Lawn Mower Engine Surging

Lawnmower 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 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:

  • 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. You are not on your 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 your mower engine is a Honda or Kohler, 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”.

Briggs & Stratton Surging

Surging B&S 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 and allows a vacuum leak.

Metal gas tank B&S

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 shortlist 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:

Gas stabilizer

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”.

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 starts.

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 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 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.

Careful of this spring it lives under the gaskets, 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 the 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 is obvious damage to the manifold or 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 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.

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, 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 the 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 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.

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. 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. 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. Or check out the carburetor deals on Amazon, link below.

Amazon Lawn Mower CarburetorsOpens in a new tab.

Carburetors aren’t so inexpensive, it doesn’t make sense to mess around with them.

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.

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 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.

Gaskets wear out and break down, and that causes surging.

Extreme Caution – 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 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 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 it’s usually carburetor gaskets that 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 manufactures 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 which will show you everything you need to know – “Briggs Manifold Replacing”.

Related Question

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