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Why Is My ATV Running Rich? (6 common reasons)

Black smoke and the smell of raw gas from your ATV sure sign your engine is running too rich. It makes sense to grab this problem early. Ignoring this symptom could be costly.

Top 6 reasons an ATV engine runs rich include:

  1. Carburetor needs adjusting
  2. Faulty carburetor
  3. Faulty Petcock
  4. Restricted air flow
  5. Sticking choke
  6. Ignition system fault

In this post, you’ll learn why your ATV engine is running rich and what you can do to fix it. In this post, we’ll look at the more common carburetor tye ATV. If your bike is fuel injected and running rich, you should begin diagnosing by checking for air filter blockage, spark plug issues, and checking the ECU for fault codes.

Check The Basics

Let’s get the easy-to-check stuff out of the way first. That way, we can eliminate a simple, quick fix.

ATV air filter removal

Air filter – Restricted airflow is also a very common cause of a rich running ATV engine. A dirty, contaminated air filter, kinked or blocked snorkel or intake ducting will prevent proper airflow. A quick way to eliminate this possibility is to remove the filter and see if it improves the situation.

Choke – You know, as your engine warms up, your choke needs to be turned off. If it’s left in the on position, it will supply more gas than the engine can handle, and the engine will run rich.

Engine mods – Engine mods like re-jetting the carburetor will drastically affect the air-fuel ratio (AFR), and the carburetor will need to be tuned.

Gas – Your engine needs fresh gas, and this may seem too obvious, but stale gas causes a lot of problems. Most people don’t know that gas goes stale after about one month unless you use a gas stabilizer.

Old gas loses its Oomph, and your engine may not start or feel slow and unresponsive and stink of gas.

Carburetor Adjustment

ATV engines are pretty simple bits of kit, but the carburetor is one of the more complex components. Your carburetor has three main jobs, and if it doesn’t get them right, you’ll know about it pretty quickly.

Your carburetor is tasked with:

1 Mixing gas and air together
2 Mixing the gas and air to a precise ratio of 14.7 parts air to 1 part gas
3 Supplying the correct amount of gas for the given engine load


But what is a rich condition? Whenever your air to fuel ratio (AFR) is below 14.7 parts air to 1 part gas, your engine is running rich.

The symptoms you’ll already be familiar with, bogging, flooding, black smoke, the smell of raw gas, hard to start, leaking gas, misfiring, backfiring, black plug electrode, etc.


Pilot circuit ATV carburetor

Carburetors are set at the correct air to fuel ratio when they leave the factory but will need to be adjusted for: engine wear, altitude, temperature, humidity, engine modifications, etc.

And that’s why your carburetor is fitted with a fuel mix screw, it may also have an air control screw. Turning these screws directly affects the AFR at idle. The closer you get to 14.7 :1, the better your engine will run.

Air screw

Some carburetors may have an air screw and a fuel screw

Re Jet

Most ATVs run a CV-style carburetor that incorporates a pilot, needle, and main jet. In addition, your carb may be fitted with an accel pump and leak jet. The needle moves up and down inside the emulsion tube, allowing more or less gas in.

Carb jets diagram

The jets are sized to the size of the motor, and having to re-jet a carburetor is common.

Most bikes have had some mods, and a rich running engine may have been incorrectly jetted.

Needle Clip Height

The height of the needle inside the tube relates directly to the gas supply and it is adjustable. A clip set too low will supply excessive gas to the engine. While it is common to move clip position to suit a given situation, using a bike at higher altitude e, modification, etc, many clips are positioned incorrectly, either causing a lean or a rich condition.

As you believe your engine is running rich, your clip may be set too low. To check you’ll need to access it and to do that you’ll need to remove the needle from the carburetor. On some bikes, you’ll need to remove the carburetor completely.

ATV carburetor clip position

A clip at the top of the needle, (e clip) is set in one of 5 grooves (usually middle).

Moving the clip down will increase gas flow to the engine.

Faulty Carburetor

The most common type of ATV carburetor is known as the CV (Constant Volatility) type. It’s pretty durable kit, but like all components, it has its weak points. Carburetors generally are more likely to get gummed up and supply too little gas than too much. But a faulty worn-out jet needle, carburetor bowl float or needle valve, and needle valve seat seal can all cause an oversupply of gas to the engine.

ATV carburetor needle valve

Check the carb bowl float level, a bowl that overfills will flood the engine, causing a rich condition. These are all parts that are easily replaced, and a rebuild kit won’t cost much.

Choke fault

Choke come in a few different forms, and some can be problematic. A choke temporally adjusts the natural lean condition caused by a cold engine. A choke does this by either restricting airflow to the carburetor or by injecting extra gas. The effect is the same.

ATV Choke

Auto chokes are known for causing some problems. Manual chokes are pretty basic and can be checked easily. Plunger style won’t cause a rich condition.

Faulty Petcock

Positioned on or close to the tank, a petcock or fuel valve controls the flow of gas to the engine. Vacuum operated Petcock valves are common and can leak, causing gas to flood the engine.

How do they work? A vacuum line from the carb to the valve applies vacuum as the engine cranks over. The valve opens, and gas flows to the carb bowl.

The petcock employs a rubber diaphragm which fails in a couple of ways. It commonly won’t open all the way, which causes a lean condition (popping and coughing) or gas leaks past the worn diaphragm and into the carb via the vacuum line. That, of course, will lead to a very rich condition.

Finding gas inside this vacuum line would be the giveaway, but you could also bypass the valve, block the vacuum line and test the bike.

Ignition System

You don’t generally think of an ignition system causing a rich condition, but it can. An incorrect spark plug, dirty or faulty, will misfire, and a misfire won’t combust the fuel mix. Similarly, a failing coil (usually when hot), coil wire, coil cap, or failing stator, pick up, or CDI unit will also cause a misfire.

In line ignition system tester

Use an inline spark tested to check for a weak ignition system component.

You’ll find the inline ignition system tool I recommend here on the ATV tools page.

Check the spark plug is gapped correctly and the correct spark plug type is fitted.

Spark plug gap ATV and Dirt Bike

I’ve covered a complete ignition system test previously in this post “Dirt bike won’t start when hot”

Valves Lash

An engine needs to breathe, and the valves make that possible. They allow fuel mix in and spent gasses out of the combustion chamber. The sequence is timed precisely, but the valves are mechanical and fall out of adjustment which causes erratic breathing (running).

Your ATV most likely runs a CV carburetor and unlike a mechanical-style carburetor, a CV carburetor slider is activated by engine vacuum, ie how the engine breaths.

Excessive valve lash

Badly adjusted valves as you know will cause your engine to breathe erratically, which in turn will cause the carburetor plunger to operate erratically.

Fuel entering the engine at the wrong time or in the wrong quantity may cause the engine to misfire and run rich. Valves should be checked and adjusted at scheduled service intervals, usually once per year.

You may find these posts helpful also:

The raw gas will wash the cylinder and blow out the tailpipe instead. If the rich condition is ignored, the raw gas will remove the protective oil from the cylinder and dilute the crankcase oil. The engine will eventually seize.

You may find the following pages and posts helpful:

ATV parts page

Why does my ATV make popping noise?

Can I use WD40 to oil my air filter?

Start a flooded bike

ATV carb leaking gas