Lawn mower timing adjustment. Lawn Mower Blades Hitting Each Other? (Try This)

TimeMaster® 30in Lawn Mower

This rotary-blade, walk-behind lawn mower is intended to be used by residential homeowners. It is designed primarily for cutting grass on well-maintained lawns on residential properties. It is not designed for cutting brush or for agricultural uses.

Read this information carefully to learn how to operate and maintain your product properly and to avoid injury and product damage. You are responsible for operating the product properly and safely.

You may contact Toro directly at for product and accessory information, help finding a dealer, or to register your product.

Whenever you need service, genuine Toro parts, or additional information, contact an Authorized Service Dealer or Toro Customer Service and have the model and serial numbers of your product ready. Figure 1 identifies the location of the model and serial numbers on the product. Write the product model and serial numbers in the space provided.

Important: With your mobile device, you can scan the QR code on the serial number decal (if equipped) to access warranty, parts, and other product information.

This manual identifies potential hazards and has safety messages identified by the safety-alert symbol (Figure 2), which signals a hazard that may cause serious injury or death if you do not follow the recommended precautions.

Easily Fix Your Bent Mower Deck (Cutting Uneven)

This manual uses 2 words to highlight information. Important calls attention to special mechanical information and Note emphasizes general information worthy of special attention.


The engine exhaust from this product contains chemicals known to the State of California to cause cancer, birth defects, or other reproductive harm.

It is a violation of California Public Resource Code Section 4442 or 4443 to use or operate the engine on any forest-covered, brush-covered, or grass-covered land unless the engine is equipped with a spark arrester, as defined in Section 4442, maintained in effective working order or the engine is constructed, equipped, and maintained for the prevention of fire.

The enclosed Engine Owner’s Manual is supplied for information regarding the US Environmental Protection Agency (EPA) and the California Emission Control Regulation of emission systems, maintenance, and warranty. Replacements may be ordered through the engine manufacturer.

Gross or Net Torque: The gross or net torque of this engine was laboratory rated by the engine manufacturer in accordance with the Society of Automotive Engineers (SAE) J1940 or J2723. As configured to meet safety, emission, and operating requirements, the actual engine torque on this class of mower will be significantly lower. Please refer to the engine manufacturer’s information included with the machine.


This machine has been designed in accordance with ANSI B71.1-2012.

General Safety

This product is capable of amputating hands and feet and of throwing objects. Always follow all safety instructions to avoid serious personal injury.

Using this product for purposes other than its intended use could prove dangerous to you and bystanders.

  • Read, understand, and follow the instructions and warnings in this Operator’s Manual and on the machine and attachments before starting the engine.
  • Use your full attention while operating the machine. Do not engage in any activity that causes distractions; otherwise, injury or property damage may occur.
  • Do not put your hands or feet near moving parts of or under the machine. Keep clear of any discharge opening.
  • Do not operate the machine without all guards and other safety protective devices in place and working on the machine.
  • Keep bystanders and children a safe distance away from the machine. Do not allow children to operate the machine. Allow only people who are responsible, trained, familiar with the instructions, and physically capable to operate the machine.
  • Stop the machine, shut off the engine, and wait for all moving parts to stop before servicing, fueling, or unclogging the machine.

Improperly using or maintaining this machine can result in injury. To reduce the potential for injury, comply with these safety instructions and always pay attention to the safety-alert symbol, which means Caution, Warning, or Danger—personal safety instruction. Failure to comply with these instructions may result in personal injury or death.

You can find additional safety information where needed throughout this manual.

Safety and Instructional Decals

Safety decals and instructions are easily visible to the operator and are located near any area of potential danger. Replace any decal that is damaged or missing.

