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Mercury Idle Adjustment?


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I have a 90 HP Mercury (year 2000). The last couple of times out it has idled pretty rough and cut out a few times while at idle. Last 2 times out it was pretty chilly (and boat is stored in unheated garage) maybe the engine is a little tempermental but even when I let the engine run for a couple of minutes and then kicked it into forward gear the engine cut out.

So I removed the cowling and on the engine I see the "Idle Screw" but my manual does not say anything about adjusting the idle or what that "idle screw" is for. Can adjusting that screw help the engine idle a little better? Does anyone know how/what adjustments can be made? I do not even know is the screw should be turned clockwise or counter clockwise to improve the engine's idle performance.

Any thoughts or has anyone utilized this screw to adjust their own Mercury engine?

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Have you cleaned your plugs or changed them out . I leave the carb screws alone and try mine by the Idling arm it helps advance your timing on the side where the cables coming in. When you move the throttle cable does it idle better if so I only adjust those two. And try to synchronize them.

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Adjusting the idle screw won't help it idle better. Like Tim said, rough idle is usually caused by dirty carbs, bad plugs, bad fuel or starving for fuel. Making it run faster won't fix it, it might hide it. I'd change the plugs and fuel filter first.

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If you motor idled well previously, but now it doesn't, adjusting timing and cables probably doesn't solve your problem. That screw has something to do with adjusting the idle, maybe speed, maybe mixture, usually in conjunction with other specific steps. You don't know how to do it (nor do I), so find someone who does ... :471_confused_face:

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  • 13 years later...

Adjusting the idle screw on your Mercury outboard can indeed help with idle performance. The idle screw is responsible for controlling the engine's idle speed. Turning it clockwise generally increases the idle speed, while turning it counterclockwise decreases it.

To address a rough idle or stalling issue, you might want to start by turning the idle screw slightly counterclockwise to reduce the idle speed. Make small adjustments, and test the engine's performance each time. Additionally, it's a good idea to ensure that your fuel system is clean, including the fuel filter, and that the spark plugs are in good condition.

Always refer to your engine's manual for specific instructions related to your model, and if you're unsure or uncomfortable making adjustments, it's advisable to consult with a professional mechanic or your local Mercury dealer. Adjusting the idle screw can be a simple fix, but it's important to do it with care to avoid causing other issues.  Just sayin...

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