Outboard not idling

[Interim]

My new boat (94 Mac S) came with a 1998 yamaha 9.9hp outboard.

I can get it to start, but it doesn't idle. If I keep nursing the throttle I can keep it running, but left alone at a given rpm it will die within 10 or 15 seconds.

Here's what I know:

It sat without running for up to a year. I replaced the fuel with 87 octane (no ethanol, no outboard fuel additives). The spark plugs look ok (not fouled, but not sparkling clean either). The engine looks clean (no gunk or dirt on the carb, etc). I sprayed some carb cleaner in the intake anyway. I have been thinking about the fuel filter, but wonder if it would run at all if it were clogged.

Any ideas I should explore before I take it to a marine engine mechanic?

--john

[Sumner]

There should be two adjustments on the carb. One is mechanical and sets how far the throttle is open/closed when you let go of it. The other is a screw that adjusts the air/fuel ratio. I'd adjust those first. I adjust the mech. one first to get the engine idling at some steady speed above where it dies. Then do the air fuel screw and set it to where it drops off a good clean sounding idle if you take it in or out further. Then usually you can back the mech. one back down to a slower idle and repeat this until the idle is at the rpm you want it and the engine runs cleanly. I do this by ear but a vacuum gauge is used by some. It takes a bit to get a feel for it.

If you can't get the carb to idle correctly with those then I'd be taking it off and apart for cleaning. The outside doesn't tell you much about the inside. From what you are telling me I'd maybe find someone who has more experience in this or at least someone to help you.

Before doing the above if there are external or internal fuel filters they need to be replaced or cleaned.

Good luck,

Sumner

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[133bhp]

Idle jet needs removing cleaning. common on small engines due to size of the jets. even sitting for a week. On mine, I shut the fuel off, run dry, then drain the bowl. Now runs every time.

[seahouse]

Sometimes backing the screw out a few turns will open the orifice and allow a trapped chunk of crud to pass through. Then pull the cord a few pulls, and return the screw to the original position. It's a fast and easy thing to do to try first. Mark the screw so you can return close to the original setting. Screwing it in and counting the number of turns in (to determine the number of turns out) is a more precise way to do it, but it risks squishing the crud in tighter.

[grady]

I have a 94 9.9 and had the same thing happen. Probably going to need a carb rebuild kit. Got mine from Sim Yamaha. Take the carb apart and soak it in MEK for a week. Blow out every port with compressed air both directions. Should fix it.

[Interim]

Thanks for the replies.

Seems like everyone thinks it is a fuel flow problem (or at least that is the place to start). I can't find a choke on this; is it an auto-choke?

I'll look at the filter and the jets. If it goes beyond that we might be on the way to the doctor.

Thanks all.

--john

[tlgibson97]

Mine has a '96 Tohatsu 50 on it. When I got it, it ran like crap. Difficult to start, would die at idle causing some rough docking at times. I replaced the filter and fuel pump diaphragm and haven't had a problem since. No adjustments on the carbs. I still run regular gas with ethanol so I'm waiting to see how long it takes to eat everything up again.

Less likey would be an intake leak screwing up the mixture. If running through the fuel system doesn't pan out then that would be the next step.

[Don T]

Hello,
I'll add my FYI to the items mentioned above.
Old drag racing axioms: Ignition must be perfect before calibrating the carb. Carb must be perfect before calibrating the ignition.
1. Carb problem (most likely - common fail point) plugged idle jet or water in the bottom of the fuel bowl. Idle jet sits lower in the bowl than the main jet.
2. Make sure it has ample fuel supply getting to the carb. Test by using a fuel container set up to gravity feed the carb. If idle becomes stable you have air leaks or a pulse pump diaphragm / check valve problem.
3. If the timing at idle is too far advanced the engine will spit back and stop itself. It is a mechanical linkage on most small engines but newer ones may have electronic advance systems. If the plugs are fouled it will act the same way.
4. If the upper crankshaft seal is leaking, air can get past it and lean out the top cylinder. Makes for a rough idle and dying. Test by shooting some combustible carb spray under the flywheel (avoid getting any in the carb intake) and see if the idle speeds up momentarily.

Good luck

[sailboatmike]

I cant believe you guys would run 87 octane, in Australia we cant even buy fuel that low in octane. standard (regular) unleaded is 91 but the majority run 97 octane in their outboards.

We find the 91 goes "off" within a couple of months, turns a horrible brown and smells like paint thinner, the 97 lasts for around 6 months without issue, and we stay away from ethanol because it attracts water, not to say what it does to the inside of a motor not built to run on it.

For the few cents per liter we save on ethanol fuel, the extra maintenance cost outweighs it about 100 to 1.

Of course we are in Australia and your fuel maybe higher quality, but I do know that most newer cars (10 years old or less) from Europe and from Australia specify a minimum of 95 octane.

[Tomfoolery]

I believe octane ratings are determined using a different method between the North America and Australia. Regular gas in Australia is rated per the RON method for an 91-92 octane rating, while in the US it's the (ROM+MON)/2 method for an 87 octane rating. Should be essentially the same stuff, though, except maybe for additives, which can vary here by State or region, and time of year.

Edit: Corrected RON numbers

[sailboatmike]

Thanks for the clarification, yes in Australia we use RON rating, but the standard 91 RON is just rubbish here, I use it in my car because I burn it rather quickly (Jeep Cherokee for a tow hack), left a few months in a tank its just horrible brown smelly stuff, good for lawn mowers maybe (I lave a chinese 4 stroke lawn mower that I recon would run of diesel without a issue )

[Tomfoolery]

That's unfortunate. I store gasoline all winter in the OB tanks, plus another 10 gallons in metal safety cans, in case I need it for the generator. It's fine in the spring, though I do transfer it to other cans for the mower/tractor and other small engines and put fresh gas in for the OB. But it's always looked clean and clear after 6 months storage with Stabil.

[sailboatmike]

Australian 97 RON fuel is great and lasts ages, its the 91 RON thats rubbish

[Interim]

I put a little Stabil in my small engine tanks, unless I know they'll get used up in a few weeks. But snowblowers in summer and lawnmowers in winter get drained.

Back to the outboard, the manual says it can run up to 10% ethanol. If it would run, that is. I'll put a new fuel filter on tonight; if that doesn't do it i'll take it somewhere for a carb rebuild. I rebuilt the carb on an old MG midget, but the consequence of failure there is walking. On the water it might be more dire.

--john

[dlandersson]

My 50 HP Merc began dying at idle last year. I used some carb cleaner and the problem is gone (for now at least).

http://www.amazon.com/gp/product/B003ZF ... ge_o00_s00