Manufacturer’s Mark

  • This mark indicates that the blade is identified as a part from the original machine manufacturer.
  • Warning—read the Operator’s Manual.
  • Cutting/dismemberment hazard of hand, mower blade—stay away from moving parts; keep all guards and shields in place.
  • Cutting/dismemberment hazard of hand, mower blade—disconnect the spark-plug wire and read the instructions before servicing or performing maintenance.
  • Thrown object hazard—keep bystanders a safe distance away from the machine; shut off the engine before leaving the operating position; pick up any debris before mowing.
  • Cutting/dismemberment hazard of hand, mower blade—do not operate up and down slopes; operate side to side on slopes; look behind you when backing up.

Traction Adjustment

  • Attention; read the Operator’s Manual—1) Loosen the knob by turning it counterclockwise; 2) Pull the cable(s) away from the engine to decrease the traction, or push the cable(s) toward the engine to increase the traction; 3) Tighten the knob by turning it clockwise.
  • Scan the QR code for more information on traction adjustment.


Important: Remove and discard the protective plastic sheet that covers the engine and any other plastic or wrapping on the machine.


Assembling and unfolding the handle improperly can damage the cables, causing an unsafe operating condition.

  • Do not damage the cables when unfolding the handle.
  • If a cable is damaged, contact an Authorized Service Dealer.

Lawn Mower Blades Hitting Each Other? (Try This)

If your lawn mower is making a terrible sound from the blades hitting each other, then I’m pretty sure you have a deck with timed blades. Even though this is a very clever design, it does have its own unique set of flaws that require precise setup and maintenance. So, let’s get the lawn mower into your workshop and inspect what’s going on underneath.

Why Do My Lawn Mower Blades Keep Hitting Each Other? (The Short Answer)

Blades on a multi-blade deck that are coming into contact with each other in the cutting deck have not been aligned on the mounting bolts correctly, are not sitting on the alignment pins/mount, or the pulleys have become misaligned on the toothed deck belt.

A Closer Look at Why Your Mower Blades are Striking Each Other

There are two main types of setups for multi-blade cutting decks: offset and timed blades. The offset blade setup uses a staggered blade positioning so that a cutting overlap is created, and the blades can’t ever touch.

In contrast, a timed setup uses blades set to specific blade positions locked into place with a timing belt. As one blade points side to side, the other is positioned front to back. As the blades rotate, the timing positions keep them in their specific orientation and maintain this spacing distance.

So, let’s see why your mower blades are hitting each other and what has gone wrong with the timing.

Incorrectly Mounted Blades

One thing that timed blades have in common is that their blades lock onto the deck spindles. This is so that if the blade mounting bolt becomes a little loose, the blades can’t turn out of position and end up coming into contact with each other.

Lawn mowers, like the Cub Cadet LT lawn tractor, use a 5-point blade mount, whereas the Toro Timemaster walk-behind uses a two-pin washer to ensure the blades are positioned correctly.

Now, if you forget to install the two-pin washer on your Toro Timemaster, there will be nothing to keep the blades correctly timed. Also, if you don’t position the blades on the Cub Cadet correctly, the blades will end up striking each other.

So, take a look under the deck and inspect the blade mounting position and hardware. For Toro mowers, you want to make sure the washers are in place. If one or two are missing, then this is likely to be the reason your mower blades are hitting each other.

For the Cub Cadet, you need to turn one blade so that it faces side to side in the deck, then check the other blade’s position. If it’s not front to back, the alignment is out. The difference between the two designs is that the Toro blade only has one position, and the Cub Cadet has five due to the five-point star mount.

So, if the Toro blades are mounted correctly, you’ll have to look elsewhere for a problem, whereas the Cub Cadet design allows you to adjust the blade even if something is off with the pulleys or timing belt.

Incorrect Alignment of the Pulleys

When toothed belts are in good condition and tensioned correctly, they do a very good job of keeping the lawn mower blades in sync. Unfortunately, like any belt on a lawn mower, over time, they become old and less effective at doing their job.

When this happens, the teeth on the belts can wear down and stretch out. As a result, even though your blades may be installed correctly, the belt could slip and cause the timing to go out of sync.

So, take a look at the belt and make sure that there isn’t any sign of damage and that the teeth haven’t worn down so much that they’re no longer doing their job. If the belt does look worn, damaged, or loose, you’ll need to install a replacement or check the tensioner.

