If your ATV begins to run poorly, idles roughly, or sputters, these are common signs of an engine misfire.
With a long list of possible causes, diagnosing a misfire can be frustrating and time-consuming. If you’re anything like me, you’d rather spend your time out on the trails than in your garage or visiting the mechanic.
That’s why I made this post so that, hopefully, you’ll get to the bottom of your ATV misfiring issue as fast as possible.
The cause of engine misfire can often be relatively easy to identify and fix. Other times, it could spell big trouble.
What Is Engine Misfire?
The process that makes a gasoline engine run and transforms fuel into mechanical movement is known as the engine combustion process.
A mixture of fuel (gasoline) and air (oxygen) at an appropriate ratio is induced into the cylinder, where the spark plug ignites the mix. The ignited fuel causes a small explosion that puts a piston inside the cylinder into motion.
- The intake valve opens, allowing fuel and air mixture to enter the cylinder (combustion chamber).
- The piston compresses the fuel/air mixture before the spark plug ignites (combusts) the fuel and air mixture.
- The compressed fuel/air mixture expands as it burns, forcing the piston down in the cylinder.
- The exhaust valve opens, allowing exhaust gasses from the burnt fuel into the exhaust pipe.
The cycle repeats itself after the exhaust gasses have exited the cylinder. This process happens incredibly fast, with hundreds or thousands of cycles each minute, depending on engine speed (RPM).
An engine misfire is when an engine, for some reason, is not achieving complete combustion as the fuel and air mixture fails to ignite inside the cylinder (combustion chamber). Misfires can happen sporadically, intermittently, or constantly on one or more cylinders.
An engine misfire can happen at various engine speeds, each with its symptoms:
- During Engine Startup: This leads to difficulty in starting the engine.
- While Idling: The engine idles unevenly or poorly.
- During Acceleration: You may notice the engine hesitates or struggles to pick up speed.
- At Constant Speed: The engine may stutter or show a drop in performance.
Symptoms of ATV Engine Misfire
Misfire problems are usually noticed when you feel the ATV isn’t running as it should. Sometimes, a check engine light may come on before you even realize there’s an issue.
The symptoms are typically more prevalent at lower engine speeds. They are usually less noticeable when driving fast.
Rough Idling or Hard to Start
When an ATV idles in a rough or clunky manner or has difficulty idling without stalling out, there is a good chance it suffers from a misfire issue.
As the air/fuel mixture does not fully combust, there may not be enough momentum to keep the engine running clean between the missed beats.
Related: ATV Won’t Stay Running or Won’t Idle
Excess Smell of Gas
A slight gasoline smell is perfectly normal, especially right after starting the ATV and before it warms up to its normal operating temperature.
When an engine misfires, it fails to burn all the fuel passing through it. This unburnt fuel exits via the exhaust system, producing a noticeable strong gasoline smell.
If an emissions test similar to those for cars were conducted on your ATV, the high level of unburnt fuel in the exhaust would likely lead to a failed emissions test.
Black Exhaust Smoke
When an engine runs rich or experiences a misfire, causing some fuel not to ignite correctly in the combustion chamber but to burn off later, it results in excessive black smoke from the exhaust muffler.
Excess Engine Vibration
In essence, an engine misfire is a very brief engine stop and start, almost like an engine hiccup. Another way to describe the engine’s behavior is that it misses a beat or stumbles.
These short engine stops caused by misfires usually result in a significant increase in engine vibration. You can feel the increase in vibration in the handlebars, gear shifter, or ATV body.
The vibration is more evident at lower engine speeds and is particularly noticeable at idle.
While other things like worn engine mounts could cause excess engine vibration, it is a prevalent symptom of an engine misfire.
Significant Loss of Engine Power
Depending on the severity of the misfire, you may experience a significant loss of engine power in the engine’s entire power band.
If one cylinder goes out entirely on a two-cylinder ATV, it goes without saying that this dramatically impacts engine power.
Jerky or Sluggish Acceleration
A brief hesitation when accelerating is an early sign of an engine misfire.
In more severe cases of engine misfire, you might struggle to reach normal operating speeds, and the acceleration may feel sluggish or uneven.
Misfires usually happen more frequently when the engine is under load, such as during acceleration or heavy pulling, and are therefore more noticeable under these conditions.
Unusual Engine Noises like Sputtering or Coughing
Sputtering is defined as soft explosions or spitting sounds and short coughing sounds.
The latter fits best when describing what a misfire sounds like.
When you get a misfire, the engine will no longer purr smoothly but run with a sputtering or hesitating feeling where it’s instead putting along.
