Lawn Mower Engine
Surging

Lawn Mower Engine
Surging

Your Problem Will Likely Be Loose Battery Connections, Flat/Faulty Battery Or Faulty Starter Solenoid.

Causes

01. Bad Gas

02. Dirty/Faulty Carburetor

03. Gas Filter Blocked

04. Gas Tank Blocked

05. Gas Line Blocked

06. Gasket Vacuum Leak 

07. Manifold Vacuum Leak 

08. Governor Control Fault

What's Surging

Lawn mower engine surging is a real pain. 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. 

 

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

 

 

 

What Causes It?

Generally it's caused by either fuel starvation or a vacuum leak in the intake system.

 

Fuel starvation is the usual cause, and as its name suggests, it's a lack of fuel supply to the engine. 

 

There are other possible causes like a governor control fault. These are less likely, and only worth looking at if vacuum and fuel tested OK.

 

 

 

Surging B&S 450, 500 or 550

If you have been directed here but you don't have one of these engines, not to worry. Diagnosing and replacing a regular carburetor gasket is quite simple, it's included in this guide "Carburetor Cleaning".

 

The Briggs and 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've come to the right place.

 

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 will remove tank/carburetor unit, clean it and replace the gaskets.

 

mower engine

Carburetor

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.

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 carburetor 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 carburetor, they can be stubborn.

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

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 carburetor 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 carburetor 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

Carburetor


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.

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 carburetor 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 carburetor, they can be stubborn.

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

Gasket Kit


mower carburetor kit mower carburetor kit mower carburetor kit

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.

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

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.

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 less common cause.

 

To quickly diagnose which problem you have, follow this simple test. It will 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. Do 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

Gas Starvation

If the test reveled 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.

 

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.

 

 

Gas Bowl

Try cleaning the gas bowl before removing the carburetor.


Ride-on mower starter

Gas Bowl


Mower 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

Gasket

Gaskets wear out and break down, that causes surging.

Gaskets


Carburetor gaskets

Gaskets wear out and break down, that causes surging.

Find 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 challanging, because you have to train your ear to notice the instant change in engine note (and not the surging).

 

Best advise, 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.

 

 

 

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 carburetor 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 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 air filter cover Mower air filter cover

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.

 

 

 

 

 

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.

Carburetor


Mower carburetor

A new carburetor comes with new inlet gaskets.

Gasket Kit


Mower 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.


mower gas cap

Carburetor

A new carburetor comes with new inlet gaskets.


Ride-on mower starter
mower starter

Gasket Kit

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

Replacing Briggs Manifold

In the majority of cases the carburetor to fuel tank gasket is the cause of the surging. However, it's not uncommon for the intake manifold to crack causing a vacuum leak (surging).

 

This section will only be relevant to you if you have found obvious damage to the manifold or you have followed the guide as above and the engine is still surging. If so, replace.

 

The manifold is basically a tube that carries the fuel/air mixture from the carburetor across the engine to the intake valve. The tubing is made from a Baekelite type material.

 

 

 

Remove & Order


mower manifold mower engine cover mower manifold damage

Manifold pipe has a few varations, so remove your old one to match it against the new before ordering.

To gain access to the manifold, we will remove the pull assembly, coil and fuel tank.

Crack seen here in the wall of the intake manifold pipe. I have also seen the metal pull assembly come loose and wear through the top of the manifold pipe.

This damage will cause surging, the fix - replace manifold pipe.

Carburetor removed as per tutorial above.

Remove


Mower engine cover mower engine cover

Remove bolt at the front and two at rear either side of the metal pull assembly cover.

Pull assembly pulls straight up, and dipstick comes with it.

No need to remove the cable attached to the front of the cover, just place the assembly out of your way.

Remove


Mower engine coil mower govenor mower manifold mower manifold mower coil fitting

Remove the coil and governor flap by removing these two bolts. No need to remove the control wire attached, just set the whole unit to the side.

Blow away any dirt from the manifold.  Remove both bolts from intake manifold.

Remove manifold and gasket. There are a few different types of manifold fitted to this engine. Use your engine number to order the correct manifold, or remove yours and match it against the new part online.

Now rebuild in reverse order. The coil air gap will need to be set, a business card does the job. Check out "Guide to fitting a coil" which includes pictures.

That's it, your Golden!

Order

Manifold pipe has a few varations, so remove your old one to match it against the new before ordering.


mower manifold
mower engine cover mower manifold damage

Remove

To gain access to the manifold, we will remove the pull assembly, coil and fuel tank.

Crack seen here in the wall of the intake manifold pipe. I have also seen the metal pull assembly come loose and wear through the top of the manifold pipe.

This damage will cause surging, the fix - replace manifold pipe.

Carburetor removed as per tutorial above.

Remove

Remove bolt at the front and two at rear either side of the metal pull assembly cover.

Pull assembly pulls straight up, and dipstick comes with it.

No need to remove the cable attached to the front of the cover, just place the assembly out of your way.


Mower engine cover bolt Mower pull assembly
mower engine coil mower governor mower manifold mower manifold mower coil fitting

Remove

Remove the coil and governor flap by removing these two bolts. No need to remove the control wire attached, just set the whole unit to the side.

Blow away any dirt from the manifold.  Remove both bolts from intake manifold.

Remove manifold and gasket. There are a few different types of manifold fitted to this engine. Use your engine number to order the correct manifold, or remove yours and match it against the new part online.

Now rebuild in reverse order. The coil air gap will need to be set, a business card does the job. Check out "Guide to fitting a coil" which includes pictures.

That's it, your Golden!