How to Stop Mower Blades from Hitting Each Other (2 Things to Try)

When carrying out any repair because the lawn mower blades are hitting each other, I like to start at the top and begin with the belt/pulleys and then move on to the blades. Then I always go back and check the timing of the whole system. This is because one repair can affect another. So, let’s start with the belt and pulleys.

Replacing a Cutting Deck Timing Belt

If you discover that your timing belt is to blame for your lawn mower blades hitting each other, then you want to remove it and replace it with a new belt. These are the typical steps to replace a timing blade belt on a Toro Timemaster, which is very similar to other sync belt systems.

  • Remove the spark plug cable and tuck it out of the way.
  • Remove the drive belt guard mounting bolts from the blade/drive pulley.
  • Remove the drive belt guard from the lawn mower.
  • Remove the drive belt from the blade/drive pulley.
  • Engage the blade engagement lever on the handlebar to disengage the blade brake.
  • Loosen off the belt tension plate.
  • Remove the old blade belt.
  • Tilt over the lawn mower.
  • Position one blade side to side.
  • Position the second blade back to front.
  • Feed the new blade belt onto the pulleys.
  • Check that the blades are still synced in the correct position. If they are not, then remove the belt and repeat the process.
  • Re-tension the tensioner plate to the manufacturer’s recommended tension setting.
  • Turn the blades by hand and check that the blades rotate without coming into contact with each other.
  • Position the lawn mower in an upright position.
  • Install the drive belt.
  • Release the blade brake.
  • Replace the belt guard and bolts.
  • Install the spark plug cable.

Tools to Install Toothed Belt on Synced Blade Systems

Now, if your riding lawn mower blades are hitting each other and flipping it over isn’t an option, then there are a few other ways to go about fixing it. The first is to remove the deck completely, or if you have a mower like the Cub Cadet, you can use the pulley markers.

If you take a close look at the blade pulleys, you’ll see some arrows showing you which way the blades are pointing. So, as long as you have the arrows pointing 90 degrees away from each other, you should be good.

Installing Timed Blades Correctly

If you have a riding lawn mower like the Cub Cadet and you just relied on the pulley markers to sync the pulleys, then you still need to check the blades. This is because the star pattern allows you to install the blades in five different positions.

So, just because the pulleys are right doesn’t mean the blades are. Now, if you tilted your mower over during the belt replacement, you should be good. But it’s still important to make sure the blades are actually installed correctly so that you don’t run into any problems. So, let’s go over these two different blade types.

Tools to Install Blades on Synced Blade Systems

Toro Timemaster Push Mower Timed Blade Pin Design

  • Position the pin washer onto the blade spindle so that the tabs point up to the spindle and the pins face downwards.
  • Twist the washer, so it locks into the spindle.
  • Position the blade onto the wash, ensuring the pins are located in the blade pin-holes.
  • Install the blade bolt.
  • Tighten the bolt to the recommended torque.
  • Confirm that both the tabs and pins are seated correctly.
  • Repeat the process for the second blade.
  • Disengage the blade brake.
  • Rotate the blades by hand and check the blades are synced correctly.

Cub Cadet Lawn Tractor Timed Blade Star Design

  • Align the pulley markers to the correct positions.
  • Position the first blade onto its blade pulley mounting position, matching the direction of the pulley marker.
  • Install the blade bolt.
  • Tighten the bolt to the recommended torque.
  • Position the second blade onto its blade pulley mounting position, matching the direction of the pulley marker.
  • Install the blade bolt.
  • Tighten the bolt to the recommended torque.
  • Disengage the blade brake.
  • Rotate the blades by hand and check the blades are synced correctly.

Why It’s Important to Fix Your Troublesome Blades ASAP

If you can hear the awful sound of your blades hitting each other, then you’re definitely going to want to change your cutting blades. Usually, you’ll find that they are damaged beyond any useful state. Even if you resync the blades again, you’ll still notice the symptoms of blade damage and unbalanced blades. So, they are not going to cut very well at all.

