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


ATV in forrest

Black smoke and smell of raw gas from your ATV is a sure sign your engine is running too rich, 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

Lets get the easy to check stuff out of the way first, that way we can eliminate a simple quick fix.

Obviously 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 effect 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 looses its OOmph and your engine may not start or feels slow and unresponsive and stink of gas.

Restricted air flow 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 air flow. A quick way to eliminate this as a possibility is to remove the filter and see if it improves the situation.

1 Carburetor Adjustment

ATV engines are pretty simple bits of kit, but the carburetor is definitely one of the more complex components. Your carburetor has 3 main jobs and if it doesn’t get them right, you’ll know about it pretty quickly.

Your carburettor 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, 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 effects the AFR, the closer you get to 14.7 :1 the better your engine will run.

Most ATV’s run a CV style carburetor which 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 simply have been incorrectly jetted.

The height of the needle inside the tube relates directly to 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.

2 Faulty Carburetor

The most common type ATV carburetor is known as the CV (Constant Volatility) type. It’s pretty durable kit but like all components has it’s 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 oversupply of gas to the engine.

Check the carb bowl float level, a bowl that over fills 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 air flow 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 enriches won’t cause a rich condition.

3 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 give away, but you could also by pass the valve, block the vacuum line and test the bike.

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

5 Valves Lash Off

An engine needs to breath, and the valves make that possible. They allow fuel mix in and spent 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 engine vacuum.

Maladjusted valve timing will cause your engine to breath 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.

John Cunningham

John Cunningham is an certified mechanic and writer on ATVFixed.com. I’ve been a mechanic for over twenty 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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