Lawn Mower Engine Surges Up and Down

The engine revs up and then down and up and then down and it doesn’t stop and is downright annoying not to mention that it affects the power output of your lawn mower.

This surging is normally caused by a fuel problem or more specifically, a lack of fuel problem. Something is preventing the gasoline from getting into the engine.

Most likely it is a plugged up idle jet that is causing the fuel not to get into the engine. Idle jets are smaller in size and plug up more easily. So the carburetor will need to be taken apart and thoroughly cleaned. You also want to make sure that there is no air leak where the carburetor attaches to the engine or in the intake as well. An air leak will cause this surging as well.

Now you can reassemble your carburetor onto the engine and then test it to see if it has fixed the problem. You should have no more surging and it should run smooth like it should in normal operation.

If it does not, you may have to replace the carburetor as that is the best course of action if you cannot get it completely clean.

Riding Lawn Mower Surges at Idle

Is your riding lawn mower not working exactly like it should. Does the engine surge up and down when you start it up and idle warming up the engine? A constant rev up and down of the engine is called surging. It is like the engine gets some gasoline and then it doesn’t and slows down.

That is actually what the engine is doing. A small bit of gasoline comes into the engine and then it doesn’t receive any and then it nearly dies and then another small dose of gas arrives and then the cycle begins over again.

This is called a lean surge when the engine is not getting enough gasoline to air fuel mixture. A lean surge can be caused by the carburetor jets being plugged up or an air leak in the intake tract. Air leaks will cause too much air to enter the intake and reduce the vacuum pulling through the carburetor which in turn reduces the amount of fuel being pulled out of the main jet. Then the surging begins.

An air leak can be in the carburetor itself or in the intake manifold or where the carburetor connects to the engine such as an intake gasket leak. Check all these areas carefully to find any air leak before you go and pull the carburetor apart to clean and inspect it.

My Lawn Mower Surges up and Down

A surging lawn mower is one that will drive you crazy! It revs up and then revs down and does this repeatedly and won’t stop. No matter what you try it still keeps on surging.

There is one main reason all this surging is going on. It is caused by a lack of fuel and in some more rare cases, too much fuel. Surging is more common in the springtime after the mower has sat for a while. The fuel phase separates and solidifies in the jets and ports and starves out the engine.

Water in the fuel can also cause this surging. It also causes a popping sound when the fuel and water are ignited.

They key to resolving this issue is to have a really clean carburetor and fuel system. The fuel tank, the fuel lines and the carburetor must be cleaned thoroughly to ensure that the problem is resolved.

If the system cleaning does not resolve the problem, check around the carburetor where it attaches to the engine for air leaks. Check for a torn or missing intake gasket and make sure the bolts are tight that hold the carburetor on.

If you still have problems with your lawn mower surging, visit our locations page for a lawn mower repair service near you.

My Lawn Mower is Surging

Carburetor Jet Plugged

A surging lawn mower is one of the most common problems that an engine has. It normally occurs in the springtime when you first pull out your lawn mower to get it ready for spring. You start it up and then it may slightly surge or it will be extreme. The engine gains RPM and then loses RPM and this cycle will continue nonstop until the condition is repaired.

The mechanics behind it is quite simple. The engine is being starved for fuel. The carburetor jets are plugged up and the engine will suck a little fuel to keep it running and then will start to run out of fuel. Then the engine governor comes into play. As the engine is losing RPM, the governor flyweights retract and will try to increase the RPM by opening up the throttle more. So a little fuel comes in again and the cycle repeats itself. This condition is known as a lean surge.

Air Leak

There could be another factor in play here. An air leak could have developed in the intact tract between the carburetor and the engine. When there is an air leak it will pull in more air than fuel and result in a lean mixture. The engine will run for a short time and then the surging will begin. There is less fuel to burn so it does not produce the power required to keep the engine running. The surging is usually worse at idle than at full RPM.


I want to throw in a third possibility and that is elevation. When you live at high elevation, your carburetor fitted engines should have the main jet replaced with one for high elevation. Everyone calls it a high altitude jet, but altitude means the distance above the ground so that is not the correct term unless you are working on an airplane. This problem can also be worse at low RPM. The cause of this problem is that a standard elevation main jet has a larger hole for the fuel to flow through and you need less fuel when you are at a higher elevation because the air is less dense. Less dense air means less air molecules to burn with the fuel mixture. So you will have the wrong air to fuel ratio and will get too much fuel and run richer than normal. This condition is known as a rich surge.
By knowing the symptoms of these individual problems, this should help you diagnose them. Take your lawn mower to a reputable repair shop to resolve my lawn mower is surging.