Before letting the blades get too far out of sync, you should be aware that damage is being done elsewhere with every spin of the blade. Here are a few problems you might encounter.

  • Pulley Bearing Damage
  • Belt Damage
  • Tensioner Damage
  • Excessive Vibration Leading to Various Mower Issues
  • Mower Deck Issues
  • Stress on the Driveshaft and Engine
  • Lawn Damage

I hope this article answers your question of “why are my mower blades hitting each other?” And if you follow my tips, you’ll have your lawn mower back in sync in no time.

About Tom Greene

I’ve always had a keen interest in lawn care as long as I can remember. Friends used to call me the lawn mower guru (hence the site name), but I’m anything but. I just enjoy cutting my lawn and spending time outdoors. I also love the well-deserved doughnuts and coffee afterward!

Leave a Reply Cancel reply

Copyright © 2010. 2022, All Rights Reserved.

Affiliate disclosure: As an Amazon Associate, we may earn commissions from qualifying purchases from You can learn more about this here.

Lawnmower Won’t Start? Do this.

A lawnmower that won’t start, especially when taken from storage, is almost always due to one problem: bad gas.

Storing a lawnmower in the fall without adding gasoline stabilizer to the fuel tank can cause the fuel to break down and plug the fuel passages. If fixing that problem doesn’t help, there are a few others that can help fix a lawnmower that won’t start, as we explain here.

How to Fix a Lawnmower That Won’t Start

Replace the Bad Gas

Over time (like the six months your lawnmower sat in your garage over the winter), the lighter hydrocarbons in gas can evaporate. This process creates gums and varnish that dirty the carburetor, plug fuel passages and prevent gas from flowing into the combustion chamber.

The carburetor bowl below formed corrosion and deposits during storage, which can easily plug fuel passages and prevent the engine from starting.

Storing equipment without stabilizing the gas can lead to deposits that foul the carburetor or injectors.

lawn, mower, timing, adjustment

Ethanol-containing gas can absorb water from the atmosphere, which can lead to phase separation, which occurs when ethanol and gas separate, much like oil and water. Ethanol that has absorbed enough moisture and has sat long enough can foul the fuel system and prevent the engine from starting.

No matter how many times you yank the pull cord and pollute the air with your advanced vocabulary, the lawnmower won’t start if it’s trying to run on bad gas.

In extreme cases, evaporation of lighter hydrocarbons can change the gasoline’s composition enough to prevent it from igniting. The gas may be fueling the engine, but it doesn’t matter if it won’t ignite.

Bad Gas in Your Lawnmower? Here’s How to Fix It

If you neglected to add gasoline stabilizer to the fuel prior to storage, empty the tank and replace with fresh gas. If the tank is nearly empty, simply topping off with fresh gas is often enough to get it started.

On some mowers, you can easily remove and empty the fuel tank. Sometimes that’s more trouble than it’s worth. In these cases, use a fluid extraction pump or even a turkey baster to remove the bad gas. You don’t need to remove all of it; but try to get as much out as possible.

Clean the Carburetor

You’ve replaced the fuel, but your lawnmower still won’t start.

Next, try cleaning the carburetor. Remove the air filter and spray carburetor cleaner into the intake. Let it sit for several minutes to help loosen and dissolve varnish and gums.

Remove the air filter and spray carburetor cleaner into the intake. Let it sit a few minutes to loosen deposits.

On some carburetors, you can easily remove the float bowl. If equipped, first remove the small drain plug and drain the gas from the bowl. Remove the float bowl cover and spray the float and narrow fuel passages with carburetor cleaner.

This kind of “quick-and-dirty” carburetor cleaning is usually all it takes to get the gas flowing again and your lawnmower back to cutting grass.

If not, consider removing the carburetor from the engine, disassembling it and giving it a good cleaning. Be forewarned, however: taking apart a carburetor can lead to nothing but frustration for the uninitiated. Take pictures with your phone to aid in reassembly. Note the positions of any linkages or the settings of any mixture screws, if equipped. If you’re at all reluctant, visit the servicing dealer instead.

