Weed Eater Primer Bulb Not Filling (Why + What to Do)

If you have a gas powered weed eater, then you probably know what a primer bulb is. You know, the little rubber button you use to prime the engine to get it started. Well, that’s what you are hoping it does, anyway. But, if you’re pressing the button and nothing is happening, and the primer bulb is not filling, then it’s going to be nearly impossible to get your engine fired up. So, instead of continuing to struggle with your weed eater, let me share with you the common causes of why your primer bulb won’t fill up and how to fix it.

Why Your Weed Eater Primer Bulb is Not Filling (The Short Answer)

The primer bulb is responsible for creating pressure within the fuel system. If the bulb is damaged or the fuel tank is not airtight, then the required pressure will not be formed. Also, if there is a blockage in the system, fuel cannot be sent to the primer bulb.

4 Possible Causes of a Weed Wacker Primer Bulb That Won’t Fill

To understand what can go wrong with a primer bulb, it’s best to understand what it does and how it works first.

When you press the primer bulb it forces air into the gas tank through the purge line. Once you let go of the bulb, the tank pressure forces fuel down the intake fuel line into the carburetor and into the bulb. For this to work efficiently, the system must have no obstructions and be airtight.

So, let’s take a look at what can make this go wrong and why the weed wacker primer bulb won’t fill.

Damaged Primer Bulb

The first place to start is with the actual primer bulb itself. If you had kept count, you would be surprised by the number of times you’ve pressed the primer button over its lifespan.

Unfortunately, this little piece of rubber can only take so much. With the repeated pressing and fuel eating away on the inside, the bulb will give way at some point. When this happens, the bulb could spring a leak and can no longer do its job.

So, inspect the bulb closely and check for holes and leaks.

Loose Filler Cap

To keep the tank sealed, a filler cap locks down in place so that air isn’t able to escape. If you have ever looked at a Stihl weed eater filler cap, you might have noticed how it squeezes down into the tank to form a good seal.

The pressure will not form in the tank if you have a loose or leaking gas cap. Normally, if you turn your gas tank upside down, it shouldn’t leak. If it does, then there is definitely a problem.

Leaking Fuel & Surge Lines

The two fuel lines are responsible for carrying the pressure to the tank and the fuel to the carburetor/primer bulb. If either of these becomes damaged, then the pressure will be lost, and the bulb will not fill with fuel.

So, give the bulb a press and inspect the lines. If you see fuel or hear any air coming from the line, you’ll have found the reason your weed eater primer bulb is not filling.

Blocked Carburetor

If you have a blocked carburetor, then your primer bulb will either not push air to the gas tank or allow fuel into the carburetor. Either way, the primer bulb will not fill with gas. To test the carburetor, you’ll need to connect a pressure tester to both the intake and purge pipes and see how they react. Let’s take a closer look at the tests.

Testing the Inflow Side of the Carburetor:

Connect the pressure gauge to the carburetor’s intake pipe and pump, pressurize the carburetor to 7psi and watch the gauge. It should hold at 7psi. If it doesn’t, then there is a leak on the intake side of the carburetor.

Then, press the primer bulb once, and the gauge should drop 2-3psi. Next, press the primer again, and the gauge should drop another 2-3psi. If the gauge isn’t dropping each time you press the primer, then there is a blockage in the carburetor.

Testing the Outflow (Purge) Side of the Carburetor

For this test, connect the pressure gauge to the outflow of the carburetor, but do not pump up the pressure gauge. This time you are going to use the carburetor as a pump. So, with the gauge and carb connected, press the primer bulb and watch the gauge.

The gauge should increase by 2-3psi each time you press the primer bulb. If the gauge doesn’t increase, then there could be a blockage on the outflow side of the carburetor.

How to Fix a Weed Eater Primer Bulb That Doesn’t Fill with Gas

I’m hoping that after finishing all the testing, you have found the root cause of your weed eater primer bulb not filling. Let’s now take a look at the steps you’ll need to follow to get your weed eater up and running and the primer filling up.

