Lawn Mower Engine Surging – Check this easy fix first!
Lawnmower engine surging is a right pain in the Jacksie; it’s an engine that runs erratically and revs up and down by itself uncontrollably. In some cases, it may only happen under certain conditions, for example, only after the mower gets hot or only when the fuel level gets low.
So what causes the lawnmower engines to surge? The most common reason for a surging lawn mower engine is a blockage in the fuel supply, but there are other possibilities:
Often you’ll find playing around with the throttle helps or applying some choke. You are not on your own; this is a regular complaint. In this guide, we will cover the diagnosis, likely causes, and solutions.
Try the easy fix first – replacing/cleaning gapping the spark plug before attempting carburetor work. If your mower engine is a Honda or Kohler, the fix is simple. Honda and Kohler’s surging is commonly caused by a blocked idle jet see “Gas starvation” towards the end of the page.
If you need more help, check out the “How to fix a surging mower video.”
If your surging mower is a Honda, check out the “Honda mower surging video.”
For many mowers, the fix is to replace the carburetor, and as carburetors are inexpensive, it just makes sense to swap it out and save a ton of messing around. You can check out the quality carburetors available and conveniently delivered to your door by Amazon.com.
Need more info on the fuel system, carburetor components, and how they work, you can check them out here.
Briggs Stratton Surging
Surging BS Classic 450, 500, or 550Some engine types are famous for surging; the Briggs Stratton 450, 500, and 550 series engines are fitted with a metal fuel tank and priming bulb-style carburetor. If you have one of these types of engines and it’s surging – You’re in the right place.
If you don’t have this type of carburetor, skip this section and jump to “Surging Test” below. These engines are fitted with a metal fuel tank and carburetor combination. The gasket sandwiched between the tank, and carburetor distorts over time, allowing a vacuum leak.
The vacuum leak causes the surging; replacing the gaskets and cleaning the carburetor/tank will leave it like new, I promise. In this tutorial, we’ll remove the tank/carburetor unit, clean it and replace the gaskets. Just some basic tools are needed, but get yourself a can of carburetor cleaner; it makes the job a lot easier.
In the workshop, I use WD40 carb cleaner, and you can check out all the tools and parts I use here on the “Small engine repair tools” page.
Tools You’ll Need
Here’s a short list of tools you’ll find useful to complete the task of fixing your surging mower. These tools aren’t essential, but they do make the whole job a ton easier; you’ll need:
Fuel treatment – Every small engine owner should use gas treatment. Most people don’t know gas goes off, and gas left in small engines can cause real problems, as you already know.
Using a gas stabilizer will keep the gas in your mower and your gas can fresh for up to two years.
Carburetor gasket – If you’re fixing the BS Classic engine, then you’ll need this gasket set.
Complete carburetor – As an alternative to replacing your BS Classic carburetor gasket, replace the complete carburetor instead; it includes the replacement gasket.
Manifold – This will only be needed if you have confirmed it has failed. Note there are a few different types of manifold pipe, so be sure to check before ordering.
You can check out all these tools on this page “Carburetor Surging Repair Tools.”
This carburetor style is fitted to a few engines and is prone to gasket failure. The job of replacing is simple and will solve the surge. The process is as follows:
Remove the spark plug wire – prevents the mower from starting.
Remove – Remove and clean the air filter and filter housing – Clean it using soapy water, and when dry, smear some engine oil over the surface of the foam. This helps trap dirt.
Remove tank bolts – They hold the fuel tank to the engine.
With fuel tank bolts removed – pull the tank unit straight out gently and remove the governor control link.
Remove the black rubber elbow crankcase breather pipe. Remove the manifold seal and keeper ring. Sometimes they will come loose and get stuck on the manifold pipe.
Remove – Remove carb screws from the carburetor and set aside.
Using a can of carburetor cleaner – clean all the ports on the surface of the fuel tank.
Empty the tank and rinse it out with fresh gas.