Consider replacing the carburetor altogether. It’s a fairly simple process on most smaller mowers and it’s often less expensive than taking it to the dealer.

Direct compressed air from the inside of the air filter out to remove debris that may be reducing airflow and preventing the lawnmower from starting.

Clean/Replace the Air Filter

With the air filter removed, now’s the perfect time to clean it.

Tap rigid filters on a workbench or the palm of your hand to dislodge grass clippings, leaves and other debris. Direct compressed air from the inside of the filter out to avoid lodging debris deeper into the media.

Use soap and water to wash foam filters. If it’s been a few years, simply replace the filter; they’re inexpensive and mark the only line of defense against wear-causing debris entering your engine and wearing the cylinder and piston rings.

An incorrectly gapped spark plug can prevent the engine from starting. Set the gap to the specification given in the owner’s manual.

Check the Spark Plug

A dirty or bad spark plug may also be to blame. Remove the plug and inspect condition. A spark plug in a properly running four-stroke engine should last for years and never appear oily or burned. If so, replace it.

Use a spark-plug tester to check for spark. If you don’t have one, clip the spark-plug boot onto the plug, hold the plug against the metal cylinder head and slowly pull the starter cord. You should see a strong, blue spark. It helps to test the plug in a darkened garage. Replace the plug if you don’t see a spark or it appears weak.

While you’re at it, check the spark-plug gap and set it to the factory specifications noted in the lawnmower owner’s manual.

If you know the plug is good, but you still don’t have spark, the coil likely has failed and requires replacement.

Did You Hit a Rock or Other Obstacle?

We’ve all killed a lawnmower engine after hitting a rock or big tree root.

If your lawnmower won’t start in this scenario, you probably sheared the flywheel key. It’s a tiny piece of metal that aligns the flywheel correctly to set the proper engine timing. Hitting an immovable obstacle can immediately stop the mower blade (and crankshaft) while the flywheel keeps spinning, shearing the key.

In this case, the engine timing is off and the mower won’t start until you pull the flywheel and replace the key. It’s an easy enough job IF you have a set of gear pullers lying around the garage. If not, rent a set from a parts store (or buy one…there’s never a bad reason to buy a new tool) or visit the dealer.

My Lawnmower Starts But Runs Poorly

If you finally get the lawnmower started, but it runs like a three-legged dog, try cleaning the carburetor with AMSOIL Power Foam. It’s a potent cleaning agent designed to remove performance-robbing carbon, varnish and other gunk from carburetors and engines.

Add Gasoline Stabilizer to Avoid Most of These Problems

Which sounds better? Completing all these steps each year when your lawnmower won’t start? Or pouring a little gasoline stabilizer into your fuel tank?

Simply using a good gasoline stabilizer can help avoid most of the problems with a lawnmower that won’t start.

AMSOIL Gasoline Stabilizer, for example, keeps fuel fresh up to 12 months. It helps prevent the lighter hydrocarbons from evaporating to reduce gum and varnish and keep the fuel flowing. It also contains corrosion inhibitors for additional protection.

I have a five-gallon gas can in my garage from which I fuel two lawnmowers, two chainsaws, two snowblowers, a string trimmer, an ATV and the occasional brush fire. I treat the fuel with Gasoline Stabilizer every time I fill it so I never have to worry about the gas going bad and causing problems.

How To Adjust the Blade Drive System on your Toro TimeMaster

You can also use AMSOIL Quickshot. It’s designed primarily to clean carburetors and combustion chambers while addressing problems with ethanol. But it also provides short-term gasoline stabilization of up to six months.

Use a Good Motor Oil for Your Lawnmower

Although motor oil has no bearing on whether your lawnmower starts or not (unless you don’t use oil at all and seize the engine), it pays to use a high-quality motor oil in your lawnmower.

This is especially true for professionals or homeowners running expensive zero-turn or riding mowers.

Lawnmower engines are tougher on oil than most people realize. They’re usually air-cooled, which means they run hotter than liquid-cooled automotive engines.

