GUIDE: How to fix a stalling boat engine?
A stalling boat engine can become a source of stress for its owner. Such a problem can be dangerous if it occurs in the open sea and the engine cannot be restarted. Must therefore fix a stalling outboard motor 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. Only the most serious cases need the expertise of a professional. First of all, you have to start by finding the cause of the problem in order to be able to provide the appropriate solutions. Indeed, there are many different timings: hot timing, starting timing, a outboard motor stalling at idle or a boat engine stalling on acceleration. In this article, we will guide you step by step to fix a stalling boat engine – regardless of the type of wedging.
Contents of the page
Why does my outboard motor 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
You can even anticipate an engine that will stall 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.
What are the different types of timing possible on an outboard motor?
Before continuing your reading, you must make sure that your wedging problem corresponds to the damage we are dealing with in this article. This guide is for you if:
- Your boat engine stalls at idle or low revs – forcing you to cruise at maximum power.
- Your hot stall outboard motor especially when accelerating.
- Your boat engine starts and stalls only a few seconds later.
NB: the solutions for repairing a stalling boat engine 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…
How to easily fix a stalling boat engine?
Before embarking on major repairs, it is necessary to carry out some basic checks. Indeed, it is possible that your stalling problem is due to a very simple failure:
Your tank is empty or almost empty
A fuel shortage can happen to anyone. There is nothing to be ashamed of! You should also know that a marine engine can stall because of a nearly empty tank. Indeed, with swell, a low level of fuel will have difficulty circulating to the boat engine.
The propeller of the outboard motor is stuck
If your boat stalls on startup, your propeller may be blocked by fishing line or other debris. A simple check is to try running the propeller in neutral. If it jams, the stalling problem of your 2-stroke or 4-stroke outboard motor comes from there.
The air screw on the gas tank is closed
Another 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 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. You should know that 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 to use in a boat engine?
The boat engine is out of adjustment
A bad adjustment of the air or richness screws may be the cause of a stalling boat engine. Especially if the outboard motor stalls at idle. Check your settings to exclude or confirm this hypothesis.
What are the most common causes of a stalling outboard motor?
If none of the above assumptions apply to your situation, then your outboard motor almost always stalls:
- Either for a damage to the carburettors
- Either for a problem with the fuel pump.
- Or both!
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 especially recurrent when you accelerate. 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. However, dirty carburettors can also be the cause of a outboard motor stalling at idle. Indeed, it is enough for one or more jets installed on the idle circuit to be clogged to cause stalling.
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 fuel coming out of the air intake.
The fuel pump diaphragm is damaged
If your carburetor is well maintained, then there is a good chance that the timing problem is due to the fuel pump. The pump membrane can be damaged after a harsh winter or simply prolonged use. Holes form and allow air to pass, which has the effect of impoverishing the air/fuel mixture. This problem is often the cause of a boat engine stalling at idle. To know if your membrane is worn, just listen. A hissing sound will be heard if there is a leak. You can also disassemble the fuel pump to visually check if the membrane is damaged. Very often, the leak is located at the level of the intake manifold.
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 a stalling boat engine
An air leak in the fuel supply circuit
A boat engine can stall due to an air bubble that has formed in the fuel system. 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 outboard motor. But the more it grows, the more your engine 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 marine 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. 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.
Rare causes of a stalling boat engine
Read this paragraph if none of the causes mentioned above seem to be responsible for the unexpected stalling of your boat engine. These hypotheses are rather rare so we recommend addressing them last.
Your outboard motor has a compression problem
Compression in a boat engine is very important as it helps to have optimal combustion of the air/fuel mixture. It may happen that your compression is too low. This leads to a partial combustion of fuel and therefore a loss of performance. If the compression drops too much, then the outboard motor cannot produce the requested power and stalls. In general, a loss of compression comes from a scratched cylinder. But the problem can also come from leaky piston rings and valves. The easiest way is to have the compression checked by a mechanic. This is the only way to tell if this is the problem that is causing your outboard motor to stall.
Your boat engine has a carburetor timing problem
Carburetor out of sync is rare and the only option to find out if your timing problem is the cause is to go to a competent marine mechanic.
What are the solutions for repairing a stalling boat engine?
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 a stalling boat engine depending on the cause:
Your tank is empty or almost empty
Fill the tank with the correct fuel for your boat engine.
The propeller of the outboard motor is stuck
Unblock the propeller by inspecting the shaft of your outboard motor. It is often fishing line that blocks.
The air screw on the gas tank is closed
You just need to open the air screw on the gas tank and you won't have any more stalling problems.
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.
The boat engine is out of adjustment
If you don't know how to tune a boat engine, we recommend going to a mechanic. Alone, you risk changing good settings without correcting the original problems. However, with a little knowledge you should be fine. First check the ignition settings (neutral and full throttle). Then, check the adjustment of the air or richness screws. The optimal settings for your boat engine can be found in the manufacturer's instructions. If you don't have the original manual, you can try setting the mixture screws to a three-quarter turn. If your outboard motor always stalls, you can increase by half a turn. If it still stalls, you can add a half turn again. (This is due to the different carburetors: some work well with a XNUMX turn setting. Others need two turns or two and a half turns)
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.
Note: If your problem is an outboard engine stalling at idle, then focus your efforts on the carburetor idle circuit. Chances are the idle jets are clogged.
How do you clean an outboard motor carburetor?
Once the carburettor has been disassembled, it is necessary clean the nozzles. Use a nozzle cleaning kit and 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)
The fuel pump diaphragm is damaged
It's easy to repair a stalling boat engine caused by a damaged fuel pump diaphragm. Just find the leak and replace the part. It's either gasket, either the intake manifold or manifold components.
Your engine'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 advise to change 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 stalling 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 a stalling boat engine.
The fuel filter is clogged
For fix a stalling outboard motor, the fuel filter must be inspected. 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 anymore.
Your outboard motor has a compression problem
A compression problem is a serious damage. A competent mechanic will have to be called in to repair a scratched cylinder or the like. We do not recommend undertaking this repair in your garage.
Your boat engine has a carburetor timing problem
As with compression issues, carburetor out of sync is a problem that requires the help of a mechanic. Only he has the necessary tools and skills required for this kind of repair. If you try to sync your carburetors yourself, you can very easily make a wrong setting. On a 2-stroke boat engine, there is a good chance that these incorrect settings will break expensive internal parts.
Conclusion to fix a stalling boat engine
We hope this article will help you fix a stalling boat engine. 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 membrane of the fuel pump and which would also have clogged the carburettors. If, despite your best efforts, your boat engine continues to stall when starting, idling or accelerating, then it's time to call in a professional.
Learn more about boat motors:
GUIDE: Which outboard motor to choose for a boat?
