Lawn Mower Engine
Surging

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
  • Dirty / faulty carburettor
  • 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.

 

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 carburettor. 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 carburettor combination. The gasket sandwiched between the tank and carburettor distorts over time, and allows a vacuum leak.

 

The vacuum leak causes the surging, replacing the gaskets and cleaning the carburettor/tank will leave it like new, I promise. In this tutorial we'll remove tank/carburettor unit, clean it and replace the gaskets.

 

Just some basic tools are needed, but get yourself a can of carburettor cleaner, it makes the job a lot easier. In the workshop I use WD40 carb cleaner, check it out on Amazon.

 

B&S Surging Repair Tools

This link takes you to a list of tools & parts needed to repair the B&S Surging fault.

Surging Repair Tools
mower engine

Carburettor

This carburettor style is fitted to a few engines and is prone to gasket failure.

The job of replacing is simple and will solve the surging.

Remove

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 air filter cover
mower carburetor mower carburetor

Remove

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

Unhook

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 engine
mower carburetor

Seal

Remove the manifold seal and keeper ring.

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

Remove

Remove all screws from carburetor and set aside.

Using a can of carburettor cleaner, clean all the ports on the surface of the fuel tank.

Empty the tank and rinse it out with fresh gas.


mower tank mower tank
mower carburetor mower carburetor mower carburetor mower carburetor

Strip

Pull the Siphon from carburettor, they can be stubborn.

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

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

Spray the carburettor 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.

Fit

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


mower tank mower tank mower tank
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 carburettor gasket and the engine was still surging.

Fit

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!


mower tank mower tank mower tank

Carburettor


mower engine

This carburettor style is fitted to a few engines and is prone to gasket failure.

The job of replacing is simple and will solve the surging.

Remove


mower air filter cover mower gas tank mower gas tank

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.

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

Unhook


mower links mower gas tank seal

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

Remove 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


mower tank mower gas tank

Remove all screws from carburetor and set aside.

Using a can of carburettor cleaner, clean all the ports on the surface of the fuel tank. Empty the tank and rinse it out with fresh gas.

Strip


mower carburetor mower carburetor mower carburetor

Pull the Siphon from carburettor, they can be stubborn.

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

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

Spray the carburettor 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.

Gasket Kit


mower carburetor kit mower carburetor kit mower carburetor kit

Pull the Siphon from carburettor, they can be stubborn.

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

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

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

Oil Seal


mower carburetor seal mower carburetor seal mower carburetor manifold

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.

Refit


mower carburetor link mower carburetor link mower carburetor links

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.

 

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

Manual

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.

Manual


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.  

 

 

Auto


mower carburetor 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.

Auto


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.

Mower air filter cover

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.

 

 

Bulb


mower carburetor primer

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.

Bulb

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.


Ride-on valve lash

Carburettor Repair Tools

This link lists all the likely tools you'll need to repair or replace your carburettor.

Carburettor Tools

Gas Starvation

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

 

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 carburettor 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 carburettor for a new one.

 

New Carburettors

This link lists all the most popular lawn mower carburettors.

New Carburettors

Gas Bowl

Try cleaning the gas bowl before removing the carburettor.


Ride-on mower starter

Gas Bowl


Mower gas bowl

Try cleaning the gas bowl before removing the carburettor.

What's a Vacuum Leak?

Air that enters the combustion chamber without passing through the carburettor 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

Gasket

Gaskets wear out and break down, that causes surging.

Gaskets


Carburetor gaskets

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 carburettor cleaner. Spray the cleaner around all carburettor gaskets, anywhere the carburettor 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 carburettor 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.

 

 

Spray


Mower carburetor Mower carburetor

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 carburettor gasket joints (where the carb connects with the engine).

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

Spray

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

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

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


Mower air filter cover Mower air filter cover

Fixing A Vacuum Leak

If a leak is detected, replace all carburettor gaskets, and as you have the carburettor 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 carburettor 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".

If

 

Model #


Mower gas cap

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 carburettor comes with new inlet gaskets.

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

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 carburettor comes with new inlet gaskets.

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


mower gas cap

Related Questions

Honda lawn mower surging fix? To fix a surging Honda lawn mower engine, clean the carburettor, 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 Lawnmowerfixed.com. 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.