They often run for hours in hot, dirty, wet conditions. Many don’t have an oil filter, further stressing the oil.

In these conditions, motor oils formulated for standard service can break down, leading to harmful deposits and reduced wear protection.

For maximum performance and life, use a motor oil in your lawnmower designed to deliver commercial-grade protection, like AMSOIL Synthetic Small-Engine Oil.

Its long-life formulation has repeatedly demonstrated its ability to safely exceed original equipment manufacturer (OEM) drain intervals in the toughest conditions. It provides an extra measure of protection when equipment goes longer between oil changes than is recommended by the OEM.

Mower Engine Valve Adjustment – Step By Step Guide

The number of different components which run in sync and make an engine run correctly is mind-blowing. Fifty times a second, in sequence with the piston, the intake valve must open and push the gas/air mixture into the cylinder and close, the coil must make the spark plug fire at the optimal moment, the exhaust valve has to open, expel the expended gasses, and close.

To adjust the valves on a lawnmower, carry out the following steps.

  • Find out the valve clearances for your lawnmower.
  • Take off the engine components, which restrict access to the valves.
  • Check the valve and pushrod action.
  • Adjust the valves.
  • Put the engine back together.

The intake and exhaust valves must be undamaged, correctly installed, and properly adjusted to ensure that the lawnmower engine works optimally. If this is not the case, the engine will display symptoms that range from being underpowered and difficult to start or may leak oil.

Mower Engine Valve Adjustment

The following sections describe the common symptoms of a faulty or badly adjusted valve are listed.

Assuming that you have narrowed down the possibility that the lawnmower valves need adjusting, the following guide details the steps necessary to adjust the lawnmower engine valves.

The adjustment measurements are different for each engine manufacturer, and these must be ascertained before you make any changes.

The most common lawnmower engines used are

Step One – Access The Valve Chamber

The following steps are needed to access the valve chamber.

  • Remove the spark plug and turn off the gas tap.
  • Remove the exhaust muffler, crankcase breather, and rocker cover.
  • Try to preserve the gaskets because if in good condition, you can reuse them.

Step Two – Check The Valve Action

Request an assistant to pull the starter cord to rotate the engine slowly, and you watch both valves open and close.

Step Three – Check The Pushrod Action

If the engine is an Overhead Cam Unit (OCU), no pushrod will be installed.

On other engines, turning the engine slowly allows you to check that both valve springs are being compressed in turn.

It is useful to check that the pushrods are secured in place and are not bent or damaged.

Step Four – Adjust The Valves

The intake valve is positioned in line with the carburetor, and the exhaust valve is in line with the muffler.

Turn the engine very slowly until the intake valve is fully compressed. It is stage One.

Gently push a blunt object into the spark plug hole and check the position of the piston. Slowly rotate the engine clockwise (by grabbing hold of the blade). When the piston pushes the blunt object out as far as possible, the engine is in the Top Dead Centre (TOD) position and is Stage Two.

Continue turning the blade until the blunt object goes ¼ inch into the cylinder. It is now in stage three, and the valve springs are unloaded, and you can adjust the valves.

Each valve has different gap settings.

Check the gap with a feeler gauge. If it fits and you feel a slight resistance, it does not need adjusting. If loose, it needs adjustment.

Release the locknut on top of the valve.

Turn the adjusting nut 1/8th clockwise while checking the feeler gauge clearance.

When the correct clearance is set, tighten the locknut, and reattach the parts in the reverse sequence.

What Do Lawn Mower Engine Valves Do?

While the combustion itself requires the piston chamber to be completely airtight, it also needs to allow the fuel/air vapor and exhaust gasses to be expelled.

Two valves that open and close function as follows;

  • The intake valve opens as the piston moves up and allows the gas/air mixture into the chamber.
  • The valve closes when the piston is at the top of its travel, making the cylinder airtight.
  • The combustion (explosion) occurs, and the piston is forced down because the chamber is airtight.
  • At the end of its cycle, the exhaust valve opens, expelling the combusted gasses out of the exhaust.
  • The exhaust valve closes, and the cycle continues.