Pull the Siphon from the carburetor; they can be stubborn. Remove both gaskets and use carburetor cleaner to clean the siphon metal filter and all ports of the carburetor. Check the primer bulb for damage; mice like to eat them.
Spray – Spray the carburetor with carb cleaner.
Remove – Remove old gaskets and discard them.
Easy Honda HRX Lawn Mower Surging Fix
Careful of this spring; it lives under the gaskets, and it can drop off and be tricky to find, as I know only too well.
The gasket is a two-part kit; the rubber-type gasket faces the tank. (carb fitted here for demo only)
The Siphon pushes back into the carb with a click. If you don’t hear the click, it’s not right – try again.
Refit the carburetor to the tank. Don’t over-tighten the screws, as this will distort the gasket. Fit manifold seal and keeper. Smear a small amount of oil on the seal; it helps it seat.
Clean the intake manifold. The grey tube in this shot is manifold. Inspect it for any signs of damage; they are prone to cracking. This will also cause a surge.
To fully inspect the pipe, you need to remove the pull assembly.
I would only do this if there was obvious damage to the manifold or if I had replaced the carburetor gasket and the engine was still surging.
This manifold is cracked and will cause a surge.
Before refitting the tank, fit the keeper ring and O-ring seal. Lube the seal before refitting the gas tank.
Offer the carb/tank unit up to the manifold and attach the governor link and spring. Now push the unit firmly onto the manifold. Fit both bolts.
Refit the air filter and spark plug wire. Use only fresh gas; make sure your gas can is clean. Gas older than three months is stale.
If, after fitting the gaskets, you still have a surge – Replace the Manifold.
As you know, gas starvation causes an inconsistent flow of fuel which in turn causes erratic running. And you also know a vacuum leak will cause erratic running, but it is a much less common cause; however, some carburetors are prone to vacuum leaks.
As engine manufacturers strive to make their engines more efficient, they have also made the carburetors more likely to clog; this has become a common issue.
To quickly diagnose which problem you have, a clogged carb or vacuum leak, follow this simple test.
You will need a helper to hold the bail lever or improvise with duct tape. CAUTION careful where you place your fingers and toes; the engine will be running, so the blade will be spinning.
Your mower will have a Manual choke, Auto choke, or a Primer bulb. Identify which type your mower has; the test is slightly different for each.
If you have a manual choke – apply half choke with the engine running.
If the engine now runs without surging – Gas starvation is the likely fault. If it runs just the same – A vacuum leak is a more likely fault.
If you have an Auto choke – Remove the air filter cover and filter – place a clean rag over the intake while the engine is running.
If the engine now runs without surging – Gas starvation is the likely fault.
If the engine runs just the same – A vacuum leak is the more likely fault.
If you have a primer bulb – you can still do the test – while the engine’s running (need a helper); give it some extra gas by pressing the bulb.
If the engine now runs without surging – Gas starvation is the likely fault.
If it runs just the same – A vacuum leak is a more likely fault.
If the test revealed gas starvation, it also showed that your problem is likely a dirty fuel jet in the carburetor, or the gas may be stale or contaminated by water.
Cleaning the main jet usually does the job.
Idle Jet Surging – Honda and Kohler use a relatively easy-to-access idle jet that clogs up and causes surging. The Kohler is easier to access than the Honda.
The Kohler is easier to access than the Honda.
Briggs has fitted a plastic carburetor to a range of engines which also clog up and cause surging.
All of these carburetors can be repaired by cleaning, which I’ve covered previously in videos (links below). Videos cover step-by-step removal, cleaning, refitting, and adjusting your carb.
Fixing this is not difficult. Sometimes you can get lucky by just draining and cleaning the gas bowl, which only takes a few minutes.
I have written a complete guide to Carburetor cleaning with pictures; it includes the gas bowl clean-out, which, as said, is worth trying first.
