Lawn Mower Engine Surges – Fix It Yourself

Lawn Mower Engine Surges – Fix It Yourself ​

Have you ever listened to your lawnmower like it was breathing heavily or panting and sounds like it’s gasping for air? These kinds of noises are what we call the lawn mower engine surges. From the word surge which means a sudden or abrupt rush of energy, when it comes to lawnmowers, these surges could mean the engine is experiencing stress due to different reasons. So if you hear that the engine sounds like it decelerates and accelerates even if it’s not in motion, there is something that you need to fix. But what are these surges and can it damage the engine of the lawnmower? 

Surges can be easily identified on a lawn mower engine revs high and low sounds. So instead of a steady hum, you could hear high and low roars and most of the time these surges can affect the performance of the machines. Not only lawnmowers are affected by engine surges but also generators, snowmobiles, leaf blowers, and other machines with small engines that have carburetors in them. So is it only the carburetor that can cause lawn mower engine surges? No. There are different causes. 

Lawnmowers like other small engines always depend on the precise distribution of fuel and air mixture in their combustion chambers. But when some parts of the lawnmower have problems, these can also contribute to the improper distribution of this mixture and some of these parts are what we are going to talk about today and we’ll tell you how to fix these.   

What Causes a Lawn Mower Engine to Surge?

The most probable cause for a surging lawnmower’s engine is a blockage in the fuel supply.However, there could also be other reasons for engine surges such as the following: 

Dirt in the Carburetor.

The carburetor can be the major contributor to why your  lawn mower engine revs high and low. So to answer the question, why does my lawn mower surge at idle and why even when the lawn mower engine surges when it is at full throttle, this is because some dirt could be blocking the carburetor’s jets. 

A carburetor jet is just a tiny hole found in the venturi .The carburetor jet plays a very important role in the distribution of fuel into the combustion chamber by simply balancing the mixture of fuel with the air that’s entering the combustion chamber through the air filter. 

When the carburetor jet gets clogged due to the gummy substance that probably came from dirty gasoline sediments, the distribution of atomized gas to the combustion chamber would not be constant. Thus the burning of the air-fuel mixture would not be constant as well and that will produce the surging sound on the engine. So in the case of lawn mower engine surges due to a carburetor jet problem, what you need to do is to clean this part first then check the other parts of the carburetor if they are freely moving or the inlets don’t have a blockage. Sounds too hard to do? Checkout How To Clean Carburetor On Lawn Mower Save Money on Repairs.

Dirty Air Filter

If the lawn mower engine still surges after clearing out the blockage in the carburetor, check the air filter. The air filter filters the air that’s coming into the engine and oxygen is needed for the fuel to get burned in the combustion process . If the air filter goes bad or gets dirty, airflow  would be limited and the burning of fuel would not be complete. Simply, the proper balance of air and fuel must be maintained so that the engine won’t get starved for air and the lawn mower engine revs high and low will stop.  

How to Fix: 

The steps are simple. Just detach the air filter and open it. If the air filter is dirty and it’s a foam filter, you can clean it with soapy water. Squeeze it to lather, put it under running water while squeezing then squeeze it again in the soapy water to remove the dirt and the oil that may have been caused by a Lawnmower Oil Leak – How to Fix Yourself click here. If the air filter is made of paper, just throw it out and replace it with a new one.    

Issues in the Fuel System

Did you know that the cap on your lawnmower’s gas tank can also cause lawn mower engine surges? And you may not believe that only that small hole in the middle of the cap can be the cause. This small hole allows the air to enter the gas tank so that there is pressure for the gas to come down the hose. However, if this hole gets blocked up due to dirt, or dust or mud during mowing operations, a vacuum will happen inside the tank and will prevent the downward movement of the gas and that will choke the engine of gas.  

Once water and dirt also enter the fuel tank through this small vent, these will enter the fuel line and will cause blockage to the pipes and the engine may choke. If you leave your lawnmower under the sun for a long time, condensation can also occur in the gas tank which will allow water to mix with the fuel while leaving your lawnmower in the rain can also cause lawn mower engine surges due to the contamination of the fuel. 

How to Fix: 

 Take off the cap, check the vent under the sunlight and get a needle and clear the dirt out. If you suspect that the gas has been contaminated, drain it all and put in fresh fuel. When draining fuel, drain the gas out also from the fuel line including stored  gas in the carburetor. You can also clear out the fuel jet of the carburetor by following the instructions above. But if you want to have a quick easy-fix  checkout lawnmower that starts and dies, follow this YouTube link. 