Pushrods or a camshaft activate the valves. The valves are opened and closed by the pushrod or camshaft.

It pushes on rockers, which manipulate the position of the valve.

The valve lash is the space between the valve tip and rocker.

What Are The Symptoms Of Faulty Valves?

If any valves are faulty or dirty, or the valve lashes are too big, the lawnmower engine may begin to display the following systems.

  • It becomes hard to pull on the starter cord.
  • Hard starting.
  • Lack of power.
  • Rough running.
  • Oil leaks.
  • Excessive Smoke.

It Becomes Hard To Pull On The Starter Cord

If the starter cord resists being pulled, it may be because the valves are not opening, and the compression within the piston chamber resists the rope’s movement.

Lawnmowers generally have a release mechanism that opens the valves early and releases the cylinder pressure, making it easier to pull the chord. While this may be the cause, it is more likely, that the valve lash (gap) has become larger through wear and tear and needs to be adjusted.

It is important to note that this is not the only possible cause, and a hard-to-pull chord may also be caused by;

  • A broken branch or some other debris prevents the blade from turning.
  • The pull cord assembly is damaged.
  • There is a faulty with the carburetor needle, causing a hydro lock.
  • The flywheel brake is damaged.
  • The flywheel key is broken.
  • Something else is wrong with the engine, possibly a bent camshaft.

Valves May Need Adjusting When The Engine Is Hard To Start

If you have checked the most obvious issues, including;

  • Bad or insufficient gas.
  • Faulty sparkplug of lead.
  • Clogged air filter.
  • A faulty or gummed-up carburetor.
  • A faulty coil.
  • A bail lever/ignition fault.
  • Bad flywheel timing.

It may be necessary to check the valves.

If the valve gap (lash) gets larger with wear, the valve sticks in place or the valve train components are damaged, it will affect the starting ability of the lawnmower.

If the valve is stuck open, there will be no compression, and the engine will not start. If the lawnmower has been stored for the off-season, it is relatively common for this to be an issue.

The Valve May Be The Issue If The Lawnmower Lacks Power

If the valve lash is too large or the valve seals are damaged, it will allow combustion gasses to escape through the valve and not be available to force the piston down.

The engine will lose power.

If the valve gap is too big, it could produce a tapping sound. It needs to be adjusted or repaired because it will cause unnecessary wear to the engine.

A Faulty Valve May Cause The Lawnmower Engine To Run Roughly

If the valve is sticking, or there is excessive valve lash, it will affect the volume of air and gas in the cylinder, which, in turn, will cause the engine t run roughly.

A simple adjustment will normally correct this.

Faulty Valves May Cause Oil Leaks From The Lawnmower Engine

The oil in the lawnmower engine lubricates the piston chamber walls to allow the piston and rings to move up and down without causing excessive wear.

If the lawnmower is leaking oil or using more than normal, it may be

Excessively Smoking Lawnmower Engine May Be Caused By Valves

If the lawnmower engine is smoking excessively, particularly when you start it or select full speed after it has been idling, it may signal that the valves are an issue.

Other causes of smoke include;

  • Oil leaking into the carburetor or exhaust (tilting the lawnmower the incorrect way)
  • The piston rings are faulty
  • The valve guides are worn or badly adjusted.


A lawnmower that is difficult to start or is not running well may result from misadjusted, faulty, or sticking valves. Accessing the valves is not difficult in most lawnmower engines, and adjustments can be easily made.

However, before you start checking the valves, first ensure that an easier-to-reach component does not cause the problem.


Jason is an expert writer and is passionate about Smart Homes and Home Improvements writing as well as lifestyle and Lawn Care. He spent the past twelve years living in Hawaii, where he worked closely alongside event planners and resort owners to perfect his knowledge of luxury products and aesthetics. He discovered his passion for DIY projects and home improvement there. Currently, he lives in Washington D.C with his family and 2 pets. View all posts