If cleaning doesn’t work out for you, go ahead and swap out the carburetor for a new one. Check out “New lawn mower carburetors page,” here, I’ve listed good quality replacement carburetors for all the most popular engines.
Carburetors aren’t expensive; messing around with them doesn’t make sense.
You might find this page helpful too – “Carburetor repair tools” I’ve listed some really useful tools that make the job easy. Some of these tools I’ll bet you already have some.
But do try cleaning the gas bowl before removing the carburetor.
Finding a Vacuum Leak
Air that enters the combustion chamber without passing through the carburetor is un-metered. This means the fuel-to-air ratio is unbalanced and, in turn, causes erratic engine performance.
When air sneaks in like this, it causes the engine to run lean (lacks gas). A lean engine runs hot, which isn’t good for an engine, especially an air-cooled one.
Vacuum leaks usually occur because of damaged gaskets. Gaskets are sealing materials fitted between the mating surfaces of engine components. Their function is to create an airtight seal.
They are commonly made from paper, felt, cork, Teflon, neoprene, metal, and rubber. The material type is dependent upon where the gasket is to be used.
Gaskets wear out and break down, and that causes surging.
Extreme Caution – You need to be careful, the engine will need to be running, and so the blade will be spinning when running this test.
A vacuum leak check is performed with the engine running and a can of carburetor cleaner; WD40 works, too, (is there anything WD can’t do?)
Spray the cleaner around all carburetor gaskets anywhere the carburetor meets the engine. The trick is to hear an instant change in engine note; that’s the sign of a vacuum leak.
This can be challenging; you must train your ear to notice the instant change in engine note (and not the surging).
Just do a small section at a time; this will allow you to pinpoint the failure area. Jumping the gun and replacing gaskets without finding the actual leak may work out for you or leave you with the same problem after the rebuild.
You’re right in thinking carburetor gaskets usually cause the problem, but other components, such as manifold pipes, can crack or become loose, causing surging.
Fixing A Vacuum Leak
If a leak is detected, replace all carburetor gaskets, and as you have the carburetor removed, go ahead and clean it. Replacement gaskets are available online; you will require the make and model numbers from the engine.
All manufacturers will have a model number printed on a sticker placed on the body or on the engine. Have a poke around; you’ll find it. Most engine manufacturers will stamp the model numbers in an accessible area. Briggs Stratton stamp their numbers on the metal engine cover.
A new carburetor comes with new inlet gaskets; I like to fit original parts where I can; they fit and are guaranteed.
If, after replacing the carburetor gaskets, the engine still surges, you’ll need to go a little further and replace the manifold intake and gasket.
It’s not a big job, and they don’t give a lot of trouble, but they do crack as they get older. I wrote a step-by-step guide showing you everything you need to know – “Briggs Manifold Replacing.”
Honda lawn mower surging fix? To fix a surging Honda lawn mower engine, clean the carburetor, gas tank, and fuel filter. Use fresh regular gas or e10. What causes a lawnmower to run slowly? The most likely cause is a throttle linkage bent out of shape by bumping into the shrubbery or a throttle spring has detached itself.
Hey, I’m John, and I’m a Red Seal Qualified Service Technician with over twenty-five years experience.
I’ve worked on all types of mechanical equipment, from cars to grass machinery, and this site is where I share fluff-free hacks, tips, and insider know-how.
And the best part. it’s free!
What causes lawn mower engine surging?
Fuel system problems or vacuum leaks through the air intake manifold are typical causes of lawn mower surges.
Here’s a rundown of the specific issues causing small engines to quickly cycle between idle speed and full throttle:
- Contaminated gasoline. Old gas or gasoline contaminated with water can cause engine surging.
- Clogged fuel tank cap vent. The fuel tank cap vent can get clogged with dirt or debris. The fuel cap vent helps keep gas flowing smoothly to the carburetor. When it’s clogged, the engine can surge.
- Dirty air filter. The carburetor won’t get a good supply of air when the air filter gets clogged with dust and dirt.
