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Go Back   SailNet Community > General Interest Forums > Gear & Maintenance
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Old 05-16-2011
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Gas engine won't idle

I've got a Volvo MB10a gas engine (1978?) in my Irwin 28 that stalls when idling and/or in neutral. The engine starts great and runs good at just above idle speed, but as soon as I back the throttle all the way down or shift into neutral it slows wayyyy down and eventually quits after about 10-15 seconds.

Any ideas on where to start trouble shooting? The carb has a air/fuel screw adjustment as well as an idle screw, but I haven't messed with either yet. Also, I haven't checked the fuel filter yet either. To do so, should I shut of the fuel line and then run the engine until she starves of fuel before I open up the fuel filter canister to check the filter?

Thanks for your help!
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Old 05-16-2011
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What has changed since he last time it ran well? How old is the fuel?
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Old 05-16-2011
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Could be something as simple as a carb adjustment idle speed and/or mixture. When was the last tuneup - plugs, etc? And as noted, old fuel could be the cause. Did you use fuel stabilizer when you winterized last fall?
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Old 05-16-2011
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The first place I would look is the idle circuit in the carb. I don't know how simple it is to disassemble, clean and reassemble, but....

The tiny jets in the idle circuit are particularly prone to getting clogged with gunk just because they're tiny. Sounds like your main jet is fine, as it runs well under load, but is being fuel starved at idle. Just a guess. Good luck.
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Old 05-16-2011
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Before you start taking things apart try screwing the mixture needle
all the way in then open it up about 1-1/2 turns. if it keeps running
screw it back in until it leans out enough to make it stumble. Then open it enough to where it idles smoothly then just a little bit more open. A
slightly rich idle mixture won't hurt anything. If that doesn't work try
mixing up 50% Barryman's B-12 fuel additive and gas in a separate
container and run it through, let it set for a day or two and see what
happens. If none of that works get out the tools and take the carb
apart. Gas left in the carb for long periods of time will eventually
evaporate leaving bad stuff in the float bowl to be picked up and stuck in
the jets. Draining the carb can help eliminate that.

Dabnis
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Old 05-16-2011
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I first noticed this issue late last season, about a week or two prior to haul out. At the time we had just completed a 3 month 1000nm cruise with no idle problems whatsoever.

I used Sta-bil fuel stabilizer on a full tank of gas at lay up time last fall. The fuel currently in the tank was mostly purchased from last September. I changed and gapped the plugs this spring before launch. The points were adjusted by a mechanic early last spring.
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Old 05-16-2011
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like above sounds like a clogged idle jet. Cleaning carb and readjusting idle is the only way your going to fix this.
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Old 05-16-2011
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either the fuel filter is clogged or the idle circuit in the carb is clogged/fouled...either will give the results you describe
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Old 05-16-2011
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so far good ideas have been given (it does sound like the idle circuit might be clogged)for future refference drain the carb at the end of the season so the fuel doesn't turn to varnish in the carb & cause things to plug up
(When you put sta-bil in the tank it must mix with the fuel & then run till it reaches the carb or it does no good for the fuel in the carb)
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Old 05-16-2011
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I would think the only hard part would be getting the volvo carb kit

When i did my A4 carb and fuel pump the auto stores still sell a one gallon soak can with a wire basket inside it which will do a much better job then trying to use spray cleaner
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