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.

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.

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 like re-jetting the carburetor will drastically AFR, and the carburetor will need to be tuned.

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.

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.

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.

Carburetors are set at the correct air to fuel ratio when they leave the factory but will need to be adjusted for: engines wear, altitude, temperature, humidity, engine modifications, etc. And so your carburetor is fitted with a fuel mix screw, it may also have an air control screw.

Turning these screws directly affects the AFR. The closer you get to 14.7 :1, the better your engine will run.

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

The height of the needle inside the tube relates directly to the gas supply and is adjustable.

To access it, you’ll need to remove the needle from the carburetor, and on some bikes, you’ll need to remove the carburetor completely.

A clip at the top of the needle, an e clip is set in one of several grooves (usually middle). Moving the clip down a groove will reduce gas flow to the engine.

Faulty Carburetor

The most common type of ATV carburetor is known as the CV (Constant Volatility) type. It’s a pretty durable kit, but like all components have 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.

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 Chokes come in a few different forms, and some can be problematic.

The basis of a choke is to temporally readjust 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.

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 Petcocks 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.

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.

Valves Lash

An engine needs to breathe, and the valves make that possible. They allow fuel to mix in and spend gasses out of the combustion chamber. The sequence is timed precisely.

The valves are mechanical and fall out of adjustment. This can cause erratic running. Valves should be checked and adjusted at scheduled service intervals.

Your ATV most likely runs a CV carburetor. Unlike a mechanical-style carburetor, a CV carburetor slider is activated by an engine vacuum.

Maladjusted valve timing will cause your engine to breathe erratically, which in turn will cause the 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.

You may find these posts helpful also:

Why does my ATV make popping noise?

Can I use WD40 to oil my air filter?

Start a flooded bike

ATV carb leaking gas

John Cunningham

John Cunningham is a certified mechanic and writer on ATVFixed.com. I’ve been a mechanic for over twenty-five years, I use my knowledge and experience to write articles that help fellow gear-heads with all aspects of ATV ownership, from maintenance & repair to troubleshooting.

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