What is a Primer Bulb?

What in the world is that?
Inspector Bill
Small Engine Primer Bulb




They’re clear, red, white, black. They look like little bubble wrap bubbles, or weird little “caps” or tiny little rubber space saucers. Sometimes they have a little hole in the middle, and sometimes they don’t and sometimes; when they go bad they have a much bigger hole (or tear) in other places.

It’s that little, rubber bulb thingy on your engine that fills with gas when you push it and it goes by many names… purge bulb, fuel bulb, purge pump…but today we’re going to refer to it as the “Primer Bulb”.

What in the world does it do?

It’s a pretty simple deal, really. The purpose of the bulb is to draw gasoline into the carburetor from the engine. When you compress the bulb, it creates pressure in the tank and the fuel line. When the bulb expands, it pulls fuel into it; filling the bulb with gasoline. When you compress the bulb again, it pushes the fuel that is already inside of the bulb into the carburetor, and pulls more fuel from the tank into it.

This is called “Priming the carburetor” and it make sure that there is enough fuel in the carburetor to cold start the engine when you pull the cord or turn the key.

Sounds too easy. What’s the catch?

Pushing the bulb too many times will fill the carburetor with too much fuel. This is called “Flooding” the carburetor, and you can usually tell when the carburetor is flooded when gasoline starts leaking into the air filter. When this happens, just walk away for a bit and give that extra gas a chance to evaporate. It might not hurt to take the spark plug out and wipe it dry.

Well, how in the world am I supposed to know how many times to push it?

Most new equipment will tell you how many times to push the primer, but it’s not an exact science. If it says push it three times and that floods the engine, next time push it twice. If that’s too much, push it once. If you continue to have problems, have a technician look at it.


Walbro 188-12-1 Primer Bulb

Thanks Pal. I pushed it, and it broke!

Remember that this little guy is just made of rubber and eventually it’s going to go bad. And by “go bad” I mean it’s going to tear or break or both. Don’t panic, it’s not a piece of junk…it’s just going to happen. It’s exposed to the elements. It’s exposed to rapidly changing temperatures and it’s exposed to ethanol heavy fuel (See: Ethanol is the Devil). The little guy never really stood a chance. But when they do tear, the engine is probably not going to start the way it should, if it starts at all. Why? Because the carburetor is getting more air than fuel, and it doesn’t like that. So, when the bulb goes bad, it’s time to get a new one.

I ain’t no mechanic!

The good news is they they tend to be relatively inexpensive and fairly simple to replace. Many can be taken off by removing a couple of screws. Some are molded into a plastic base. In that case, the entire piece will need to be replaced, but most of these just “snap” into the machine housing with a couple of plastic tabs. Easy peezy…

Once again, when in doubt have a technician look at it.


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Bill Brown

Bill is the head of content creation for the LawnMowerPros Blog and DIY section. He’s been in the Outdoor Power Equipment Industry for years and he’s still learning new things everyday. You can often find him creating featured articles, DIY guides, videos, graphics and much more.