Replacing the Primer Bulb

To replace the primer bulb, you’ll need to gain access to the carburetor. For weed eaters like Stihl models, this will involve removing the air filter cover and the main cover of the engine.

So, once you have clear access to the primer bulb, go ahead and remove the screws from the primer bulb plate and set them to one side. Next, lift the bulb with the plate attached. Now, remove the bulb from the plate and install a new bulb. Just be careful not to cause any damage when threading the bulb into the plate.

Once the bulb and plate are assembled, place them back onto the carburetor and reinstall the screws. Finally, reattach the air filter cover and the main engine cover. That’s it, five minutes, and you’re done.

To avoid problems with the bulb in the future, I recommend changing the bulb whenever it becomes very soft, very hard and when you can’t see through it. These are a few telltale signs the bulb is on its way out.

Tools & Parts to Replace a Primer Bulb

• Screwdriver
• Replacement Primer Bulb

Replacing the Filler Cap

Well, there’s not much to really say here. If you have a bad seal on your fuel cap, then all you need to do is swap it out for a new one and discard the old one. Job done.

Part to Replace a Weed Eater Fuel Cap

• New Fuel Cap

Replacing the Fuel Tank Lines

Swapping out the line can be a bit fiddly as there isn’t much room to work, so I always opt for removing the carburetor. Now, if you think you have room in your particular weed eater, then leaving the carb in place is definitely the quicker option. But on my Stihl weed eater, I have to remove the carburetor. So, let me walk you through the steps you will have to follow.

Steps to Replace the Fuel Tank Lines to the Carburetor

1. Remove the engine cover.
2. Remove the air filter cover.
3. Remove the air filter.
4. Remove the bolts holding the carburetor to the engine.
5. Remove the vent hose from the air filter backplate.
6. Remove the throttle cable.
7. Remove the carburetor.
8. Remove the two lines from the carburetor.
9. Remove the two lines from the fuel tank.
10. Measure and cut two new lines.
11. Attach the new line to the fuel tank.
12. Attach the new lines to the carburetor.
13. Position the carburetor on the engine.
14. Attach the throttle cable.
15. Connect the vent pipe to the air filter back plate.
16. Install the bolts back through the air filter back plate and through the carburetor.
17. Install the air filter.
18. Install the air filter cover.
19. Install the engine cover.

Tools & Parts for Replacing Weed Eater Gas Tank Lines

• Screwdriver
• Pipe Cutter (Sharp Knife)
• New Fuel Line

Clearing the Carburetor Blockage

The process for clearing a blockage on a weed eater’s carburetor, because the primer bulb won’t fill with gas, is a fairly lengthy process with a lot of steps. Therefore, I have put each step in a list you can follow. So, if you have a Stihl or even a Homelite string trimmer, these steps should have you covered and give you a guide to what is required.

Steps to Clear a Blockage in a Weed Eater Carburetor

1. Remove the engine cover.
2. Remove the air filter cover.
3. Remove the air filters.
4. Remove the air filter housing nuts and back plate.
5. Release the tank pressure by loosening the gas cap.
6. Remove the carb fuel intake line.
7. Remove the carb fuel return line.
8. Remove the throttle cable.
9. Remove the carburetor.
10. Remove the carburetor pump plate.
11. Remove the primer bulb & plate.
12. Remove the pump diaphragm.
13. Hold down the pump needle.
14. Spray all the passageways with carburetor cleaner.
15. Reattach the pump diaphragm
16. Reattach the primer bulb & plate.
17. Reattach the carburetor pump plate.
18. Reattach the carburetor.
19. Reattach the throttle cable.
20. Reattach the carburetor fuel return line.
21. Reattach the carburetor fuel line.
22. Position the carburetor.
23. Reattach the air filter housing and nuts.
24. Reattach the air filter cover.
25. Reattach the engine cover
26. Retest using your pressure gauge.

Tools & Parts to Clear a Weed Eater Carburetor

• Screwdriver
• Carburetor Cleaner