Do You Need a Primer Bulb on Your Outboard?

Taking your boat out on the water is usually carefree until the outboard motor fails to start. Most of the time, the issue is easily fixed, but the usual responses from online experts involve a primer bulb. As not all motors come packaged with one, you might wonder if you need one and if it is truly the cause of your boat’s issues. 

The truth is most outboard motors do not need a primer bulb to function. They can do much of the functionality internally. However, a primer bulb will improve the performance and longevity of your outboard motor. 

While most outboards work fine without a prime bulb, there are a few that do require them. Bu reading further, you will learn how you can tell if your motor needs a primer as well as a few performance tweaks. 

What Happens If You Put Too Much Oil In An Outboard Motor?

Which Outboard Motors Need a Primer Bulb?

Modern outboards are marvels of precision engineering and innovation. Dependable, quiet, and efficient, they let us enjoy being out on the water hassle-free. They are internal combustion engines though. As such, they require fuel to start and run. 

Because of this, all outboards use a diaphragm-type fuel pump to draw in fuel while the motor runs. As long as there is fuel in the tank, your motor should keep going without any issues if you properly maintain it. However, your outboard needs something to help it start. 

Starting or priming a motor is a design challenge. The internal pump requires flowing fuel to maintain the pressure required to draw more fuel out of the tank. But the fuel does not flow when the motor is off. Therefore, you need a separate device, called a primer, to jump-start that flow.

To that end, outboard manufacturers have three primer options:

  • Carburetor
  • Injector 
  • Primer bulb

What is a Primer Bulb?

Carburetors and injectors are semi or fully automatic devices that work similarly to the ones found in cars and trucks. On the other hand, primer bulbs are fully manual, low-pressure pumps that sit between the inboard, built-in fuel tank, and the motor assembly. They can either draw the fuel directly into the combustion chamber or through a carburetor. 

Primer bulbs are just two one-way valves with a flexible, rubber enclosure that looks like a chicken egg between them. Pressing and releasing the rubber creates the pressure gradient the valves use to draw fuel through the bulb. You can do this manually or electronically. This action repeats until the motor’s internal pump takes over. 

Only Small Outboard Motors Need Primer Bulbs

Primer bulbs prevent the motor from ceasing upon startup, ensuring the longevity of the whole assembly. As such, most outboards have them attached to their motors in some way. Some models have the bulbs spliced into an external fuel tube while others have them inside the main assembly. However, not all outboard models need them.

Generally, the only outboard models that need a fully working primer bulb are the low-power ones. These models come with motors that can only produce 60 horsepower or less and require that you use their primer bulbs to ignite them. This is usually done by squeezing the bulb repeatedly until it is firm to the touch. 

On the other hand, high-powered outboard motors require no primer bulbs to start, and in some cases should never have one. Clocking in at 75 horsepower and above, these motors have built-in electric primer pumps that do the job for you. You just have to put the key into the “run” position and wait until you hear a beep. 

Primer Bulbs Extend the Life of Your Outboard Motor

While all high-power outboard motors can self-prime themselves, a few of them are hybrids. These models usually have semi-automatic electronic starters that work fine without their primer bulb under certain conditions. As such, you can run these outboards without primer bulbs if you must, but you never want to keep them that way. 

Automatic motor starters rely on ambient heat from the surrounding environment to vaporize fuel directly into the cylinders. However, there is not enough heat during cold weather for this to happen. As such, the engines can cease up or otherwise become difficult to start. You would then need the primer bulb to jump-start the engine. 

Without the bulb, these models could easily cease up in the middle of nowhere, stranding you far from shore. Because of the potential issues, most of these outboard models come with an electrically controlled primer bulb as a failsafe. You just press a button, and the electric controller does the rest. 

This ensures your motor always has fuel when it needs it even if the weather changes while you are out on the open water.  

How to Troubleshoot Your Outboard’s Primer Bulb

Primer bulbs are simple enough that they should work without issue with minimal maintenance. However, they will eventually break down. At which point, your outboard will not be able to keep its fuel primed up. Fortunately, most primer bulb problems are easy to repair once you find them.

 With that said, primer bulbs can break down due to:

  • Problems with the bulb itself
  • Air leaks in the fuel line
  • A combination of the above

Air leaks are the most common problem and can occur in either the fuel line or the bulb. The air can cause the bulb to run dry or collapse, preventing the normal flow of fuel. If left unchecked, the engine could randomly run out of fuel and die. 

You must visually inspect the fuel line and the bulb to find the cracks. The crack can be anywhere along the line, so be prepared to check everything. You will know when you found the leak when you see busted or missing hose clamps or holes, or miscellaneous splices in the hoses. As such, focusing on where the hoses connect to something may quicken your search. 

You can fix most leaks by cutting out the broken section of the hose and splicing in another with a coupling barb. Try to keep the new hose the same as the old one to maintain the fuel pressure in the line. If you must, you can also replace the whole primer bulb with a new one. 


A primer bulb ensures your outboard has enough fuel in its motor to start, but not every model of outboard needs one. They are usually reserved for low-powered engines. However, if your owner’s manual to see if your model needs ones for emergencies. 

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