Lawn Mower Engine

Lawn mower engine surging is a right pain in the Jacksie, it's an engine that runs erratically, 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 lawn mower engine surging? The most common reason for a surging lawn mower engine is a blockage in the fuel supply, but there are other possibilities:


  • Bad gas
  • Bad spark plug
  • Dirty / faulty carburetor
  • Gas filter blocked
  • Gas tank blocked
  • Gas line blocked
  • Gasket vacuum leak
  • Manifold vacuum leak
  • Governor control fault

Often you'll find playing around with the throttle helps, or applying some choke. Your not on you own, this is a regular complaint. In this guide we will cover the diagnosis, likely causes and the solutions.


Try replacing/cleaning & gapping spark plug before attempting carburetor work.

If you need more help, check out the "how to fix a surging mower video", its just one of the 40 plus videos included in the lawnmower repair video library. You'll easily follow along and as surging is a common mower problem, this video alone will save you a ton on mower repair bills.


If your surging mower is a Honda, check out the "Honda mower surging video".


Surging B&S Classic 450, 500 or 550

Some 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 type engines and it's surging - You're in the right place.



If you don't have this type carburetor, skip this section and jump to Surging Test. These engines are fitted with a metal fuel tank and carburetor combination. The gasket sandwiched between the tank and carburetor distorts over time, and allows 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 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, check it out on Amazon.



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:


Sockets and ratchet - Used to remove the carburetor from the mower. A good flexible set with a wide range of tools will pay for itself very quickly.


Gas and oil siphon - Super useful tool for removing gas and engine oil. Handling these chemicals can be messy and spills are common. The siphon makes the job look easy, no more removing oil drain bungs, just siphon the oil out through the dipstick hole.


This is one of my favorite tools because it saves time and it's mess free.


Carburetor cleaner - Recommended as it does a pretty good job at cleaning the carburetor. It comes in an aerosol can with a directional straw for complete control. It's specially formulated to remove gumming and varnish deposits on carburetor parts.



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 B&S Classic engine, then you'll need this gasket set.


Complete carburetor - As an alternative to replacing your B&S Classic carburetor gasket, replace the complete carburetor instead, it includes the replacement gasket.


Manifold - This will only be needed if you have confirmed that 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", the pictures link to Amazon where you can check the prices.


mower engine


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 surging.

mower air filter cover


Remove the spark plug wire. This prevents the mower from starting.

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.

mower carburetor mower carburetor


Remove these two bolts. They hold the fuel tank to the engine.

mower engine


With fuel tank bolts removed pull tank unit straight out gently and remove the governor control link.

Remove black rubber elbow crankcase breather pipe.

mower carburetor


Remove the manifold seal and keeper ring.

Sometimes they will come loose and get stuck on the manifold pipe.

mower tank mower tank


Remove all screws from 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.

mower carburetor mower carburetor


Pull the Siphon from carburetor, they can be stubborn.

Remove both gaskets and use carburetor cleaner to clean siphon metal filter and all ports of carburetor.

mower carburetor mower carburetor

Check the primer bulb for damage, mice like to eat them.

Spray the carburetor with carb cleaner.

Careful of this spring it lives under the gaskets, it can drop off and be tricky to find, as I know only too well.

mower tank mower tank mower tank


The gasket is a two part kit, the rubber type gasket faces the tank.

The Siphon pushes back in with a click. If you don't hear the click its not right - try again.

Refit the carburetor to the tank. Don't over tighten the screws, as this will distort the gasket.

mower carburetor mower intake manifold

Oil Seal

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 the manifold. Inspect it for any signs of damage, they are prone to cracking. This will also cause surging.

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 I had replaced the carburetor gasket and the engine was still surging.

mower tank mower tank mower tank


Offer the carb/tank unit up to the manifold and attach the governor link.

Now push the unit firmly on to the manifold. Fit both bolts. Refit 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 surging - Replace the Manifold.

Your good to go? Nice work!

The Surging Test

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 is a much less common cause.


If you want to skip this test, you can as I'm willing to bet you've got a plugged up carburetor. Some carburetors are prone to it. As engine manufactures strive to make their engines more efficient, they have also made the carburetors more likely to clog.


To quickly diagnose which problem you have, follow this simple test. It will help you reveal where the fault lies, gas starvation or vacuum leak.


You will need a helper to hold the bail lever or improvise with duct tape. Be careful where you place your fingers and toes, the engine will be running so the blade will be spinning.


Your mower will have Manual choke, Auto choke or a Primer bulb. Identify which type yours is, the test is slightly different for each.


mower throttle lever


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 the more likely fault.

Auto Choke


If you have Auto choke - Remove the air filter cover and air filter - place a clean rag over the intake while the engine is running.  


Mower air filter cover


If the engine now runs without surging - Gas starvation is the likely fault.

If it runs just the same - A vacuum leak is the more likely fault.

Primer Bulb

If you have a primer bulb, you can still do the test - while the engine's running (need helper) give it some extra gas by pressing the bulb.


Ride-on valve lash


If the engine now runs without surging - Gas starvation is the likely fault.

If it runs just the same - A vacuum leak is the more likely fault.

Gas Starvation

If the test revealed gas starvation then it also showed you that your problem is likely a dirty fuel jet in the carburetor or the gas may be stale or contaminated by water.


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. Briggs have 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. It's all covered in the video library. It shows you step by step how to remove, clean, refit and adjust your carb. It's a detailed guide, engine specific, easy to follow and only regular tools are needed. You can check it out here.


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 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.


Some carburetors are so inexpensive, it doesn't make sense to mess around with them, just go ahead an swap them out.


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 tool's I'll bet you already have.


Ride-on mower starter

Gas Bowl

Try cleaning the gas bowl before removing the carburetor.

What's 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 occur usually because of damaged gaskets. Gaskets are sealing materials fitted between 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. 


Mower gasket kit


Gaskets wear out and break down, that causes surging.

Finding a Vacuum Leak

A vacuum leak check is done with the engine running and a can of carburetor cleaner. Spray the cleaner around all carburetor gaskets, anywhere the carburetor meets the engine. The trick is to hear an instant change in engine note.


This can be challenging, because you have to 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 pin point 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 it's usually carburetor gaskets that cause the problem, but other components such as manifold pipes can crack or become loose causing surging.


Extreme caution, the mower will be running during this test, so be careful where you place fingers and toes.


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 on line, you will require the make and model numbers from the engine. 


Most engine manufactures will stamp the model numbers in an accessible area. Briggs & Stratton stamp their numbers on the metal engine cover.


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 which will show you everything you need to know - " Briggs Manifold Replacing".

Mower air filter cover Mower air filter cover


For this test you will need a can of carburetor cleaner. Have a helper hold the bail lever, so the mower stays running.

To test, simply spray the carb cleaner around the carburetor gasket joints (where the carb connects with the engine).

Listen for an increase in engine idle speed - this indicates a vacuum leak.

mower gas cap

Model #

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.

A new carburetor comes with new inlet gaskets.

I like to fit original parts, they fit and are guaranteed.

Related Questions

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 lawn mower to run slow? The most likely cause is a throttle linkage bent out of shape by bumping into shrubbery or a throttle spring has detached itself. 

Auto Technician and Writer at | Website

John Cunningham is an Automotive Technician and writer on I've been a mechanic for over twenty years, I use my knowledge and experience to write "How to" articles that help fellow gear-heads with all aspects of mechanical repairs, from lawn mowers to classic cars.