Lawn Mower is Surging (Revs Up and Down) – What to do

A lawn mower that uncontrollably revs up and down indicates a problem with the engine. It may happen at full throttle, when the engine is hot, or at any other time. In many cases, you can troubleshoot and fix the root cause of the issue in your own garage but complicated fixes need a professional service.

Lawnmower engine relies on a precise mixture of fuel and air to produce power. If there is anything that will make either of these exceedingly rich or lean, then it will typically result in engine surging. When a lawn mower surges, it sounds as if its engine reaches full speed, only to decelerate sharply.

Lawn Mower is Surging? (Causes and Fixes)

Engine surging commonly happens when the engine is running on an unbalanced fuel-air mixture ratio. This could be due to problems in the fuel line, faulty air system, issues in the carburetor, or bad gasoline. The following are potential reasons for mower surging and how to fix them.

1. Bad or old gasoline

Gasoline can degrade in as little as 30 days. Upon longer storage, fuel in a lawn mower can go bad when not stabilized. Contaminated or old fuel in a lawn mower can make lawn mower not start or may cause poor engine performance issues like sputtering, stalling, and surging.

How to Fix

If you did not empty the fuel tank or stabilize the fuel before storing your lawn mower for winter or a holiday then you need to drain the bad fuel and add a fresh one.

For a fuel tank that is nearly empty, topping off with fresh gasoline is often enough to solve the problem. If there is a substantial amount of bad fuel in the tank, simply empty the fuel tank. In this case, use a siphon or fluid extraction pump. Alternatively, disconnect the fuel line connecting the tank to the carburetor and empty the gas into a can.

2. Dirty clogged air filter

Air filter helps in filtering the air before it moves into the carburetor where it mixes with fuel. Over time air filters get clogged with dirt or small debris. This automatically reduces the flow of air into the carburetor. This means that your lawn mower will be running on a rich fuel mixture which results in surging.

How to Fix

You should inspect and clean your lawn mower air filter if they are dirty. If your lawn mower air filter is soaked in oil as a result of incorrectly tilting over the mower then you should opt to replace it with a new one. Similarly, for a filter that has overstayed more than the recommended time in the user manual.

Start by locating the air filter on your lawn mower, usually under a plastic cover on the side of the engine. Remove the housing cover and extract the filter. Thoroughly wash the foam filter in a wash sink or with a garden hose to clean out all the dirt. Use a rug to clean inside the housing as well.

If your filter is damaged or badly clogged, then replace it with another one. Once done, place your clean or new filter back into the housing and fit it back on the lawn mower. Surging should stop when you run your lawn mower engine if the air filter was the problem.

3. Bad spark plug

Spark plugs may get dirty or sooty and as a result, fail to produce and deliver the sparks needed for ignition and combustion of the air-fuel mixture. A bad lawn mower spark plug can spark inconsistently and in the process, cause an engine surge. This may also happen with a damaged or loosely fitted spark plug.

How to Fix

You’ll need to remove the spark plug and inspect it. First, disconnect the spark plug cable and remove the installed spark plug using a plug wrench or a plug socket. If dirty, simply drench its tip with a spray-on plug cleaner and gently brush it with a wire brush to remove the soot or dirt.

Replace the spark plug if its electrode or thread is worn out or damaged. Once done, insert your new or clean spark plug back into the socket and gently tighten it. Reattach the ignition cable and start your lawn mower for testing. If spark plugs were the problem, your mower should now run smoothly.

4. Clogged carburetor

The carburetor is where fuel and air are mixed in a regulated ratio before the mixture is delivered into the combustion chamber of the engine. In the carburetor, there are small jets where fuel and air pass through. When these jets are clogged with dirt or small debris it will automatically result in the engine surging.

How to Fix

You’ll need to remove the carburetor from the engine and inspect it. To unclog the jets, you will have to clean the carburetor and its internal components. This means locating where the carburetor is on the lawn mower and removing it to clean. You’ll need pliers and a screwdriver.

To start, disconnect the spark plug cable, shut off the fuel flow and remove the air filter including its housing. This now gives you access to the carburetor and its components. Open the carburetor and clean the innards, including the jets, float, and fuel cup. Use a carburetor cleaner solution or WD-40.

Once done, reinstall all the components of the carburetor including the fuel line, gaskets, and throttle linkage. Fit back the air filter and reconnect the spark plug cable. Start the engine to test your lawn mower. If that is where the problem is the engine should stop surging.

5. Air or vacuum leaks

A carburetor is tightly sealed to the engine using gaskets. Damaged gaskets or a loose carburetor sucks in air through the resulting openings around the damaged gasket. This affects the air-fuel mix ratio and the internal suction created for a smooth flow of gasoline in the carburetor. When this happens the engine is thrown into surging.

How to Fix

Inspect and replace damaged gaskets between the engine and the carburetor. Further, tighten the bolts for a good fit of the carburetor to the engine and other attachments.

Final Thought

Engine surging can happen in both ridings or walk-behind lawnmowers. As long as it’s a combustion engine, the size doesn’t matter. Surging and hunting can affect both the performance and life of a mower engine. Fortunately, it’s something you can figure out and fix or seek the service of a professional technician.

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