it's running well I wouldn't be too inclined to mess with it. With a 2 stroke a little rich is definitely better than too lean under load (for lubrication purposes). Won't matter as much at idle though.
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Old 26-05-2022, 08:12   #3
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Re: Outboard, too rich, too lean???

The dark art of carb tuning. I use the factory settings as a starting point, then tune until the engine tells you its happy. On my dirt bikes, they never came correct from the factory. I honestly think most ship based on the best emissions ratings rather than performance. I suspect the same goes for OB's. On dirt bikes, I used have to change pilot and main jets, then adjust the needle height and open the air intake, sometimes even drill out the exhaust to balance air in/out.
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Old 26-05-2022, 08:28   #4
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Re: Outboard, too rich, too lean???

Today’s oil is way ahead of two strokes.
You need less than any 1999 manual may say.
It sounds like fuel starvation not oil mix.
Assure you have a clean plug. Set a meter on Oms resistance and check the spark plug wire. A number like 120-140 is a crap spark lots of leakage.
Fuel left in float bowel too long turns into resin. Fuel pumps membrane or rotary give little warning and just fail. Half the fuel pump are flat and have an inner and outer membrane. They are common on twin and triples. The inner membrane has a tiny cut and the boat seems to have lost power doesn’t like to idle etc.
Last thought is neutral safety switch. Should be on the top of the engine. A micro switch to ground the spark during shifts. It’s to take the load off the clutch.
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Old 26-05-2022, 15:25   #5
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Re: Outboard, too rich, too lean???

If it runs good do not touch it, its running.
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Old 27-05-2022, 02:28   #6
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Re: Outboard, too rich, too lean???

If it ain't broke.....Likely the idle air jet, which us simply a drilled hole in the carb body is clogged. The usual setting of the idle mixture screw is closer to 1.5 turns. You're probably not helping your fuel economy by running it the way it is but other than possibly fouling plugs more frequently it may run fine. If whatever is blocking the passage comes loose you'll once again have running issues. It's too rich now which is why the mixture screw is near closed by virtue of idle air being restricted. Personally I'd clean the carb but shoe makers and shoes, plumbers and plumbing I'd bet I put it off until the engine starts stalling. YMMV.
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Old 27-05-2022, 04:11   #7
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Re: Outboard, too rich, too lean???

Quote:
Originally Posted by bcripps View Post
So anyone have an opinion? Don't mess with it? Or try turning the screw out to enrich the mix?

As posted upthread there may be other carb problems (partial blockages of non-adjustable jets or bypasses) at work. Carb problems tend to be cumulative--disassembly and cleaning every few years is sort of a routine maintenance item.


Mixture on carbureted engines is always a compromise and varies depending on atmospheric conditions. On a hot day you have to set the carb leaner than on a cool day to get it to run right. Altitude plays a role too, if you ever go to lakes well above sea level.
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Old 27-05-2022, 05:41   #8
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Re: Outboard, too rich, too lean???

E10 vs. gasoline. Which is it running on? E10 will require about 3% more fuel for the same mixture (1/6 turn is about 11% more area). And there is the matter of manufacturing tolerance. As Jammer said, these things can also be cumulative.
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Old 27-05-2022, 10:24   #9
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Re: Outboard, too rich, too lean???

Okay, thanks everyone. A lot to chew on here, some of which I don't understand completely. But hopefully the carb is okay. I bought it about six months ago as a backup. Installed it to verify fit and left it in place. Cleaned up the original, bagged it, and put it in stores.
With the mixture screw backed out only 1/6th of a turn, I can't help but feel the engine is running lean. But when I pulled the plug this morning, I found it to be a nice ash brown. So...
Tomorrow I'll back the mixture screw out another ¼ turn and if the engine still runs smooth, I'll leave it at that.
Thanks again, everyone. I'm learning just by reading your posts.
Bry
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