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What’s Wrong with Your Mercury 20 Hp 4 Stroke? 4 Most Common Problems

mercury 20 hp 4 stroke problems

It’s frustrating to launch your boat and realize the motor won’t start. You have looked at all the possible reasons. But you’re still not quite sure about what’s wrong with your Mercury 20 Hp 4 Stroke.

So, it strikes you what are the common Mercury 20 Hp 4 stroke problems?

Well, there are quite a few. A common problem that you can face is when the outboard motor puts off or idles irregularly. It mainly happens due to the scarcity of pressure in your fuel pump. It can also happen when if the fuel tank air vent is closed and constricted.

This was just an overview of one problem that you might face. There is more to it. To know more about these problems and how to fix them, read along.

4 Common Problems with Mercury 20 Hp 4 Stroke

Mercury 20 Hp 4 Stroke

The Mercury 20 HP 4 stroke motor is equipped with an easy start and great fuel efficiency. Overall it delivers an excellent performance.

This outboard motor provides EFI with its advanced technology. Within the limits of this horsepower, no other outboard motor on market offers the same service. But still, customers encounter a few issues while using it.

Problem 1 : Outboard Motor Puts off Or Idles Irregularly

According to the Mercury 20 Hp 4 stroke customers, there are quite a few issues. One of the very common issues is often the motor puts off or idles irregularly. Also, read about Mercury 150 Four Stroke Problems.

While the motor is in neutral or the gear is not engaged, it can run very sporadically.

A lot of reasons can lead to this specific tumult. Therefore, you should identify the particular reason first behind this occurrence.

Reasons And Solution

It basically happens when the fuel pump is out of pressure or there isn’t enough pressure. Therefore you should first take a look at the fuel pump and try to solve the issue. This is similar to the problem of Mercury’s outdoor fuel pump.

Go through the repair service that comes with your outboard before you start the test. You will get to know about the specific specs once you have read the manual. And you will also become familiar with the procedures to fix the fuel pump’s problem.

Firstly, you need to turn off the motor’s power before you test the fuel pump. The cowl cover, the plug, and the spark plug wire boot must be detached and withdrawn.

Next, the compression gauge reading should be evaluated.  The compression gauge should be replaced if the value exceeds or is below 30 PSI. If the diaphragm is impaired or valves are unidirectional the fuel pump will be impacted. Therefore, carry out a proper investigation and reinstate the flawed part.

This issue can also arise when there is a blockage in the fuel tank is constructed.

So, the air vent positioned on the fuel tank needs a proper checkout. If the air vent is not open, it should be turned around to the open position. You should always ensure that the fuel tank air is kept open and away from friction.

Problem 2: Outboard Doesn’t Respond Upon Acceleration

Another common problem is when the motor refuses to speed up upon acceleration.

Often it can be noticed that the outboard motor stalls or staggers. It typically occurs when the outboard motor is being accelerated. And the motor also loses balance while running and spurs.

A lot of reasons can make this happen as the reasons are linked with each other.

Therefore, let’s try to identify the reasons and then get on with how to solve them

Reasons and Solution

Your outboard motor won’t run smoothly if there is no spark in the ignition system. It will be the same even if there is a little bit of spark. It can also happen if there is a problem with the ignition switch.

But first, you need to check if the ignition system is the chief offender behind this. You can verify this using a spark detector. There are plenty of spark detectors available in the market.

Next. a 3/8 inches gap should be left on the spark detector. And get your motor going to check if there is a spark in each motor cylinder. Your ignition system will be considered okay if sparks are visible in both of the cylinders.

Take a look at the yellow and black wires if no spark is observed in any cylinders. The wires should also be tested. The wires will also need a replacement if no spark is seen on the wires.

In such cases, the wire should be detached from the switch box and restored.

Carburetors can also be a potential reason leading to this occurrence.

Use the spark tester to observe the circumstances of the spark with similar procedures. If your carburetor has an overabundance of dirt it indicates two things. First, no amount of spark is observed in the carburetor. And also there is an enormous amount of grime that is stopping the valves from closing.

First, you should detach the fuel cap and evacuate the dirty old fuel. Then, the carburetor should be withdrawn and cleaned with a carburetor-cleaning solution.

