What if a Lawn Mower Primer Bulb Doesn't Fill?

Lawn mower primer bulb
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  • Intermediate
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A primer bulb is a small button or bulb usually located adjacent to the carburetor of an engine. The primer bulb’s main function is to prime the engine for combustion. When the bulb is pressed and released, it creates a vacuum that draws fuel into the intake line, which then deposits it into the carburetor. When a primer bulb isn't filling, the engine won’t start because no fuel gets to the carburetor. This can be frustrating, but before you call a mechanic, here are a few troubleshooting ideas.

Cracked Bulb Surface

If there are any cracks on the surface of the bulb, the vacuum needed to suck fuel and deposit it into the carburetor will be absent. In this case, you may just need to replace the primer bulb. Your local hardware store should have it in stock. Replace the assembly according to the owner's manual.

Cracked Rubber Ring Seal

Most primer bulbs have rubber rings around them to seal them. Again, this is to ensure that the vacuum required for sucking fuel into the carburetor is present. If you find cracks in the seal, simply replace the ring. In most cases, you'll need to replace both the primer bulb and the ring.

Air Leak in Intake Line

Inspect the connection of the primer bulb going to the carburetor. If you are pressing the primer bulb and it returns to the original position but does not draw up gas into the fuel line, then you probably have an air leak in the intake line.

The reason could be that the fuel line is the wrong size and is not fitting tightly enough or that the line with the filter isn’t submerged in the gasoline in the tank. If you find this to be the case, replace the fuel line with one that's the proper size and ensure the line is submerged in the gasoline in the tank.

gas intake line on lawnmower

Reversed Lines

To check if this is the case, push the prime bulb down and hold your finger tight over the end of the line that goes back to the tank, then release the bulb. Do this a couple of times and see if it will fill. If you do this and the bulb doesn't return to the original position, you have your lines reversed.

That would give you air entering as a leak from the discharge side of the primer, resulting in the primer bulb not filling. To fix this, simply fix the lines to undo the reversal, referring to the diagram in the owner's manual (you can usually find these online if you don't have the original).

Check Valve is Messed Up

If you put your finger over the suction side and press the primer, it shouldn't return to the original position until you let your finger off and it can suck air through. If it does return to its original position, the check valve is messed up and the primer isn't pumping anything, just pushing in and out. It's possible to get new primer bulbs with bad check valves straight from the package. Just ensure that your check valve is working properly and maintain it by always keeping the pipes free of debris.

small carburetor in hand

Ball in Primer Base is Stuck

If the ball in the primer base is stuck, the primer bulb may not fill. In this case, all you may need to do is to remove the entire carburetor and clean it with a carb cleaner. It's always best to use the carb cleaner recommended by the manufacturer.

If you've checked all of the above troubleshooting ideas and none of them work, it may be time to call in the mechanic. Check out our forums for more troubleshooting ideas here and here.