How to Fix a Boat Engine That Stalls at Idle

How to Fix a Boat Engine That Stalls at Idle

Un outboard motor stalling at idle 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. 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. 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. In this article, we will guide you step by step to fix a boat engine that stalls at idle.

How to easily repair a boat engine that stalls at idle?

NB: the solutions for repairing a boat engine that stalls at idle 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…

Before embarking on major repairs, it is necessary to carry out some basic checks. Indeed, it is possible that your stalling problem at idle is 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 and the outboard motor will stall at idle.

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 at idle on first sea trip, 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?

The boat engine is out of adjustment

A bad adjustment of the air or richness screws may be the cause of an outboard motor stalling at idle. Check your settings to exclude or confirm this hypothesis.

What are the most common causes of an outboard motor stalling at idle?

If none of the above assumptions apply to your situation, then your outboard motor almost always stalls at idle:

  • Either because of a damage to the carburettors
  • Either because of 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. 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 more quickly on two-stroke boat engines than on 4 stroke outboard motors.

When spark plugs are dirty, they produce a weaker spark. That little spark can be enough to cause a boat engine to stall at idle. 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. An outboard motor that stalls at idle may be caused by spark plugs that have not been gapped properly.

Other Possible Causes of a Boat Engine Stalling at Idle 

The fuel supply circuit is blocked

A solid plug may form. 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 at idle comes from the fuel system, just test the fuel pressure at the end of the chain.

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 causing your outboard motor to stall at idle.

Your boat engine has a carburetor timing problem

Carburetor out of sync is rare and the only option to find out if your idle timing problem is the cause is to go to a competent marine mechanic.

What are the solutions to fix an outboard motor that stalls at idle?

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 boat engine that stalls at idle 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 and you will no longer have a stalling problem at idle.

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 at idle, 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.

NB: For one outboard motor stalling at idle, it is necessary to concentrate on the idle circuit of the carburettors. 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 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 remember to buy new carburettor seals because they tend to get damaged with each disassembly. We recommend to replace 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...

The fuel pump diaphragm is damaged

It's easy to repair a boat engine that stalls at idle due to a damaged fuel pump diaphragm. Just find the leak and replace the part. It's either the joint, 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 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.

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 boat engine that stalls at idle.

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 for Fixing a Boat Engine That Stalls at Idle

We hope this article will help you fix a boat engine that stalls at idle. The most important thing is to identify the cause of the untimely stalling. There may be several problems 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 at idle then it's time to call in a professional. 

Learn more about boat motors:

GUIDE: Which outboard motor to choose for a boat?

GUIDE: How to Fix a Smoking Boat Engine?

Which boat engine consumes the least?