- Worn or damaged air intake gasket. The mower surges when the carburetor sucks air through an unsealed gap in the air intake manifold instead of through the air filter.
- Dirty carburetor. Clogged fuel jets inside the carburetor commonly cause the lawn mower engine to surge. Clogged jets can’t provide the right mix of air and fuel to the engine.
Troubleshooting a lawn mower that is surging
To find the cause of a surging engine, first check the basic issues described above.
- If fuel in your mower’s gas tank is older than six months or you left the lawn mower out in the rain, move the mower to a well-ventilated area and drain the tank. If you see water in the drained gas, you’ll also need to remove the bowl from the bottom of the carburetor and dispose of that gas.
- Check the air vent on the fuel tank cap. Clear the vent hole if it’s clogged.
- Replace or clean the air filter if it’s dirty.
If these basic troubleshooting tips don’t help, then you’ll likely need to make one of the repairs described below.
How do you fix a pulsating lawn mower?
Move the lawn mower to a well-ventilated area and take these steps to fix the engine.
- Remove the air filter and its housing. Check the air filter housing gasket and replace that gasket if it’s worn or damaged.
- If that gasket is okay, check the gasket and seals between the carburetor and the engine. Replace any worn or damaged seals or gaskets.
- If external carburetor seals and gaskets are okay, then you’ll need to rebuild or replace the carburetor. Replacing the carburetor is a much easier repair that rebuilding the carburetor. Follow the steps in our How to replace a lawn mower carburetor repair guide/video to install a new carburetor. If the replacement carburetor for your engine is unavailable, or if you prefer to rebuild the carburetor, order the rebuild kit for your carburetor and follow the steps in our How to rebuild a lawn mower carburetor repair guide/video to clean and rebuild the engine’s carburetor.
How much does fixing a surging lawn mower engine cost?
If you can troubleshoot the surging engine and don’t have to buy parts, the cost of fixing your lawn mower typically isn’t more than the price of replacing the gas.
If you have to buy parts such as an air filter, carburetor or carburetor rebuild kit and you complete the repair yourself, the cost of fixing a surging lawn mower won’t be much more than the cost of the repair parts. The most expensive part that you would have to buy would be the carburetor. You’ll pay about 50 for the replacement carburetor on many common lawn mower engines.
When you have to take the lawn mower to a repair shop to get it fixed, then the repair will typically cost between 50 and 100 to fix a surging engine.
Why Does My Lawn Mower Rev Up And Down Then Die? 5 (Simple) Solutions
So you are here because you want to know why your lawn mower revs up and down, and then dies?
Or maybe the engine is just revving up and down without dying?
Either way, it is an annoying problem to have but the good news is you can usually fairly easily pinpoint what is causing the issue.
And normally it is pretty easy to fix as well.
So let’s take a closer look at how to remedy this problem.
Why Does My Lawn Mower Rev Up And Down Then Die?
If your mower is revving up and down and/or dying, you should check for any clogged components. Most notably this will be the air filter and carburetor, or the diaphragm in the carburetor might simply need replacing. Also, check the spring attaching the governor to the throttle, and make sure your gas isn’t water contaminated.
Just for information, you might have heard this problem called several other things. Sometimes it is referred to as a lawn mower hunting and surging, sometimes as RPM hunting, sometimes as governor hunting, and sometimes simply just surging and revving.
They all mean the same thing and can all be resolved in the same way.
ISSUE #1: Check the Air Filter
The first place to start when trying to determine why your mower engine is revving up and down is to check the air filter.
A dirty or clogged air filter can cause the fuel mixture in the mower to be too rich.
That is because if your air filter is clogged up, it means there is more gas in the carburetor than normal.
All you need to do here is replace the air filter. This is a pretty easy job and the reason why it is the first thing you should check.
ISSUE #2: Check the Carb
The root cause of any engine hunting and surging often involves the carburetor.
