Repairing an outboard motor that stalls on acceleration
Un boat engine stalling on acceleration can become a source of stress for its owner. Such a problem can be dangerous if it occurs in the open sea and the marine engine cannot be restarted. It is therefore necessary to repair a marine engine that stalls without delay. We advise to cancel any sea trip until the outboard motor is working properly. Fortunately, such damage is common and can often be repaired without the help of a mechanic. First of all, you have to start by finding the cause of the problem in order to be able to provide the appropriate solutions. In this article, we will guide you step by step to fix an outboard motor that stalls on acceleration.
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Why does my boat engine stall?
All thermal engines are built according to the same principle: a mixture of air and fuel is injected, ignited by a spark and the combustion creates the movement. This basic principle is valid for all marine engines: 2-stroke boat engine, 4-stroke outboard engine, diesel engine, etc. So a combustion engine stalls if:
- There is not the right amount of air
- There is not the right amount of fuel
- There is no spark to generate combustion (or the spark is too weak)
You can even anticipate an outboard motor that will stall when accelerating if you detect the warning signs such as:
- Your boat engine is consuming much more gasoline than usual.
- You notice a lack of power when you try to accelerate.
- Your outboard motor has poor idle performance.
How to easily repair an outboard motor that stalls when accelerating?
Before embarking on major repairs, it is necessary to carry out some basic checks. In fact, your stalling problem on acceleration may be due to a very simple failure:
The air screw on the gas tank is closed
A basic check is to see if the manifold's air inlet is open. Otherwise, gasoline cannot flow well. The outboard motor may idle, but it will stall when you go up the revs.
The gasoline in the tank is old
This is the classic problem at the start of the season. All boaters who have not drained their boat engine properly are going to have stalling issues on resumption. So if your outboard motor stalls when accelerating on first outing, then you know the problem is with the gasoline. Indeed, new fuels have been created for the car and they are not made to stagnate for several months. It only takes a month for gasoline to start degrading!
The gasoline in the tank is of poor quality
If your boat engine has started to stall after refueling, chances are the problem is fuel-related. The gasoline may be of poor quality or you have put the wrong fuel in your outboard motor. You should know that new vegetable fuels such as E10 are not suitable for marine engines. For more information on this subject, you can read our article: what gasoline for a boat engine?
NB: the solutions for repairing an outboard motor that stalls on acceleration are the same for all brands: Yamaha, Mercury, Suzuki, Johnson, Honda, etc… And whatever the power of your 2-stroke or 4-stroke engine: 6hp, 15hp, 25hp, 30hp, 50hp, 60hp, 75hp, 100hp, 150hp, 250hp, etc…
What are the most common causes of an outboard motor stalling on acceleration?
If none of the above assumptions apply to your situation, then your outboard motor stalls on acceleration almost always due to carburetor failure.
The most common causes are:
Your outboard motor's carburetors are clogged
Carburetors mix fuel and air before injecting the resulting mixture into the cylinders of the marine engine. If this mixture is incorrectly made due to a lack of air or gasoline, then the outboard motor stalls. This stalling problem is recurrent when accelerating. It's logical: a boat engine doesn't need a lot of fuel when idling, but as soon as it revs up, its fuel consumption increases. If gasoline is not available, then the boat engine stalls.
The carburettors of your outboard motor can be clogged due to a lack of regular maintenance or due to poor wintering (as explained above). Indeed, every spring, all outboard motor owners who have not drained their carburetors line up at the marine mechanics! If the carburetor bowl is not cleaned properly, the carburetor will be full of paraffin a few months later. So remember to add products to the carburettors that prevent the gasoline from solidifying into a small solid deposit each fall.
To find out if the problem comes from the carburetors, there is a simple manipulation. Once the engine is running, remove the air box attached to the carburetor. If the carburetor is clogged, you should see gasoline coming out of the air intake with each acceleration.
Your outboard motor's spark plugs are worn or dirty
The spark needed to burn fuel in a marine engine is produced by spark plugs. These spark plugs get dirty naturally because carbon is deposited on them with each combustion. However, dirt accumulates faster on two-stroke boat engines than on 4-stroke outboard engines.
When spark plugs are dirty, they produce a weaker spark. This small spark may be enough to make the marine engine operate at idle, but as soon as the influx of fuel becomes significant – especially when accelerating – it no longer manages to initiate combustion. In this case, the boat engine stalls. If you are a novice, know that there are color charts of candles to recognize which ones are dirty.
However, your spark plugs may be new and still be the source of the problem! Indeed, in addition to being in good condition, the spark plugs must be well adjusted. A stalling engine may be due to spark plugs that have not been gapped properly.
Other Possible Causes of an Outboard Motor Stalling on Acceleration
An air leak in the fuel supply circuit
Un boat engine may stall on acceleration because of an air bubble that has been created in the fuel circuit. The fuel can no longer flow because it is blocked by this bubble. Most of the time it is the rubber membrane of the fuel fittings that is worn by a unsuitable fuel and let air through. At first, the bubble is small and only reduces the performance of your marine engine. But the more it grows, the more your outboard motor will tend to stall when accelerating. The last stage is complete blockage of the duct. In this case, the outboard motor will stall for a few seconds when starting. To find out if your fuel inlet hoses are porous, you must check if the seals between the tank and the engine are not cracked.