Without realizing it, you’ve likely developed an excellent ear for how your ATV should sound like. When the engine misfires, you tend to notice that the engine somehow sounds off.
Unstable Engine Speed – Power Pulsing
If you are experiencing an engine that is “chasing” up and down in RPM, leaving it unable to maintain a stable engine speed, you have what is called “power pulsing.”
The engine power will drop off and return in a pulsating manner. A misfire issue typically causes this phenomenon.
Random Engine Stalls
Many issues can cause an engine to stall out, but most would make the engine stall consistently.
Engine stalls caused by a misfire are unique because they usually happen more randomly.
Increased Fuel Consumption
An engine that misfires burns fuel inefficiently and uses significantly more fuel than usual.
The Check-Engine-Light Turns On
On modern ATVs, various sensors monitor if there is an issue, such as an engine misfire. One of the sensors registers if there are unexpected drops in engine speed.
As misfires cause short engine drops, this may set off the Check-Engine Light, signaling an issue you should address.
Popping Sounds or Backfiring
People often confuse misfire with backfire, but these are two different phenomena.
Misfire is when the combustion inside the cylinder fails, while backfire is when fuel ignites outside the cylinder, either on the intake or exhaust side.
However, backfire or popping sounds are often a by-product of misfires.
The misfire causes unburnt fuel to be pushed into the exhaust, where the leftover fuel may ignite when it comes in contact with glowing hot metal components.
A backfire on the intake side happens when fuel ignites while the intake valve is open.
What Causes an ATV to Misfire?
An engine misfire is caused by one or more of these usual suspects:
- A disruption to the air/fuel mixture
- An ignition (spark) problem
- Sensor and control module errors
- In rare cases, a mechanical issue
Please note that some causes in our list are unique to fuel-injected (EFI) ATVs, while others only apply to carburated engines. The list is not meant to be complete but covers most causes.
Ignition-Induced ATV Misfire
Anything that prevents the spark plugs from firing at the right time or intensity can lead to a misfire.
Water in the Spark Plug Boot
Let’s start gently with one possible cause that is common and easy to check and fix if necessary.
If you’ve been out riding in heavy rain or playing in wet mud, water may have gotten into the spark plug boot, causing a short. A spark plug that shorts out will undoubtedly lead to an engine misfire.
Remove the boot from the spark plug (it pulls straight off), clean it, and allow it to dry completely before reinstalling the spark plug boot. You can speed up the process with a hair dryer.
A pro-tip is packing the spark plug with di-electric (non-conductive) grease before re-connecting it to the spark plug. This may help prevent the same issue from happening again in the future.
Faulty Spark Plugs
Spark plugs can go bad with no evident sign of damage. Even if the plugs look okay, it may be worth replacing them before spending time and money on other causes.
Dirty or Fouled Spark Plugs
Spark plugs foul up when there is too much fuel, too little fuel, the engine running hot, and other operation imbalances.
A dirty spark plug will not ignite the fuel properly, possibly leading to misfires.
Tip: Learn how to read the spark plugs, which can tell much about the engine’s operating condition.
Wrong Type of Spark Plugs
While spark plugs may look similar, they differ in important parameters such as heat range and thread reach. An engine needs a spark plug type specified in the owner’s manual to run correctly.
Incorrect Spark Plug Gapping
The gap of a spark plug is the distance between the center electrode (the firing part) and the ground electrode/side electrode. Too wide of a gap may lead to an engine misfire.
The distance is set and measured using a gap tool.
A typical spark plug gapping ranges from 0.6mm to 0,9mm. Please refer to your ATV owner’s manual for the correct spark plug gapping. Re-gap the spark plug if necessary.
Damaged or Loose Spark Plug Wires
Cracks or breaks in the wire rubber insulation can lead to shorts, and breaks in the wire conductor will prevent the flow of electricity from the ignition coil down to the spark plugs.
Examine the wires in the dark and watch for shorts/tiny sparks. Also, try misting the wires with water to provoke sparks if there are any cracks.
Remove the wires and bend them lightly to look for cracks in the insulating rubber.
Also, ensure that the spark plug wires are properly connected to the ignition coil and that the rubber boot connector is correctly seated on the spark plug thermal nut.
Faulty Ignition Coil
The spark plug gets its electric charge from the ignition coil. Ignition coils may deteriorate over time until eventually failing. Sometimes, a faulty ignition coil may show up with a fault code. Other times, they break with no warning.
If you feel up for it, you can check if the ignition coil is good by testing the windings for resistance using a multimeter.
