Why Your ATV Run or Idle Rough and How to Fix It

Rough running or rough idling are terms often used to describe an ATV that’s unable to maintain a steady or smooth engine speed (RPM) but runs irregularly, sputter, misfire, or at decreased engine speeds. 

When an ATV runs or idles rough or sputters, it is usually caused by poor fuel, faulty spark plugs, or a carburetor issue. 

Note: All troubleshooting and testing described in this guide are on a general basis. Please refer to the service manual for specific information and instructions when working on your ATV. 

How to Troubleshoot an ATV That Runs Rough

When your ATV engine runs rough, people often tell you it’s the carburetor. While they might be correct, assuming the cause is rarely the best strategy.

When troubleshooting an issue with multiple possible causes, starting with the least effort and least expensive item on the list makes sense.

An experienced mechanic may narrow the search by listening and observing how the engine runs. But it is probably more feasible for the typical home mechanic to work systematically through the possible culprits. 

While there is no definite rule that fits all situations, I recommend troubleshooting in the following order:

1. Ensure the fuel is good. 
2. Inspect or replace the spark plug. 
3. Clean the carburetor
4. Troubleshoot other ignition system issues
5. Rebuild and tune the carburetor
6. Troubleshoot mechanical issues.

Hopefully, this strategy will help you identify the problem without spending time, money, and frustration on things that are not defective. 

Common Causes & Fixes for an ATV That Runs Rough

As you probably know, an ATV needs fuel, spark, and air in the right amounts and at the right time to run. Any issue disturbing the carefully tuned balance may affect how the engine runs.

The list below includes the most common causes for an ATV that runs rough and a suggested solution to each problem. The causes are not necessarily arranged by their likelihood of happening. 

1. Fuel Quality Problems

The ATV needs clean, high-quality fuel of the specified grade to run correctly, and poor fuel can leave the ATV running rough or prevent it from starting.

Old Fuel Gone Bad

If your ATV runs rough after sitting, chances are the fuel inside the tank and carburetor has gone bad due to old age.

Gasoline can go bad over time due to components of the fuel evaporating, leaving the mix out of balance. Old fuel can leave the engine running rough and can gum up the carburetor so that it no longer mixes fuel and air correctly. 

Water Present in Fuel

Water can get into the gas from condensation through a leaky gas cap or the tank vent. 

Here are nine symptoms that can indicate water is present in the fuel.

Contaminated Fuel

Dirt and debris in the fuel can clog up the fuel lines or the small passages inside the fuel lines, carburetor, or fuel injectors.

Incorrect Fuel

Always run your ATV on the fuel grade and quality as specified by the manufacturer in the owner’s manual. 

Check this post to learn what type of gas to use in your ATV and what can happen if you use the wrong type or grade.

Solution: Drain the gas tank and carburetor bowl of old fuel and add fresh fuel. You might also need to clean or rebuild the carburetor after running bad or contaminated fuel.

2. Fuel Delivery Problems

If the engine is not getting a steady flow of fuel, it cannot run properly. Any issue disturbing the fuel supply system can lead to rough running. Some possible culprits include:

• Plugged or faulty fuel tank vent.
• Kinked or blocked fuel lines.
• Failing fuel pump.

Solution: 

• On carburated ATVs, disconnect the fuel line by the carb to check for a healthy fuel flow.
• Trace the fuel lines and ensure fuel flows freely. 
• Inspect the gas tank vent to ensure the tank doesn’t build a vacuum as you burn fuel.

3. Fouled or Defective Spark Plugs

A healthy spark plug should have a light brown or brown color. The plug is fouled if the plug’s firing end looks black (wet or dry) or grey or has dark deposits or a melted coating. 

Replacing a fouled spark plug may fix the issue temporarily, but you need to identify and fix the underlying root cause for a permanent solution. 

The most common cause of a fouled spark plug is gasoline, engine oil, or coolant contamination. 

Please refer to this guide for troubleshooting an ATV that keeps fouling spark plugs.

Solution: Identify and repair why the spark plugs got fouled in the first place and replace or clean the fouled plugs. 

4. Incorrect Spark Plug Gap or Heat Range

The spark plug gap is the opening between the center and ground electrodes and must be set according to the manufacturer’s specifications. 

Also, the spark plugs you’re using need to meet the specified heat range, or it will affect how the engine runs. 

Solution: Refer to the owner’s manual for the proper specs, and use a spark plug gap tool to set the gap to spec. Replace the spark plug if it’s the incorrect heat range. 

5. Ignition Timing Issues

The ATV’s engine timing system controls when the ignition should fire the spark plugs, fuel injection on EFI models, and engine speed (RPM).

Timing system design varies, but when your ATV runs rough, there are two common culprits to look into

Damaged Trigger Wheel or Loose CPS Sensor

The trigger wheel is a slotted metal wheel, typically mounted to the back of the flywheel. 

A sensor, the Crankshaft Position Sensor (CPS), continuously detects the crankshaft position and feeds this information to the ECU, which uses this data to calculate timing, etc. 

Solution:

1. Inspect the trigger wheel and look for bent teeth.
2. Ensure the CPS sensor has not come loose.
3. Replace or repair parts as necessary. 

Faulty Pickup Coil

The pickup coil, also known as the pulsar or pulse trigger, is located on the stator, and its purpose is to help the timing of the ignition. 

