A surging lawn mower engine can turn an easy Sunday morning mow into a real nightmare. Not only does the sound of the engine revving up and down become annoying pretty quickly, but also the intermittent lack of power makes mowing difficult. So what is going on and causing your mower to surge? Let’s find out.
This article may contain affiliate links, meaning I may get a small commission if you make a purchase through links on this site at no additional cost to you. Visit our affiliate disclosure page to learn more.
Why a Lawn Mower Engine Surges (My Overview)
Lawnmowers typically surge because the fuel mixture switches back and forth from rich to lean. In other words, in one minute, the mixture has too much air and not enough fuel, then too much fuel and not enough air. The following issues usually cause this back-and-forth switch between a good mix and a bad mix, making your lawn mower surge:
- Bad Gas
- Blocked Fuel System
- Dirty Carburetor
- Loose Carburetor
- Stretched Governor Spring
How to Tell in Your Lawn Mower is Surging
You can tell a lawn mower engine is surging by the repeated increase and decrease of the engine revs or sound. The faster the engine runs, the higher the pitch, and the lower the speed/revs, the lower the sound. So you’ll hear your engine quickly speeding up and then slowing down or the sound of the engine going from high to low repeatedly as the engine surges. Adding to this, you’ll also feel the power increase and decrease as your lawn mower surges up and down.
Lawn Mower Engine Revving Too High (A Different Issue)
A lawn mower over-revving is when the engine is either provided with a fuel mix that is too rich or too lean. So, instead of the lawn mower engine surging from high to low, the engine just stays high and over-revs. Even though similar problems typically cause both over-revving and surging, they are, in fact, different. So, you need to tackle the two different issues separately. Most importantly, if your lawn mower engine is surging or your lawn mower is revving too high, it can suffer irreparable damage if you don’t fix it.
Common Causes of a Surging Engine (A Closer Look)
Now it’s time to take a closer look at the common causes of why your lawn mower is surging. I’m pretty sure that one of these issues will be the reason why you’re having trouble with your lawn mower.
Using Bad Gas
Does gas go bad and cause problems? Yes, it does, and yes, it will. Gasoline left unprotected and allowed to degrade is what we call bad gas. Basically, the gas degrades and can no longer combust efficiently. So, one minute the fuel could burn, then it might not. The result is the engine fires one minute and then doesn’t the next. When this happens, you’ll hear your lawn mower surge.
Blocked Fuel System
Any issues in the fuel system, whether in the fuel lines, fuel filter, fuel pump, or even the gas cap, can affect fuel flow, causing the engine to surge.
Gas Cap Problem
Typically a lawn mower gas cap has vents to prevent back pressure from forming in the gas tank as fuel is consumed. So, if the vents malfunction and do not allow air in as needed, back pressure can form, preventing fuel from flowing through the fuel system to the carburetor. Sometimes what you will find is the vent will intermittently release pressure and cause the engine to surge. In really bad cases, the vent won’t let any air through, and the engine will stall. So the best option is to clear the blockage or replace the gas cap completely.
Clogged Fuel Lines
Clogged fuel lines are usually caused by debris, such as grass clippings working their way through the gas tank into the fuel line or from fuel gum caused by bad gas. Either way, the flow of fuel to the carburetor is restricted and causes your lawn mower engine to surge. So, the best approach is to clean out the clogged fuel line.
Blocked Fuel Filter
Just as the fuel line can get blocked, so can the fuel filter. Again the disruption in fuel flow can cause your mower engine to surge. The surge can be caused by there being momentarily enough fuel and then not enough fuel in the carburetor. So, the quickest way to overcome this issue is to replace your lawn mower fuel filter as soon as possible.
Faulty Fuel Pump
Lawnmower fuel pumps typically run into two different issues. The diaphragm either gets clogged with debris or is damaged from regular wear and tear. If you have been running your mower with a dirty fuel filter for a while, it probably facing one of these issues. For example, it’s likely junk has worked its way to the pump and clogged it, or the diaphragm in the pump has split due to the repeated back-and-forth motion needed to pump the gas through the fuel system. Either way, the block or damage will disrupt the fuel flow, and your lawn mower will likely surge. To get around these issues, you could try cleaning your lawn mower fuel pump or installing a replacement fuel pump, depending on the specific issue.
Dirty Carburetor
The next component on your lawn mower that could be causing the surging is the carburetor. If junk managed to get through the fuel systems, it could be blocking the carb and preventing gas from getting to the combustion chamber. Typically, you’ll find that debris will block up the fuel float or the carburetor jet. Also, even if you have managed to keep junk out of the fuel system, you’ll still probably have a build-up of fuel gum or fuel impurities getting in the way. So, you’ll want to clean the carburetor and remove the blockage. Don’t worry, this doesn’t mean you have to strip down the carburetor completely. Usually, you can just remove the fuel float and the jet with the carburetor still attached to your lawn mower. So, even if you don’t know too much about engines, you can still clean your carburetor, as this is a pretty straightforward repair.
Loose Carburetor
Sticking with the carburetor for a moment, you’ll want to check that the carb is securely attached to your mower. It is pretty common for the carb’s mounting bolts to work themselves loose and allow additional air into the cylinder head. This alters the fuel-to-mix ratio, switching between rich and lean as the carburetor rocks around on its mounts. So, locate your lawn mower’s carburetor and check whether it’s securely fixed to the engine. If it’s not, you’ll need to remove the air filter cover and tighten the bolts. Usually, this is enough to fix a surging lawn mower engine. However, if the carburetor has been loose for a while, the gasket could be worn/damaged and needs replacing.
Stretched Governor Spring
The last item you’ll want to check is the governor system and the governor spring. The governor is a connecting rod from the engine. It controls the throttle plate inside the carburetor, which controls the engine speed.
For example, the carburetor throttle plate is wide open when the engine is off. When the engine starts and runs at high revs, the throttle plate closes, reducing the air into the engine and preventing it from overspeeding and damaging itself. The governor spring allows the governor to return to the open position. The engine will surge up and down without this spring in place and correctly tensioned.
So to check if this is the problem, you’ll need to inspect the spring and ensure it’s taught when the engine is off. Plus, you’ll want to ensure it isn’t so tight it stops the governor from moving.
A quick way to tell if the governor spring is the cause of your surging lawn mower engine is to see if the throttle lever connected to the carburetor moves while your mower is running. To do this, place your lawn mower on your driveway, start the engine, and allow it to run without load. The governor should stay still. If your engine is surging, and the carburetor throttle lever is moving, you’ll likely have found the problem. All you need to do is replace the spring, and the problem is solved.