Ensure that you do not take out the main jet while the carburetor is immersed in the solution. Let the carburetor be dipped in the solution for an hour. And then you can think of putting out the main jet.

Now, the fuel jet should be immersed and cleaned in the solution similarly.  To get rid of the excess gunk, spray pressurized air on the fuel jet.

Get a new jet and get rid of the old one if it seems to be dysfunctional.

Problem 3: Motor Losing Power More than Usual:

One of the common issues the users complain about is that the motor often crackles. Or it repeatedly keeps losing its power on a daily basis.

This issue basically pops out when the motor has been out of use for a long time.

Anyway, to get rid of this problem completely first you need to identify the reasons. And then you need to take action accordingly.

Reasons And Solutions

Motor Losing Power More than Usual

When the fuel condition of the motor is in a bad state these kinds of problems arise.

If your fuel mix contains ethanol that’s where your motor is struggling and encountering this. But you can totally fix this by using the fuel mix recommended by Mercury. You can take check out the troubleshooting johnson boat motors for better insight.

Mercury advises their consumers to use a Regular 87 Octane Minimum (R+M/2).

Often we see that the customers do not look properly into their fuel condition. This results in fuel getting cloudy and filthy. You need to change your fuel on a regular basis before using it.

A premium quality 10-micron fuel and r can stop your problems. Using a water-extracting filter will keep the water away from your motor.

Besides, whenever you’re filling the tank with new fuel utilize a fuel stabilizer of good quality.

These are the problems that you will face with your Mercury 20 Hp 4 stroke.

Problem 4: Check the Fuel Pump:

Fuel pump problems are not uncommon with small engines, especially those using gasoline. The problem can stem from a number of sources, but the most common cause is a failed fuel pump. When this happens, the engine will not be able to get the fuel it needs to run effectively.

In addition to causing poor running, a faulty fuel pump can also lead to other problems, such as decreased engine performance and emissions. If you’re experiencing any of these symptoms, it’s important to take action and find a solution for your fuel pump problem.

Reasons And Solutions

If you’re seeing decreased engine performance or emissions, it’s likely that the fuel pump is at fault. However, there could be other factors at play as well so it’s important to have your engine checked out by a professional. If you only notice the reduced performance when you’re driving in cold weather, then it might be time for a new thermostat or cooling system on your vehicle.

Once you know what the issue is, you’ll need to find an appropriate solution. One option would be to replace the fuel pump outright; however, this could be costly and might not be necessary if other solutions don’t work.

Another option would be to try fixing the problem yourself; however, this requires knowledge about mechanics and could also result in further damage or loss of functionality if done incorrectly. Either way, it’s important that you take action soon so your engine doesn’t get damaged even more.


How often should you change the oil in a 4-stroke outboard

Are Mercury 4-stroke outboards of good quality?

Yes, Mercury 4-stroke outboards are of really good quality. Mercury Marine has been very popular in the market for its outstanding performance and reliable features. The best part is they don’t require high maintenance and can be repaired easily

How long will a 4 stroke outboard last?

A normal outboard engine, whether two-stroke or four-stroke, should last 1,500 hours. This will sustain 7-8 years looking at the annual application of 200 hours per year. Nevertheless, replacing your oil every 50 hours of operation can extend the life of your machine.

How often should you change the oil in a 4-stroke outboard?

Most four-stroke outboards need oil and oil filter replacements every 100 hours. As a basis for the formulation for off-season storage, the oil is changed. Professional technicians recommend it if you intended to preserve your boat for the cold season.

Are Mercury 4 strokes engines reliable?

Mercury four strokes engines have been around for over 50 years and are still a popular choice for many small engine enthusiasts. They offer good fuel economy and easy starting but are not without their challenges.

Mercury engines can be quite reliable if they are kept clean and properly serviced, but they can also suffer from oil leaks, blown head gaskets, and rod bearing failure. If you’re looking to buy or restore a mercury four-stroke engine, it’s important to do your research first and make sure that the engine is fit for your specific use.


That was our take on the most common mercury 20 Hp 4 stroke problems. We hope now you are enlightened about all the reasons that are leading you to these problems. And the better outcome is you know how to fix them now.

A special tip for you will be is to make sure a fuel problem isn’t the culprit. Fuel Gauge problems can sometimes be mistaken for ignition problems.

Good luck with your boat.

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