An air leak to the carburetor, an incorrect air-to-fuel mixture adjustment or a dirty carburetor can all be to blame.
often than not it is the last of these that is causing the problem. Dirty, clogged jets in the carb, starts off a chain reaction.
The clogged jet means that the engine is not getting enough fuel, the RPM drops, the governor opens the throttle, air and fuel rushes in, the engine surges then drops, and the cycle is then repeated.
To fix this you simply need to clean out the clogged-up jet. Don’t do this using wire as you could damage the jet, instead buy some dedicated carb cleaner or gives it a blow with some compressed air.
It could also be that the rubber diaphragm in the carb needs replacing, as this is how a lot of smaller engines regulate fuel pressure.
When these diaphragms get old they can often dry out and stop working. A new diaphragm should not cost any more than 10, make sure you have the correct model and number to order the right diaphragm as there are more than you would think!
Sometimes the needle valve on the diaphragm can get stuck too. Adding a little seafoam to the gas tank will often loosen the needle.
FIXED!! DOES YOUR LAWN MOWER SURGE AND BACKFIRE? ALREADY BEEN THROUGH THE CARB? THEN YOU MUST WATCH!
Some particular brands are more prone to problems with the carb than others.
If you have a carb on the tank Briggs and Stratton engine that is revving up and down, often it is because the carb diaphragm is getting old and brittle and needs replacing.
Honda builds solid motors, but their carburetors are very finely engineered and very sensitive to gas quality.
If the engine of your Honda lawn mower is revving up and down you might want to disassemble and clean the carb.
Or you could try dropping the float bowl and loosening the vertical jet from below. Then blow it with some compressed air to clean it and reattach.
There is also the option for any of these issues of simply replacing the carburetor. They are fairly inexpensive and can be replaced by a professional if necessary.
ISSUE #3: Check the Governor
Most small engines are equipped with a governor. This stops the engine from over-revving by reducing the carburetor’s throttle opening
Essentially it is there to govern the speed of the engine, and keep it running at a safe speed.
The governor is an air vane that is attached to the throttle wire by a spring, this spring brings the engine back to neutral.
As the engine runs faster, the air from the cooling fins on top of the engine pushes against the air vane. The air vane move the throttle and slows the engine down.
Then as the air decreases the spring reopens the throttle and the engine RPMs increase.
So a lawn mower engine that is continually hunting and surging, indicates there could be a problem with the governor.
And more often than not it stems from the spring that connects the governor to the throttle.
If the spring is dirty, weak or damaged in some way, or has worked itself into the wrong place the governor will not be able to regulate the speed of the engine effectively.
Check the spring to see if it is broken, and move the throttle shaft in the carb to see if it is sticking.
Replacing the spring usually solves the problem.
ISSUE #4: Check the Spark Plugs
Dirt in the spark plugs can cause them to lose contact with each other and misfire.
This could not only make the engine surge, but it could also lead to excessive wear on pistons or valves.
Take a look and replace them if necessary.
ISSUE #5: Has It Been in Storage For a Long Time? Check the Fuel…
Have you just got your mower out for the season? Is it revving up and down?
The ethanol in modern gas is notorious for attracting water. Water sinks to the bottom of the fuel tank, where it gets sucked into the carb and through the engine.
Even just a small amount of water can cause problems for a lawn mower engine.
You will need to get rid of the bad gas and possibly clean the carb too.
Using ethanol-free gas and mixing in some stabilizer should eliminate the chances of this happening again.
How to Fix a Surging Engine: An Overview
Hopefully if you have turned on your mower only to find it constantly revving up and down/hunting and surging and also possibly dying, then this has helped you solve that problem.
Start off by checking the air filter, as that can often be the cause, and if it is it is one of the easiest and cheapest parts to replace.
If that doesn’t solve it, the two main parts to check are the carburetor and the governor.
Often water contaminated gas can be the culprit as well, especially if you don’t use ethanol-free gas.
Either way, I hope this hopes you solve the problem and you are out there mowing your lawn in no time!