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Primer Bulb Not Priming: 7 Likely Causes and Fixes

The primer bulb on your tool may be fastened directly onto the carburetor or connected to it through a port that technically serves as the suction line. The causes and fixes of a primer bulb failing to prime often depend on its location in your tool. 

When a primer bulb isn’t priming, it could be because it’s cracked, broken, or damaged and needs replacing. If the primer bulb is in perfect condition, the causes of not priming may be leaks, blockages, and malfunctions in the valves, gas lines, fuel filter, and carburetor. 

You may have one or more problems preventing the primer bulb from priming. Thus, inspect the usual suspects sequentially, detect the causal issues, and opt for the appropriate fixes. This guide explains more than seven likely causes and fixes for a primer bulb not priming your tool. 

1. The Primer Bulb Has Leaks

A primer bulb should be in pristine condition to work flawlessly. Its ultimate purpose is to prime the carburetor. Technically, priming is the process of providing the carburetor with sufficient gas so that the fuel and air can mix and be ready to combust when you start the engine. 

What Is a Primer Bulb? 

A primer bulb is a flexible button made of rubber or plastic. The bulb-like button is used on many handheld tools and other equipment to regulate the air pressure and suction inside the carburetor, fuel lines, and gas tank. The primer bulb is integral to priming, thus starting the tool’s engine. 

Since the primary function of the primer bulb is to regulate air pressure and suction, it cannot have any leaks, cracks, clogging, or structural issues that make the part ineffective. Therefore, you must replace a primer bulb that’s: 

  • Broken 
  • Worn out 
  • Cracked 
  • Damaged 
  • Leaky 

1.1 When the Primer Bulb Is Fastened Directly Onto the Carburetor

You can instantly spot visible cracks or other evident signs of damage. However, a leak may be a tad tough to detect. Inspect the primer bulb closely under a bright light to find any tiny pores causing air to leak. Also, you must check the seal, gasket, or ring of the primer bulb. 

A leaking or damaged primer bulb that is fastened directly onto the tool’s carburetor is not a serious problem. Also, many primer bulbs are not durable because of the flexible material quality. Besides, a primer bulb will suffer wear and tear due to frequent use, like all such parts. 

How To Fix

Get a new primer bulb to replace the damaged one. Ensure that you select the right type and size for your tool by referring to the model and part numbers on the manual. Unscrew the plate from the carburetor, remove the old primer bulb, and install the new one. You may not need to fix anything else unless there is another broken or malfunctioning component, like the valve. 

The remedy is similar if you have a broken gasket, seal, ring, or primer bulb plate on the tool’s carburetor. You have to replace the damaged part. However, if the primer bulb, its seal, and plate are fine, you are looking at one of the other causes and therefore fixes explained here. 

1.2 When the Primer Bulb Is Connected to the Carburetor and Return Fuel Line

Many handheld tools and other equipment have the primer bulb connected to the carburetor through a hose or line and port. Such primer bulbs have another port to connect to the fuel line. This fuel line returns the gas to the tank. However, you may have a leaky or broken primer bulb. 

How To Fix

Disassemble the tool, as required, to remove the primer bulb. Get a new compatible primer bulb, connect the ‘in’ port to the carburetor and the ‘out’ port to the return fuel line going to the tank. Complete the installation, and the primer bulb should prime unless you have other issues. 

How Do I Tell if My Primer Bulb Is Working?

Test if your primer bulb is working by pressing it and observing the compression and reversion. If the primer bulb is compressing and reverting to its normal shape without any aberration, the part is working, and you have other problems preventing the priming. 

A primer bulb will not compress and revert flawlessly if: 

  • The part has leaks. 
  • One or more valves are jammed. 
  • The suction line through the carburetor is clogged. 

Also, you can do other tests to check if the primer bulb is working, but another component is not. 

One test applies to primer bulbs with two ports connected to the carburetor and return fuel line, respectively. Unplug the fuel intake line from the carburetor. Press and release the primer bulb a few times. Check if gas flows out of the fuel intake line. If you see some fuel, the primer bulb is working fine, but the gas is not flowing into the carburetor due to some blockage or broken part. 

2. The Sealed Unit Has Leaks

Like the primer bulb, the entire section of your tool comprising the gas tank, fuel lines, and carburetor form a sealed unit. There are valves, gaskets, diaphragms, and other components regulating the flow of air and fuel. Any leakage in this sealed unit of the larger tool will prevent the primer bulb from priming the carburetor and, in the process, the engine. 

How To Fix

The leakage may be in the fuel lines, connecting ports, or elsewhere. You have to take apart the wing section of the tool housing the air cover and filter, carburetor, fuel lines, and primer bulb. Check the fuel hoses and replace them if they are leaking, damaged, or worn out. Inspect all the gaskets and replace broken rings or seals. In some cases, epoxy glue may fix a broken ring.

3. The Primer Bulb Valve Is Jammed

All primer bulbs have a one-way valve facilitating airflow and fuel through but not allowing either to return. There is another hole in the base plate of the primer bulb that allows both fuel and air to flow into the section. This air and fuel mixture is pumped into the system when you press the bulb. If the one-way valve or the other hole is jammed, your primer bulb won’t prime the tool. 

How To Fix

Check for blockages at the valve and hole. Primer bulbs installed directly on the carburetor do not have valves or holes. These parts are on the carburetor. Primer bulbs connected to the return fuel line and the carburetor have the one-way valve on the ‘out’ port. This valved port is connected to the return fuel line. Check if it is blocked by debris and residual buildup. 