Solution: Test the pickup coil and replace it if necessary. 

This is how you test a pickup coil:

• Remove the stator side cover. The easiest way to locate the stator is to trace the wires from the voltage regulator and take note of where they enter the engine casing. 
• Use a multimeter and test the coil for resistance. 
• Look up the specs for your specific coil online or in the user manual for reference. 

6. Defective Ignition Circuit

The ignition system consists of several components contributing to a healthy spark at the tip of the spark plug. 

Worn or Defective Spark Plug Wires

Inspect the spark plug wires and ensure they are not damaged, cracked, or show any signs of failing. 

Solution: Defective spark plug wires need to be replaced.

Faulty Ignition Coil

Ignition coil design varies depending on specific vehicle needs. To test the condition of your particular coil, you need to look up its specifications online or in the service manual. 

Primary side test (power-in side):

• Disconnect the ignition coil and remove it from the ATV to de-energize it. 
• Place your multimeter in the ohms position.
• Position the probe on each of the coil’s primary side terminals to measure the resistance.
• A standard resistance reading on the primary side of an ignition coil reads anywhere from 0.2 ohms up to 5.0 ohms.

Secondary side test (spark plug side):

• Dual lead coil: Place one test probe in each spark plug cap.
• Single lead coil: Place one test probe in the spark plug boot and place the other on the primary side positive probe. 
• The resistance should read about 5-15 ohms higher than on the primary side due to the resistance in the spark plug wires and spark plug cap. 

Faulty Regulator Rectifier

A healthy voltage regulator’s output should be 13.5V at minimum when the ATV idles and rise to 14 or 14.5V when revving the engine. 

Solution:

1. Ensure all connections are clean and tight.
2. Read the regulator rectifier charge output using a multimeter.
3. Replace parts if necessary. 

Faulty Stator

The stator generates power to run the ignition system and keeps the battery charged. 

Solution: Test stator condition and repair if faulty. You’ll find instructions on how to test the stator in this guide.

Bad CDI or ECU

The CDI or ECU module is central in calculating and deciding when the ignition coil should fire the spark plugs. 

CDI modules are relatively affordable to replace, while an ECY will set you back several hundred dollars. 

Damaged Wiring

A wire with damaged cable insulation may intermittently short out when it touches ground, interfering with the ignition system.

Solution: Inspect the ATV wiring harness for signs of damage or modifications, even if it’s not evident that it’s related to the ignition system. Repair or replace damaged wiring. 

Poor Engine and Battery Grounds

Poor main grounds can interfere with the ignition. 

Solution: Check, clean, and repair all engine or battery grounds.

7. Low Battery Voltage

While some ATVs will run just fine with a bad battery, others need a fully charged battery for the ATV electrics to operate correctly. 

Depending on vehicle designs and technology, as battery voltage gets low, some ATVs start having electrical issues that might affect the ignition system. 

Solution: Recharge the battery to 12.8V. 

8. Carburetor Problems

This applies only to ATVs with a conventional carburetor, not electronic fuel injection models (EFI).

Dirty Carburetor

Before you tune the carburetor, it’s a good idea to give it a cleanse to see if that solves the issue. 

In less severe cases, you might get away by running a couple of gas tanks with carb-cleaning additives like Sea Foam.

However, complete carb disassembly and deep cleaning are required in more severe cases. 

To clean the carburetor, take it apart and use a carb-cleaner, a small brush, and compressed air to remove all contaminants. 

Ensure the float and needle valves move freely and no needle jets are clogged. Some carbs are vented, and the vent tube may get clogged. 

Faulty or Worn Carburetor Internals

The components inside a carburetor may wear out or become damaged due to corrosion, old age, and use.

Sometimes, you must rebuild the carburetor to replace things like the float valve (needle valve), main jet, dried-out o-rings, or cracked seals.

You can buy complete rebuild kits with all the necessary parts for most carburetors.

If you don’t feel like rebuilding your car, consider buying a new complete unit. Remember to tune the new carb for the ATV to run correctly. 

Improper Carburetor Tuning

Carburetors need to be perfectly tuned for the ATV to run correctly.

Tuning can be done by adjusting the carburetor or changing jets and needles inside the carb to different sizes.

Basic carburetor adjustment:

• Locate the fuel/air mixture adjustment screw, typically located in front of the float bowl.
• Turn the screw clockwise until it seats lightly. 
• Back out 1 ½ to 2 full turns as a starting point. 
• Start the engine and allow it to idle with the choke off. 
• Turn the screw in until the engine begins to stutter.
• Turn the screw out until the idle smooths out. Continue until the engine starts to stutter again. Count the turns.
• Divide the number of turns in half and turn the screw this amount of turns back in. 
• The engine should now run smoothly. Adjust the idle screw if necessary. 

9. Vacuum Leak

On some ATVs, the fuel pump and carburetor operate by vacuum. Holes no bigger than a pinhole can lead to rough idle.

Solution: Thoroughly inspect the vacuum hoses and replace them if damaged. 

10. Clogged Air Filter

Inspect the air filter to ensure it’s clean and properly installed. Replace if necessary; get quality replacement air filters over at KN Airfilters.

Also, check for cracks in the air intake rubber boot, which could bring the air/fuel mix out of balance.