Most Common Problems With Mercury 25 HP 4 Stroke

The Mercury 25 HP 4 stroke outboard motor model delivers constant easy starting, crisp throttle response, maximized fuel economy, and overall outstanding performance. 

Though this advanced, compact Mercury outboard motor offers EFI that no other outboard or boat manufacturer offers at this horsepower rating, often users experience some troubleshooting issues while using it.

In this article, I will reveal the most common problems that may occur with the Mercury 25 HP 4 stroke outboard motor and let you know how to deal with those problems.

Outboard Idles Roughly Or StallsLow pressure on the fuel pumpFuel tank air vent is closed or restrictedDamaged or restricted fuel lines and fuel filters
Outboard May Hesitates Upon AccelerationIgnition system has no spark or even weak or low sparkIf the Carburetor is overflooded
Motor May Experience Sputtering Or Powe Loss IssuesBad fuel condition

1. Outboard Idles Roughly Or Stalls:

According to Mercury 25 HP 4 stroke many users, sometimes the motor idles rough or stalls.

Users claimed that the outboard motor runs irregularly or unevenly while they put it neutral or out of gear. 

However, various reasons may cause this particular commotion, but first, you need to do the proper investigation.

Possible Fix 

It may occur when your fuel pump has low pressure, so you need to perform a test to inspect the fuel pump and fix the problem.

Before performing the test, make sure to read the repair service provided with your outboard. The manual will guide you to select accurate specs and learn the exact procedures to follow to fix your fuel pump’s issue.

First, start the fuel pump testing by turning off the outboard engine’s power. Then, disconnect and remove the cowl cover, the plug, spark plug wire boot.

Now, you should check the compression gauge reading, and if the gauge reading shows less or above 30 PSI, the compression gauge needs to replace. The fuel pump gets affected by the damaged diaphragm or one-way valves so, check them and replace the faulty part.

The problem can also occur if the fuel tank air vent is closed or restricted.

Therefore, check and inspect the air vent located on the fuel tank. If it is not open, turn it on the open position. Remember, the fuel tank air vent should always stay open and free from friction.

Another possible reason behind this problem can be the damaged or restricted fuel lines and fuel filters.

Therefore, inspect all the fuel lines for any clogged or damage issues and also check whether any fuel line connection is loose or not. If you notice loose connections, tighten them appropriately. But do not overtighten any connections. If the fuel lines are clogged or severely damaged, you should clean or replace the damaged fuel lines.

While inspecting the fuel filter, if it is broken or restricted because of excessive corrosion or other contaminates, either you should clean or replace the fuel filter with a new one.

2. Outboard May Hesitates Upon Acceleration:

Another common problem that Mercury 25HP four-stroke outboard users often complain about is the motor’s visible hesitation on acceleration.

Sometimes, they notice that the outboard motor struggles or hesitates when they put on the acceleration, and even the engine starts to run unevenly or surges.

Several reasons can lead to this particular commotion since these types of issues are interconnected,

So, let’s find out what are the possible reasons and how to handle those.

Possible Fix 

When your ignition system has no spark or even weak or low spark, your outboard motor will hesitate or run unevenly.

To verify whether the main culprit is the ignition system, use a spark tester or detector to test the spark. You can get the spark tester or detector from any auto supplies or parts stores, and you do not need any specific type of spark tester for the Mercury 25 HP four-stroke.

Now, you need to set 3/8 inches gap on the spark detector and crank your motor to investigate the spark condition of each motor cylinder. If you see the spark in both cylinders, that means your ignition system is okay.

But if you see no or poor spark on one or both cylinders, you need to inspect the yellow and black wires. Use the tester on both wires, and if you cannot see any spark, you need to replace the wires.

In that case, remove the wire from the switch box and replace it.

Another possible reason can be the carburetor as well…

Now, test the spark condition on the carburetor with the same spark tester and follow the same method. If you see no spark in there and found an excessive amount of sticky gunk insider of the carburetor, that means your carburetor is overflowed with the gunk and preventing closing the valves.

First, you need to drain the old, dirty fuel by removing the fuel cap. Then, you need to remove and dip the carburetor in a carburetor-cleaning solution to clean the carburetor.

But remember, no need to remove the main jet from the carburetor while soaking it in the solution. Wait and let it soak in the solution for a minimum of an hour. After that, you can remove the jet from the carburetor.  

Use a flathead screwdriver to eject the main jet from the carburetor and remove the main jet. Now, rinse and dip the fuel jet, just like the carburetor.  Then, spray compressed air on the fuel jet to remove the excess gunk and clear the path for the fuel flow. 

If the jet seems damaged, replace it with a new one and put it back inside the carburetor.

3. Motor May Experience Sputtering Or Powe Loss Issues:

Some of the Mercury 25 HP 4 stroke motor users have complained that the motor sometimes sputters or constantly loses more power than usual.

This particular issue occurs when they take out their outboard to use after several months of not using.

However, to fix the issue permanently, you need to understand what is causing this sudden commotion. So, let’s find out the reason and solution.

Possible Fix 

Most outboard motors usually experience this issue for the bad fuel condition.

If your motor uses ethanol-blended fuel mix or even 10% ethanol in the fuel mix, this problem can arise. But you can easily avoid this commotion by using the Mercury recommended fuel mix.

Mercury recommends using either unleaded Regular 87 Octane Minimum (R+M/2) or 90 RON 10% Ethanol Maximum for all their outboard motor and especially, for the 25HP 4 stroke one.

However, sometimes consumers do not clean or change the fuel regularly or before every use, which leads the fuel to turn dirty or bad. That’s when your engine will start to lose power.

You can also prevent the problem by using a high-quality 10-micron fuel and water separating filter. This high-quality filter will keep water out of your outboard motor and fuel.

Also, use a high-quality fuel stabilizer whenever you add new and fresh fuel into the fuel tank.

What The Majority of Users Feel About the Mercury 25 HP 4 Stroke?

The majority of the Mercury 25HP four-stroke users are satisfied with the overall performance, and the motor performs excellently in shallow or even in cool water. Some users also mentioned that this compact electric motor is faster and torquey than other competitor’s motors. 

However, it is undeniable that some common issues arise while using this outboard motor, and users have already confirmed that. But fortunately, none of those issues are severe and easily fixable.

Final Thoughts

The Mercury 25 HP 4 stroke outboard motors feature an advanced technology that made them easy to install and operate. These compact outboard motors come with reliable starting, low vibration, and quick acceleration to perform with exhilarating speed.

However, just like other Mercury outboard motor lineup models, the Mercury 25 HP 4 stroke outboard motors are not entirely faultless, and users should be well aware of those problems.

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