Clean the valve if it is jammed. Replace a broken or malfunctioning valve. Some primer bulbs may not have a detachable one-way valve, so you need to replace the integrated part. Also, check the other hole that allows air and fuel to flow into the primer bulb. Clean this hole to ensure there is no blockage. You can use compressed air or an appropriately sized poking tool. 

4. The Fuel Lines Are Blocked

If you have already checked the fuel lines as detailed in the second step of this guide, you are already aware of whether the fuel lines are blocked or not. The same inspection can tell you if there is leakage, blockage, or both. However, unlike leaks, you can clean the blockage so that you can continue to use the same fuel lines. The only exception is worn-out or damaged hoses. 

How To Fix

Detach the fuel lines from the gas tank, carburetor, and primer bulb. Look for debris buildup inside. Use a slender brush to clean the hoses. Also, you may use an aerosol cleaner after getting rid of the gunk. Exercise caution so that you do not damage the hoses or fuel lines. 

5. The Fuel Filter Is Clogged

Like the valves and hoses, the fuel filter in your tool’s gas tank may have gunk buildup. Thus, the filter will not allow fuel to flow out of the gas tank as it normally does. You may have this problem if you store a tool with a loaded gas tank. Always empty the fuel tank before storing. 

How To Fix

Empty the gas tank before taking out the fuel filter. You can use pliers to grasp the fuel intake hose inside the gas tank and bring the filter out. Check and clean it, if necessary. Use a brush with soft bristles and soapy water to scrub and clean the filter. Don’t use abrasive detergents and cleaning agents. A gentle regular soap should work well. Rinse, and air dry the filter (source).

6. The Carburetor Is Not Working

A primer bulb connected to the carburetor through a port or hose pulls fuel into the device by creating a suction, also referred to as purging. Thus, the primer bulb is a purge bulb, too. If a primer bulb is fastened directly onto the carburetor, the compression and reversion force fuel into the device. Such models don’t have the primer bulb connected to the return fuel line. 

A carburetor has many tiny but vital components, a few of which may prevent your primer bulb from priming. The most common issues are: 

Stuck valves 
Failing or broken metering parts, such as the:

  • Lever 
  • Pin 
  • Needle  
  • Spring 
  • Diaphragm 
  • Gasket 

6.1 The Carburetor Is Clogged

A carburetor has several channels to regulate the flow of fuel and air. If any channel or tube is clogged, the gas will not flow through the passages of the carburetor and to the main jet when you press the primer bulb. Usually, gunk and grime can clog one or more parts of the carburetor. 

How To Fix

Inspect the carburetor and clean all its: 

  • Pores 
  • Inlets  
  • Outlets  
  • Venturi  
  • Main jet 
  • Metering unit 

Inspect the diaphragms and remove grime or residual buildup if you see any. Essentially, you must ensure that the gas flows through the intake line into the carburetor and to the primer bulb. 

Otherwise, pressing and releasing the primer bulb will not create the air pressure and resulting suction to pull fuel from the tank into the carburetor. Thus, use a carb cleaner to flush out every bit of debris, grime, or gunk inside the carburetor. Also, you may opt for ultrasonic cleaning and compressed air. Let the carburetor air dry before reassembling & testing the primer bulb (source).

6.2 The Carburetor Valves Are Stuck

The carburetor on your tool may have three or more valves, depending on its type and design. All these valves operate only in one direction, thus regulating the flow of air and fuel. Any stuck valves lead to a primer bulb not priming. Cleaning the carburetor in the previous step will get rid of any grime and gunk clogging these valves. However, you may have a broken valve or more. 

How To Fix

Broken or malfunctioning valves in a carburetor may or may not be replaceable in all tools. For instance, a small string trimmer and large lawnmower or snow blower don’t have identical carburetors. Replace the broken valve or diaphragm, if possible. Otherwise, consider repairing or buying a new carburetor. You may consult a local technician about fixing a broken carburetor. 

6.3 The Metering Unit Is Broken

The carburetor’s metering unit or chamber should be flooded as you prime the tool for the fuel to flow into the primer bulb. Only then will the metering valve open and facilitate the gas to flow into the main jet to be discharged to the venturi in the carburetor. 

Your primer bulb will not prime if the metering lever, pin, needle, diaphragm, and valve are broken, or any of these parts are not working normally. Thus, take apart the carburetor to check if all these vital components are working alright. 

How To Fix

Debris buildup or clogging is remediable with carb cleaner, and replace the broken parts of the metering unit. For some tools, you may have to get a carburetor rebuild kit if each component is not available separately. 

However, you can shop for carburetor rebuild kits comprising the components for your metering unit, such as: 

  • Diaphragm 
  • Gasket 
  • Needle 
  • Spring  
  • Pin 
  • Lever 

7. The Fuel Lines Are Swapped

This problem is relevant only if your primer bulb is connected to the carburetor and gas tank using two separate ports. Check if the fuel intake hose is connected to the primer bulb’s ‘out’ port. Such errors may happen while replacing the fuel lines or reassembling a tool. 

How To Fix

Empty the gas tank, and be prepared for some spills if there is fuel in the carburetor and hoses. Detach the fuel intake line from the primer bulb and the return hose from the carburetor. Fit the fuel return line snugly to the primer bulb. Connect the fuel intake hose to the carburetor. 


Look for leakage and anything clogging the primer bulb, fuel lines & filter, and carburetor. Then, inspect the valves, gaskets, diaphragms, and ports to ensure they are alright. Accordingly, choose a relevant solution. Finally, check the carburetor’s metering unit to replace broken parts.