Fuel-injected ATVs typically have pen-style ignition coils, one for each cylinder. Carburated ATVs, on the other hand, use conventional cylindrical shaped ignition coils.
Engine-Control Induced ATV Misfire
Modern ATVs use a range of sensors and computers to monitor and adjust the air/fuel ratio, among other things.
If one of the units in the control loop or the wiring between them fails, you will likely experience a misfire.
Components such as ECUs are relatively expensive to replace. It’s often worth it to thoroughly check for more straightforward issues like faulty sensors or wiring before investing in a new module. This approach could save you money and time.
Damaged Wiring Harness
The various valves, sensors, and computers are connected with electrical wires. If one or more of these wires get damaged or come loose, the other components no longer know what to do.
Wiring issues can be challenging to troubleshoot but are usually relatively cheap to repair.
An ECU (Electric Control Module) controls engine management on fuel-injected ATVs. If the ECU fails, the ATV will no longer know how much fuel and air it needs to run correctly.
The CDI (Capacitor Discharge Ignition) is a component in carbureted ATVs that tells the coils when to send electricity to the spark plugs.
Like any electrical component, it can fail, leading to a faulty ignition system and an engine misfire.
Faulty Camshaft Position Sensor or Wiring
The camshaft position sensor measures the position and rotation of the camshaft. It feeds this data to the ECU, which uses this information to determine when to fire the spark plug for optimal engine performance and fuel economy.
Camshaft sensors may fail but, luckily, are relatively cheap and easy to replace.
Faulty Idle Air Control (IAC) Valve
The IAC valve is located on the throttle body of fuel-injected ATVs. The valve works electronically with the ECU to regulate airflow to the engine.
If the valve fails, the airflow will be off, causing an operating condition that is too rich or lean, which, you guessed it, could lead to engine misfire, amongst other things.
Faulty or Dirty Mass Airflow Sensor (MAF)
The purpose of the MAF sensor in a fuel-injected engine is to track how much air enters the engine. This information is sent to the ECU, which signals the IAC valve to regulate airflow according to the ECU calculations.
If the sensor gets dirty or fails, it may send incorrect data to the ECU, which cannot adjust the proper fuel/air ratio.
Faulty Oxygen Sensor (O2 Sensor)
The O2 sensor is another used to fine-tune the airflow in a fuel-injected vehicle. Like the other sensors, it can break, leading to an imbalance in the combustion process.
Fuel-Related ATV Misfire
For optimal performance, an engine requires a precise amount of fuel.
- Excess fuel, known as running rich, can result in incomplete combustion.
- Conversely, too little fuel, known as running lean, might prevent the engine from firing altogether.
Fuel-related misfires differ from the other types because they typically occur more acutely and are more prominent at idle than cruising speeds.
Bad Fuel or Fuel Contaminated With Water
Fuel can go bad over time. Bad fuel can lead to a gummed-up fuel delivery system or carburetor and may not combust properly.
If water gets into the gas tank from heavy rain or being submerged, you should drain the fuel to prevent a misfire. Fuel that is contaminated with water is not able to go through the injectors.
If there is only a tiny amount of water in the fuel, say from condensation, there are additives you can try that are made to remove water from the fuel.
Clogged Fuel Filter
Some ATVs have an in-line fuel filter, and some have a mesh screen before the fuel valve (petcock valve). A clogged screen or filter can prevent enough fuel from reaching the cylinder for combustion.
Inside the carb on a carburated ATV are jets with tiny holes that the fuel needs to go through. These holes clog up quickly if the ATV is run on bad or contaminated fuel or if it’s left sitting for extended periods.
Dirty or Clogged Fuel Injectors
Fuel injectors may clog up wholly or partially due to bad fuel or fuel contaminated with dirt or debris. Fuel additives such as injector cleaner can be worth a try before replacing the injectors.
A partially clogged fuel injector may lead to an intermittent engine misfire. A fuel injector may clog up entirely, not allowing fuel into the cylinder. In this case, the affected cylinder has a continuous misfire.
Faulty Fuel Injector Solenoids
Fuel injectors may fail. You can test the fuel injector solenoids by measuring resistance using a multimeter.
Faulty Fuel Pump
An ATV can have an electrical, mechanical, or vacuum-operated fuel pump. If the fuel pump loses pressure, the engine may not get enough fuel for the combustion process to work correctly.
Fuel lines may develop tiny leaks due to corrosion, material fatigue, or mechanical impacts. This could lead to an imbalance in the fuel/air mixture or contaminated fuel.
Improperly Adjusted Carburetor Fuel/Air Ratio
On carburated ATVs, the fuel/air ratio is not set by a computer but mechanically by the carburetor.
