The primer bulb of a lawn mower plays an essential role in starting the lawn mower’s engine. It can be quite difficult to get your lawn mower up and running with a defective primer bulb.
If you recently discovered that your lawn mower primer bulb has hole, you will need to replace it with a new one.
In this article, I talk about the working process of a primer bulb, the easy-to-follow steps you can take to replace a damaged primer bulb on your lawn mower, and how to start a lawn mower without a primer bulb.
How Does A Primer Bulb Work?
A primer bulb is a small rubber bulb located adjacent to or at the bottom of a carburetor that primes up or prepares the mower engine before it starts. It is usually made of rubber or plastic.
A primer bulb performs an important function by filling the carburetor with fuel when the button/bulb is pressed a few times.
How A Primer Bulb Works
- The primer bulb is pressed in and released repeatedly
- A vacuum is created as the primer bulb is released
- This created vacuum draws fuel from the tank and sends it into the carburetor to get the engine started
Why Does A Lawn Mower Have A Primer Bulb?
A lawn mower makes use of a primer bulb because of the small size of the engine’s carburetor.
A carburetor mixes the appropriate amount of air and fuel needed for combustion before sending it into the combustion chamber.
As the engine is about to start up, the carburetor needs fuel from the fuel tank. By pumping the primer bulb, fuel is extracted from the fuel tank of the lawn mower, and sent into the carburetor, which then sends it to the combustion chamber and starts the engine.
The primer bulb is especially important if you occasionally run the carburetor dry of fuel before you store your mower. It fills the empty bowl on the carburetor (which will be dry), with the fuel needed to ignite the engine.
Vented And Non-Vented Primer Bulbs
Primer bulbs are of two different styles:
- Vented primer bulb (Hole)
- Non-vented primer bulb (No hope)
Vented Primer Bulb
As the name implies, the vented primer bulb has a tiny hole at the center that allows a tiny supply of air into the carburetor. It is usually used by lawn mower engines that make use of an air vent system.
The disadvantage of this model is that water and dust can enter the bulb through the vent/hole, which is then forced into the carburetor.
Non-Vented Primer Bulb
Non-vented primer bulbs are the most common types of primer bulbs used for lawn mowers. They are completely covered and do not contain a vent or hole that allows air into the carburetor.
When purchasing a new primer bulb to replace the old one, ensure to get the exact type that was originally used on your mower to avoid altering the air-fuel ratio that the engine requires.
How Do I Know If My Primer Bulb Is Bad On My Lawn Mower?
You can tell if your primer bulb is cracked or broken by checking for the following:
1. Visible Cracks And Holes On The Bulb
Carrying out a thorough visual inspection is the easiest way to check whether a primer bulb is defective, cracked, or needs to be replaced. Inspect the primer bulb thoroughly for signs of any damage like splits, holes, and cracks.
These cracks and splits appear on the bulb as a result of regular use, resulting in wear and tear. Heavy impact on the side of your lawn mower can also cause cracks to appear on the primer bulb.
If cracks are present, the primer bulb is bad and you need to replace it to keep your lawn mower’s engine running efficiently.
2. Damage On The Outer Ring Of The Primer Bulb
The outer ring attaches the primer bulb to the body of your lawn mower. Check for any signs of damage on the outer ring whether in the form of cracks or indentations.
Cracks on the outer ring will make the prime bulb loose allowing dust and air into the carburetor. This can cause problems with combustion and will not allow the engine to run smoothly.
If cracks are present, the primer bulb and its ring will need to be replaced with a new one.
3. Bulb Refuses To Pop Back After Being Pushed In
The bulb pops back up after being pushed in under normal operating circumstances. This is an indication that it has fed fuel into the carburetor.
Additionally, you should encounter some resistance as you press the bulb in. The bulb is cracked if there is no resistance and the primer bulb doesn’t pop back up.
4. Shorter ‘Snap Back’ Time
Snapback time refers to how long the primer bulb takes to snap back after being pushed in. Push it in and check to see if it is easier to push in than it was before.
Time is required for the snap back after pressing it in. If it snaps back immediately, there might be a hole in the primer bulb and it will need to be replaced.
How To Replace A Lawn Mower’s Primer Bulb That Has A Hole
Lawn mower primer bulbs usually get damaged or worn out after several seasons of use. A damaged primer bulb will need to be replaced before starting up the engine.
Replacing the primer bulb of your lawn mower requires little to no technical skills and can be done with tools you most likely have at home.
Tools You Need For This Job
17mm round socket
New primer bulb
How To Replace A Primer Bulb – A Step By Step Guide
Step 1: Locate the primer bulb attached to the carburetor at the side of the lawn mower
Step 2: Remove the plastic cap protecting it (some lawn mowers do not have this)
Step 3: The primer bulb is clipped into the carburetor on both sides. Use a flat screwdriver to disengage the clips pushing in and out.
Step 4: Use your pliers to pull out the bulb once it has been disengaged
Step 5: Insert the new primer bulb into the round metal socket with your fingers
Step 6: Line up each clip on each side of the primer with the primer holes on the side of the carburetor
Step 7: Push the primer bulb in until you hear a click, signifying that it is locked in
Step 8: Remove the round socket and replace the plastic cap on the primer bulb
How To Start A Lawn Mower Without A Primer Bulb
Did you notice a crack in your primer bulb? Is your primer bulb damaged or missing?
You can start your lawn mower without a primer bulb using the guide below:
The Starter Fluid Method
For this method, you will need:
- A screwdriver
- Starter fluid
Step 1: Preparation of the lawn mower
The first step is to prepare your lawn mower by placing it on a flat surface to avoid movement during the operation.
Step 2: Remove the air filter
The next thing to do after securing your mower is to remove the air filter. The air filter is housed by a rectangular, plastic box located next to the carburetor.
Gently remove the air filter housing with your screwdriver and take out the air filter. Take note of the position of the air filter before taking it out.
Step 3: Spray starter fluid into the air intake nozzle
The air intake nozzle is the hole behind the air filter which leads directly into the carburetor. Spray the starter fluid into this hole for about 2-3 seconds and close it quickly with the air filter.
Step 4: Quickly replace the air filter
After spraying starter fluid into the air intake nozzle, the next step is to put the air filter back into place.
The speed with which you put the air filter back in place is important because starter fluid is a highly volatile liquid and as such, evaporates quickly into the atmosphere. This is why you should cover the air nozzle as fast as you can after spraying your starter fluid.
Also, be careful to not damage the air filter. There should be a balance between speed and care when carrying out this step.
Step 5: Start your lawn mower
Finally, start your lawn mower. It should come up right away but if it doesn’t, try carrying out the steps again more carefully.
The Syringe Method
This is an alternative to the starter fluid method. This method requires only a syringe and is easy to carry out. It can be used if the primer bulb is missing completely from the lawn mower.
Step 1: Insert a new syringe into the primer bulb hole
Step 2: Pump the syringe about 8-10 times to get suction and draw enough fuel into the carburetor
Step 3: Start your lawn mower
A damaged primer bulb might be the reason you experience difficulty in starting your lawn mower engine. A primer bulb with a hole or cracks on it will make it almost impossible for your lawn mower engine to ignite.
By following the steps outlined in this article, you can replace your primer bulb in no time and get your lawn mower running again.