“How do I fix lawn mower surging?” This was a comment left on a video on my YouTube channel the other day. Surging can happen in any small engine, but in this situation we’ll talk about how to fix a lawn mower surging.
What is Engine Surging
When a lawn mower engine “surges,” the throttle alternates between revving and dying. The RPMs run up and then fall off. This pattern repeats itself over and over. It’s the engine’s way of telling you that you have to fix the lawn mower surging. When an engine is surging, it is running inefficiently. As the RPMs fall off, the power production is reduced. Overall, this results in damage to the engine and poor performance.
What Causes Lawn Mower Surging
The number one cause of a surging engine is an interruption to the appropriate air/fuel mix. This can be caused by a number of things, but it’s important to understand the concepts of how your engine works. The engine runs optimally on a predetermined mix of air and fuel. When this proportion is changed, the engine can run inefficiently or not at all.
If the air/fuel mix becomes too “rich”, or there is too much fuel vapor combined with the air, then the engine can run too hot. In this case, it’s not fully burning all of the fuel vapor, and you might even smell an odor of gasoline.
If the air/fuel mix becomes too “lean,” or there is not enough fuel vapor combined with the air, then the engine becomes starved for fuel and will eventually die.
When the engine is surging, it’s oscillating back and forth between too rich and too lean – never quite finding the sweet spot to run optimally.
Tools Needed (Commissions Earned)
Why is my Lawn Mower Surging
Fortunately, there are a few things you can check to diagnose what’s causing the engine to surge. The number one problem is a dirty carburetor. Before you check there, however, go ahead and pull the air filter. This is a quicker inspection, and you have to pull it to get at the carburetor anyways.
Air Filter
The air filter (obviously) filters the air coming into the engine. If the filter is dirty, this restricts the air coming into the engine. This will result in a “rich” air/fuel mix. If the filter is dirty, go ahead and clean it or replace it.
Carburetor
If the air filter is clean, go ahead and remove the carburetor bowl. If there is sediment in the bowl, chances are that your carburetor is in need of a good cleaning. You can try to spray some carburetor cleaner into the jets with it in position, but I prefer to just remove the carburetor altogether.
Oftentimes, the culprit is a dirty or clogged low idle jet. Let’s run through a scenario. You fire up your cold engine with the choke and it starts great. Then, as you back the choke off it starts to surge. If you put the choke back on, or even half on, it smoothes out and runs fine. This points to a plugged low idle jet.
What’s happening is that when the choke is on, or even partially on, it’s restricting the air and creating a “rich” environment. As you turn off the choke more air is introduced. The engine is expecting the low idle jet to provide the necessary fuel, but since it is plugged the air/fuel mix becomes too lean.
It’s important to fix this problem and not just run the engine with the choke on. As we talked about in a previous post, this can create additional problems.
Vacuum
To provide the appropriate air/fuel mix, the engine is designed with specific amounts of vacuum and breathers. In order for fuel to reach the carburetor, there’s a small vent hole on the cap of the fuel tank. If this is plugged, then fuel will be stuck in a vacuum and cannot reach the carburetor. This will result in a lean mixture.
Furthermore, if there are air leaks in the gasket material at the carburetor, air intake gasket, or any of the breather tubes, this will allow too much air into the mix as well.
Remember
Knowing how to fix a surging lawn mower engine is important for any diligent homeowner. As this process involves a little bit of diagnostic work, it’s vital that you understand how the different parts of the engine work together. You’ll have to inspect and diagnose defects with multiple components in order to identify how to fix lawn mower surging.
Simple maintenance on your machine can prevent lawn mower surging. When in doubt, clean the carburetor again.