Vacuum Leaks in the Carburetor

So you’ve cleaned the carburetor jet, cleaned out the air filter, and checked the air vent on the fuel’s cap but still the lawn mower engine surges, the last thing you may want to check again is the carburetor. But this time, it could be that the carburetor is just loose which lets the air suck  in through its joints and crevices or around the points where it is attached to the engine. 

When this happens, the extra air can throw the air-fuel mixture out of balance and will prevent the right air pressure needed to get the fuel into the carburetor. To double-check, wiggle the carburetor with your hand and see if you can feel for any small movement. The gaskets on the joints that connect the engine block and the carburetor can also be the issue if it’s old or damaged.  

How to Fix:    

Close the fuel line and release the gas tube leading to the carburetor and detach the carburetor to check on the gasket. If it’s worn out, change it. Then tighten up the carburetor securely to ensure no leakage.  

Poorly Fixed or Dirty or Worn Out Spark Plug

We cannot leave discussing the spark plug if your lawn mower engine still surges. The spark plug provides the spark to ignite the air-fuel mixture in the combustion chamber of the engine. If you have done all of the above troubleshooting, the last thing you can do is check the spark plug. detach it from its socket and thoroughly inspect it. Are there carbon deposits on its terminals or is the spark plug worn out or corroded? Or maybe it’s just not firmly fixed in its seat which can cause up and down revving as the flow of electricity stops and goes.

How to Fix:

 All you need to do is check the spark plug, clean with a wire brush if there are carbon deposits on the terminals, or insert sandpaper in between the center and side electrode and do a sliding motion. Replace the spark plug if it’s worn, old, or corroded then fix the spark plug securely in its seat. You can also check the spark plug’s gap using a gauge and follow the right gap distance based on your owner’s manual. Check  How To Change A Spark Plug On A Lawn Mower here.

Why Does My Lawn Mower Surge at Idle

Some of our readers leave us comments about their lawnmowers like “why does my lawn mower surge at idle?” And some people get bothered by this because they think that lawn mower engine surges can only happen during mowing operations.  

Lawnmower surges during idle are also called the “lean surges” which means while the lawnmower is at its stationary status and the gears are not moving, there are still the causes of the surges that are starving or choking the engine of its fuel. So the causes can be the same as above: carburetor jet blockage, dirty air filter, blockage in the fuel system, air leaks in the carburetor, or defective or loose spark plug. 

How to Fix: 

It doesn’t matter if the lawnmower is at  idle or moving, the distribution of proper air-fuel mixture must remain to constantly burn it in the combustion chamber. So to solve the problem of a lean surge, begin inspecting the unit and you can start by trying to fix using the troubleshooting guides we have mentioned above.

Why the Lawn Mower Engine Surges When It Is at Full Throttle?

Puzzled why a lawn mower engine surges when it is at full throttle? We have already discussed that surges can happen during engine idle. But did you know it could surge on full throttle ? With walking behind lawn mowers the fix is the same weather it happens on idle or full throttle.  

However, the case with the riding lawnmowers can be quite different if the lawn mower engine surges when it is at full throttle. If you have cleaned  the carburetor jets, changed the air filter and the spark plug and you have drained the old gasoline but the engine still surges at full throttle, the problem can come from its governor system. 

The lawnmower governor acts like the cruise control system of your engine and it’s there to maintain the speed of your machine regardless of the engine load. But if you’re going uphill with your lawnmower, the governor will automatically adjust the throttle of your engine so you don’t need to manually adjust the throttle every time you come across a thick patch of grass or come to higher ground. There are three types of governors for small engines: mechanical, electronic, and pneumatic. 

How to Fix:

Adjust the governor to loosen the screw that’s located at the bottom of the governor’s arm. Then push the governor arm to let the throttle wide open followed by turning the bottom clip that is connected to the shaft counterclockwise. The governor shaft will now be set on top of the spool then tighten the screw.  

Why you need to adjust the governor is when the engine consistently surges especially during throttle. And if your engine just surges slow then fast, then adjusting the governor could solve the problem considering that you have already  done the main troubleshooting tips discussed above. 

If you need more troubleshooting tips about lawnmowers and how to detect problems, Visit our lawn trouble shooting blogs here.

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