Will An Outboard Run With Low Compression?


Replacing an outboard is an expensive job and task to do. Outboards aren’t cheap and it’s hard to hear the words, “blown up.” But what does that mean, what exactly is, “low compression?”

Will An Outboard Run With Low Compression. Yes, an outboard will run for a little while with low compression. If the compression is too low though, it will not run at all.

Here are all of the things that you need to know about low compression on an outboard engine, what they mean, and how they affect the running of the engine!

What Should The Compression Be On An Outboard?

Compression numbers are going to vary greatly, depending on certain variables. Things like whether the engine is a two-stroke or a four-stroke.

How old is the engine? What brand and horsepower is the engine? Then there are also things that you wouldn’t commonly think about. Like putting the control handles in the wide-open throttle (WOT) position.

Compression on an outboard

Putting the handles in neutral and in WOT are going to open up the throttle plates and allow the engine to get more air.

For a general number scale for compression. Two-stroke engines should be somewhere between 100-120 PSI per cylinder.

Then four-strokes are going to be somewhere between 150-210 PSI! This is where the throttle position comes in. On say a 250 HP Yamaha four-stroke.

If you do a compression test, (click here to learn how to perform a compression test.) and you have the controls closed. You will see a good compression reading of 150-160 PSI.

Then, if you put the handles in WOT. You will see those compression numbers come up to around 180-190 PSI!

One thing that you want to make sure of, is the difference between the cylinders. You don’t want to see more than 10% difference between the different cylinders.

So if you have 110 PSI on one cylinder, you don’t want to see another cylinder with 70 PSI!

What Causes Low Compression?

There are many, many different things that can cause low compression. This will also vary depending on whether you are talking about a two-stroke or a four-stroke.

On a two-stroke, you can have a leak in the head gasket. The rings on the piston could fail. The rod bearings on the crank shaft could also fail and begin to create an issue.

Low compression on a four-stroke

We’ll cover whether you can fix these issues in a little bit. Because when it comes to a four-stroke. The issues can be the same as those of a two-stroke.

But now you have the introduction of valves. Carbon build from low idle can cause the valves to not seat properly. Which will bring the compression to a point.

It will also increase the leak down of the engine. Which simply means that when the cylinder has pressure in it (compression) that it leaks a certain percentage of that compression out.

This can be on either the exhaust side of the valves. Or it can be on the intake side of the valves.

What Are The Symptoms Of Low Compression?

You can probably see that low compression has many different factors that affect it. This goes for the symptoms as well.

Most commonly, when you just have low compression. You will have a loss of power. Say one or two thousand RPM’s from your top end.

Symptoms of low compression

It could also be a poor hole shot. As well as rough running idle and a loud noisy running engine when it is idling.

These are also the signs for many other mechanical issues that could be going on with your engine. So don’t just to the conclusion that you have low compression if your engine is just running rough or poorly.

Here are more resources of articles that we have put together to help you figure out what is going on with your engine:

Why Does My Outboard keep Stalling & Why Outboards Run Out of Fuel

Why Won’t My Outboard Boat Engine Turn Over!?

Can You Fix Low Compression?

This is the number one question that anyone that has an engine with low compression asks. The answer is yes, sometimes it can be!

Depending on when the issue is found and the severity of it. Sometimes you can do things to bring up the compression and fix the issue.

You can disassemble the engine and change out bearings, rings, rods, and other components that could have failed. Replacing the head gasket and honing out the cylinders is common.

Fixing low compression

This is the usual fix for two-strokes and usual cases for fixing the issue. You just need to make sure that you weigh the costs of doing this work. Sometimes the cost of the parts and the mechanics time.

Can be more costly to do than it would be to replace the engine. It all depends on what the issue is and how bad it is.

When it comes to four-strokes and valve issues. Like the carbon build up. There are products that can be purchased and added to the fuel to help clean out that carbon.

The two most common and effective additives are Mercury Quikcleen and Yamaha Ring Free. Which we have linked to on Amazon so you can check them out if you would like to get a bottle or two.

They are great to run in the engine regularly to keep it clean. But you can also mix them up heavily and run that mixture through the engine to help burn out any big carbon build ups.

Will A Motor Run With Low Compression?

Since you can sometimes fix an engine with low compression. You should know that it will actually run as well!

Obviously it will depend on how low it is and how bad it is. Also what the issue is too.

If it is just low because of carbon build up or even a bad bearing. The engine will start and run, to a point.

Just be aware that it is definitely not good for the engine to run like this and if you run it for too long. Then you will eventually blow the engine up, destroying it, permanently!

So if you suspect having low compression in your engine. Check it out before you just run it until it blows 🙂

Check Us Out!

Now that you know if an outboard will run with low compression. Here are some other extremely helpful and informational articles you will like!

Why Does My Boat Battery Keep Dying? How To Fix It!

How To Change Outboard Prop Shaft Seals, The Easiest Way!

The Best Guide To Learn How To Dock Your Boat Safely!

We’d also like to invite you to check us out on Our YouTube Channel! We create all kinds of how-to and DIY videos to help you learn more and more about your boat and how it works!

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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