Where Is The Primer On My Mower?

Using the primer bulb is key to successfully starting some mower engines. Sometimes just finding the primer bulb is the hardest part. You are in the right place, five minutes from now you’ll be mowing the yard.

A lawn mower primer bulb is located on the side of the carburetor, close to or on the air filter. The air filter is positioned on the opposite side to the engine muffler.

In this post you’ll learn how to find your lawnmower primer bulb, you’ll also learn how and when you should use it.

Mower primer bulb and keeper ring

Primer Bulb

What Does Primer Bulb Look Like?

The primer bulb on a lawn mower is often brightly colored, red is common but black is pretty common too.

Primer bulbs are made from a durable rubber, however they are usually recessed into the air filter housing to protect them from damage.

The primer bulb is held in place using a plastic keeper ring.

Mower primer bulbs

New Primer Bulbs

Primer Bulb Location

Not all mowers are fitted with a primer bulb. But all mowers will employ some type of system that adds extra gas on cold starts.

Mower engine

no primer bulb

If you can’t locate the primer, it is likely your mower doesn’t have one. Your mower may have a manual choke or an auto choke system fitted.

As auto choke is only fitted to higher end mowers, manufacturers usually proudly display a badge “Auto Choke”.

Auto choke

Auto choke

Alternatively your mower may have a manual choke lever which will also be marked choke or display the choke symbol (see below).

Mower choke lever

manual choke lever

Primer bulb on the carburetor

Primer on The Carburetor

Older mower primer bulbs are often located on the carburetor itself.

Mower primer bulb

Primer on The Air Filter Housing

Most modern mower primer bulbs are located on the air filter housing.

How To Use The Primer Bulb?

Press and release the primer bulbs three times, that’s usually plenty of extra gas to start a cold mower engine. Primer bulbs are basic bits of kit, they commonly suffer from wear and tear, they become perished and mice find them irresistible.

If a hole develops in the rubber bulb, it won’t syphon gas and distribute it to the engine.

Replacement bulbs are available are are easy to replace. Remove the keeper ring and remove the bulb.

mower carb primer bulb

Press 3 times

When To Use The Primer Bulb

Mower engines run at their best when the air to fuel is 14.7 to one – that’s 14.7 parts air to one part gas.

It’s referred to as the air fuel ratio or AFR. It’s important to add extra gas when the engine is cold, as cold air is denser and therefor is richer in oxygen.

The increased oxygen upsets the fuel ratio, and the fix is to add extra gas. Three basic types of fuel enrichment systems are common, the primer bulb, manual choke plate and auto choke. the primer bulb will be a prominent feature on the air filter housing.

The primer bulb is only needed to start a cold engine. Warm engines don’t usually require extra gas to start.


Auto Technician and Writer at | Website

John Cunningham is an Automotive Technician and writer on Lawnmowerfixed.com. I've been a mechanic for over twenty years, I use my knowledge and experience to write "How to" articles that help fellow gear-heads with all aspects of mechanical repairs, from lawn mowers to classic cars.