The fuel supply circuit is blocked
In the best case scenario, your fuel system is clogged with an air bubble as described above. But it may be that the plug is not air but a solid plug. In this case, the problem is serious. Often this happens when you start your boat engine for the first time after months of inactivity. It starts with old fuel which will stick together to create a plug. But it could be something else! In all cases, the result is the same: the gasoline cannot circulate correctly in the circuit. In the most extreme cases, the plug is so big that gasoline no longer even flows to the fuel filter. To find out if your problem outboard motor stalling from fuel system, just test the fuel pressure at the end of the chain.
The fuel filter is clogged
It is recommended to change your fuel filter regularly. If this has not been done, then the fuel cannot be properly routed to your boat's engine. The dirtier the fuel filter, the more the boat engine will lose efficiency, speed and power. It will start to stall on acceleration and if the filter is not cleaned or changed quickly, the outboard motor will no longer start.
The air filter is clogged
A poorly maintained air filter may cause the outboard motor to stall on acceleration. For injection engine owners, the air flow sensor is also a problem if it is dirty. A brief examination will allow you to judge the condition of the filter or sensor.
What are the solutions to fix an outboard motor that stalls when accelerating?
Once the cause of the stall has been identified among all the hypotheses mentioned above, the problem must be repaired before you can go back to sea. We have listed the solutions for fix an outboard motor that stalls on acceleration depending on the cause:
The air screw on the gas tank is closed
You just need to open the air screw on the gas tank to no longer have a stalling problem when accelerating.
The gasoline in the tank is old
You will need to clean the carburettors of your outboard motor and bleed the entire fuel circuit. We describe the process to be followed in the following paragraphs.
The gasoline in the tank is of poor quality
The system must be completely drained: fuel circuit and carburettors included ! Once the emptying is done, fill your feed with SP98 and never E10. For more information read our article: what gasoline for a boat engine ?
How to bleed the fuel circuit of a boat engine?
Start by filling your tank with the correct fuel for your boat engine. Then disconnect the fuel supply to the engine and remove the hose seal. Finally, use the bulb to pump in the new fuel. It will push poor quality gasoline out of the circuit. You can stop pumping when about half a liter of fuel has been ejected.
How to bleed the carburetor of an outboard motor?
Bleeding a carburetor is very simple. Simply unscrew the emptying screw under the tank. Then retighten the screw and use the bulb to pump new fuel into the carburetor.
NB: Before tightening the carburettor drain screw, we recommend completely filling the tank with cellulose thinner. You can use a syringe to introduce the thinner through the intake hose (the pipe that comes from the fuel pump). Leave the product in the tank for 10 minutes and purge again by opening the emptying screw.
Your outboard motor's carburetors are clogged
A dirty carburetor should be thoroughly cleaned. You have to dismantle it completely and take care of each part: the jets in priority, the float, the needles, etc... If you don't know how to do it, you can call a mechanic. But if you're a handyman you should be able to get by.
How do you clean an outboard motor carburetor?
Once the carburettor has been disassembled, it is necessary clean the nozzles. Use a special nozzle cleaning kit et carburetor cleaner. Most of the time you will find your jets clogged with paraffin. If the nozzles are too damaged, they will need to be replaced.
Also think about buy new carburetor gaskets because they tend to get damaged with each dismantling. We recommend replacing it on this occasion.
CAUTION: Never use a sewing needle to unclog your outboard motor jets. It risks breaking in the nozzle and damaging it. You will then have to change it...
- View a sprinkler cleaning kit
- See a good carburetor cleaner (to be used only on sprinklers and not on plastic areas)
Your outboard motor's spark plugs are worn or dirty
Check if your spark plugs are in good condition. You can try to clean them by passing emery cloth over the electrodes. But often that is not enough... If the spark plugs seem too damaged or dirty, we recommend changing them.
On the other hand, if the problem comes from spark plug gap then you will need to set them correctly. Adjusting the gauge cannot be improvised and if you do not know how to do it, we recommend that you seek help from a marine mechanic. In terms of spark plug gap, it is recommended to space the electrodes from 0,8mm to 0,9mm.
An air leak in the fuel supply circuit
You have to check all the hoses to find the vent. The easiest way is to look at each hose clamp. This is the perfect place for an air bubble to form. Once the faulty clamp has been identified, it must be tightened or changed. To remove the air bubble, the fuel supply circuit must be bled. As soon as gasoline comes out, the air has been ejected and your throttle stall problem is solved.
The fuel supply circuit is blocked
Check all the connections of the fuel circuit to find the plug… You will have to clean the whole circuit, hoping to find the plug quickly. Unfortunately, there is no other solution for fix an outboard motor that stalls on acceleration.
The fuel filter is clogged
To fix an outboard motor that stalls on acceleration, inspect the fuel filter. Is it clean? If the answer is no, the solution is to clean it or to change it.
NB: We recommend cleaning the fuel filter only if you are stuck at sea. It is best to replace it with a new filter. In addition, for a long crossing, it is crucial to have a suitable fuel filter in your toolbox.
The air filter is clogged
Check the condition of the air filter (or air flow sensor) and if it is clogged try to clean it. If it's too damaged, buy a new one. Replacing an air filter is very easy to do. In the future, it is enough to maintain the air filter periodically so that your outboard motor does not stall any more under acceleration.
Conclusion to Repair a Boat Engine That Stalls on Acceleration
We hope this article will help you fix an outboard motor that stalls on acceleration. The most important thing is to identify what is causing your outboard motor to stall unexpectedly. There may be several causes such as poor quality (or unsuitable) fuel which would have damaged the fuel pump and which would also have clogged the carburettors. If, despite your best efforts, your boat engine continues to stall when accelerating, then it's time to call in a professional.
Learn more about boat motors:
GUIDE: Which outboard motor to choose for a boat?