A carburetor that is not tuned correctly will not deliver the right amount of fuel and air to the cylinder for ignition, leading to an engine misfire.
Carburetors are tuned by manually adjusting the adjustment screws or replacing internal components, such as the jets, to a different size.
Air-Related ATV Misfire
Fewer issues can arise with air (oxygen) compared to fuel or spark problems in an engine. However, too little or too much air, often due to insufficient fuel, can still cause the engine to misfire.
Clogged Air Filter
It would be best to service air filters regularly to prevent them from clogging up with dirt, dust, and debris.
A clogged air filter will obstruct the airflow, causing too little air to enter the engine. Without enough air, the engine cannot combust the fuel, leading to a misfire.
Air Intake Leak
The air intake rubber boot on the carb may crack over time, causing excess air to be drawn into the carburetor. Also, look for loose connection clamps, which would have a similar effect.
Cracked air intake is less common, but they may come loose where they connect.
Depending on the design of your specific ATV and its components, it may or may not have various vacuum hoses and pipes.
The vacuum is generally used to actuate components such as a fuel pump or to provide sensor readings.
Look for nicked or damaged vacuum hoses or hoses that have come loose.
Mechanically Induced ATV Misfire
Misfires caused by mechanical failures are the most complex and expensive to repair. Luckily, they are also the least common on our list.
The Engine Timing is Off
ATVs have a timing chain that ensures the crankshaft (piston) and camshaft (valves) work in tune. In rare cases, the chain can jump one or more sprocket teeth or stretch, which also messes with the timing.
Any damage to the valvetrain components (valves, springs, and lifters) can lead to an engine misfire. The most common are possibly bent valves that are not sealing correctly.
Poor Cylinder Compression
The air inside the cylinders must be compressed at a specific rate for the combustion process to work correctly.
Reduced compression can be caused by a leaky cylinder head gasket, burnt-out valves, worn piston rings, or cylinder wear (galling).
To test the engine compression, you can do a so-called leak-down test.
Mechanical Valve Failure
If the intake valve doesn’t open, you are not getting fuel into the cylinder.
And if the exhaust valve doesn’t open, there will be a build-up of burnt fuel and fresh fuel mixed in the cylinder with nowhere to go.
How to Fix an ATV Engine Misfire?
With such a long list of potential causes, it’s good to have a troubleshooting plan.
Unless signs indicate a particular issue, my strategy is always to begin with what’s cheap and easy and work my way down to expensive and complicated.
While the list of possible culprits is long, some are more common in ATVs than others.
ATVs and other power sport vehicles with lightweight, high-power engines are typically exposed to rugged use, making them run hotter than on-road vehicles.
The most common causes of misfires in ATVs are bad spark plugs, spark plug wires or coil, bad fuel, or a faulty fuel delivery system. That’s where I would begin.
The other causes in our list are not as likely but may also be worth looking into if the most likely ones turn out fine.
Can You Drive an ATV With a Misfire?
Riding an ATV with misfiring issues can strain the engine and its components extra.
While misfires may occasionally fix themselves, most of the time, they won’t go away without proper intervention.
If you don’t hear any severe metallic noises that suggest a mechanical failure in the engine, you should be able to safely drive the ATV back home under its own power.
However, you are strongly advised to address the issue before you put on additional miles.
Is ATV Misfire Expensive to Fix?
The cost of fixing an ATV misfire issue depends entirely on what is causing the problem. If bad spark plugs cause the misfire, you can get new ones for $10 to $20. However, if the ECU is faulty or the engine has a mechanical issue, it may cost you several hundred dollars.
How to Prevent Engine Misfire
While there is no way to guarantee that your ATV will never develop an engine misfire, you can considerably reduce the risk with some precautions.
- Proper maintenance. Replace oils, filters, spark plugs, and fuel injectors according to the manufacturer’s maintenance schedule.
- Use only high-quality oils according to spec.
- Use high-quality fuel and fuel stabilizers when the ATV sits for long periods.
- Make the engine work hard now and then. This will burn off deposits from the spark plug tips. Ensure the ATV reaches operating temperature before any hard acceleration or pulling heavy loads.
- Use a torque wrench and torque according to spec each time you work on the ATV engine.
When an ATV Misfire During Acceleration
One of the most common causes to look into when an ATV misfires during acceleration is faulty or fouled spark plugs or bad spark plug wires.
When an ATV Misfires Only at Idle
Usually, if your ATV misfires when idling but runs well at other times, the issue is likely an improper mix of fuel and air in the engine.