Outboard Idling Rough? Here’s Why & What To Do!

It’s really important that your outboard is idling at the right RPM, there’s nothing worse than a rough idle when you are trying to use your boat!

Outboard Idling Rough? A rough idle on an outboard is generally caused by a failed spark plug, a clogged or stuck open fuel injector, a clogged or dirty carburetor, or an engine that has an issue with the timing when dealing with a 2-stroke outboard.

Knowing why an outboard idles rough is only the beginning, here is what you need to do to figure out why and how to fix it!

What Should Your Outboard Idle RPM Be?

In order to get started down this road, you are going to need to know what your idle RPM should be.

This way you will know whether you are dealing with a high idle, or a low idle, as well as where you want the engine to idle.

Outboards will typically idle somewhere between the 550-950 RPM range. Depending on whether you have a two-stroke or a four-stroke. But we’ll get to that next.

This can be easily figured out though by looking at the tag that is on the engine. Most outboards will have a tag on the belly pan, or on the bracket.

That will tell you some information, including the engine idle RPM that the engine should run at.

Which is going to give you a starting point. That we will use in a minute.

What Kind Of Outboard Do You Have?

Now it’s important to note what kind of outboard you have. Mainly whether it is a two-stroke or a four-stroke, as well as how it operates.

Whether that be a carbureted two-stroke, a carbureted four-stroke, a fuel-injected two-stroke, direct injection two-stroke, or a fuel-injected four-stroke or direct injection four-stroke.

(Here is an article that will teach you about Fuel Injection Outboards Vs Carbureted Vs Direct Injection Outboards.)

These are your main types of power when it comes to outboards. We aren’t going to touch on electric engines or propane engines here today.

Though the principles will be similar for the propane engine. But completely different for an electric engine, because they operate in a completely different way.

What Maintains Your Outboard Idle RPM

Knowing what kind of power you are running on your engine will tell us what controls your idle RPM.

Because it can get tricky when trying to figure out a rough idle on certain models. Based on what is controlling that RPM.

But almost all engines maintain the idle RPM. By controlling the air that is being fed to the engine.

Most of the two-strokes and the carbureted engines control the idle RPM through idle stop screws and idle airscrews.

The throttle stop screws will mainly just adjust the opening of the throttle plates feeding the air to the intake manifold.

And the carburetors idle air screws will regulate that air that is going into the carb.

Then on the direct injection two-strokes, we will also be looking at those idle stop screws.

But when it comes to the fuel-injected four-strokes, and the direct injection four-strokes.

We are going to be looking at electronic throttle valves. Which maintains the idle RPM through the computer on the engine.

Controlling the electric throttle plate. As well as other components, such as idle air controllers, which regulate the air going into the intake manifold.

What Causes Your Outboard To Idle Rough?

But this is where it can get tricky. Because the cause of a rough idle isn’t always just a problem with one of these components.

Though yes, 50% of the time there is a clog in the idle air screws, a problem with the idle air controller, or the adjustment on the throttle stop screws is out.

These will all cause you an issue with the idle, and most of the time a rough idle, or a no start or running issue.

But you will find that many other times a rough idle is due to failed spark plug, a clogged or stuck open fuel injector, a clogged or dirty carburetor, or an engine that has an issue with the timing when dealing with a 2-stroke outboard.

When we have an issue with one of these components, it is commonly causing the engine to lose a cylinder. Making the engine not run on all cylinders, which we talk about in this article linked here.

Losing one cylinder can cause the engine to lose 100 to 200 RPM.

Then once it loses that, a lot of times, on the four-strokes, the engine will add fuel and open up the throttle. Or the idle air controller to get more air into the engine.

Which will cover up a lot of the rough idle that you would normally see. And this can make it tricky to figure out what you have going on.

What Can You Do To Fix Your Idle

So in order to fix the rough idle on your outboard. We need to isolate and figure out where our problem is.

We’ll do this by first doing a drop test. Where we just simply unplug a fuel injector, or the ignition coil while the engine is running. Which will drop out that one cylinder.

And we’ll drop out each cylinder until we find one cylinder that doesn’t affect the engine’s running condition.

This lets us know that that cylinder is where our problem is.

Then we can remove the spark plug to see what is going on. We can test the spark plug to see if that is the issue. Then we can run a spark test to see if that cylinder is getting spark.

And if both of those things are correct, then we can look at the fuel injector to make sure it is giving fuel.

As long as you have both spark and fuel, then we can move on from there.

When it comes to the carburetor, we can mix a little gas and oil in a bottle and just squirt a little fuel into the carb when it’s running. To see if that levels out the idle.

That will let you know if the carb is not supplying fuel to that cylinder.

And this will leave us with just having an air supply issue.

Which can be a problem with the idle screw adjustment, the idle stop screw adjustment. Or an issue with the idle air controller.

But you’ll need to look into the service manual for whatever engine you are working on to see the procedure for setting and testing those components!

Check Us Out!

We hope that this article has sent you in the right direction to figuring out and fixing your rough idling outboard!

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Here are some other articles that you might find helpful as well!

Aaron Hilligoss

Aaron has been working in the Marine Industry for over a decade and holds certifications for Yamaha and Mercury Marine. It is not uncommon for him to own and be working on at least three different boats at any given point in time!

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