How to Tell if your Motorcycle or ATV is Running Lean or Rich

When you’re trying to tune a motorcycle or ATV, it’s essential to determine if your machine is running too rich or too lean and adjust accordingly. Neither is good for the engine, so it’s best to know where it's at. 

When it comes to rich vs lean, all we're really talking about is the ratio of fuel to air inside the combustion chamber. Watch the video above or read on below to learn about how to tell whether your motorcycle, ATV or side-by-side's engine is running rich or lean. 

What is a Stoichiometric Ratio?

The ideal ratio is called the stoichiometric ratio, which is basically the sweet spot where you have the right amount of air and fuel mixed together to get complete combustion. That sweet spot ratio is 14.7 to 1. What that means is 14.7 parts of air to 1 part fuel. 

All lean or rich means is the ratio inside the cylinder gets to one side or the other of this optimal combustion proportion. With carbureted engines, you really can't get inside the engine with an O2 sensor to analyze the mix in real time, so you have to look for symptoms to know what's happening inside that cylinder. 

What is a Rich Running Engine?

A motor that runs rich means that compared to that ideal stoichiometric ratio, there’s too much fuel in the mix. The percentage of air is too low and the percentage of fuel is too high. When the spark lights up this air-fuel mixture, you run out of air before all the fuel is burned up, which results in incomplete combustion. 

In a nutshell, a rich-running ATV or motorcycle has a lower percentage of air and a higher percentage of fuel than the stoichiometric ratio. So when combustion occurs in a rich condition, the process runs out of air before it runs out of fuel. This results in an incomplete combustion, and the residual fuel often produces smoke from the exhaust.

For an engine that's running rich, some of the symptoms include: 

• Uneven acceleration 
• Engine bogs before it picks up 
• Having to keep the throttle pinned to gain speed and RPMs 
• Engine runs better in cold conditions than in warm conditions

Because cold air is more dense, it helps burn a little bit more of that extra fuel, so that could be a sign that the engine is running too rich. 

Other signs of a rich-running engine include:

• Black smoke coming from the exhaust
• Black soot around the end of the tailpipe 
• Exhaust and/or spark plug smells like fuel 
• Spark plug is completely black

All of those symptoms mean your motorcycle or ATV is pushing unignited fuel through the exhaust system and then out the pipe. And that also means the fuel efficiency might be affected. If the machine is running just fuel into the cylinder and out the exhaust without lighting it, it’s wasting fuel. 

What is a Lean Running Engine?

On the other hand, a lean running engine is when those percentages are reversed. There’s too much air and not enough fuel in the mix, and therefore the combustion doesn't pack enough punch to get full power. 

In a lean condition, there’s a higher percentage of air and a lower percentage of fuel than the stoichiometric ratio. 

For an engine that's running lean, some of the symptoms include: 

• Hard to apply quick throttle, or it won't accelerate like it should. 
• Engine surging and then lagging as you hold a steady partial throttle. 
• Exhaust backfires on deceleration
• Engine runs better in warm conditions than in cold conditions

Because warm air is less dense, it helps bring the air-fuel ratio closer to where it needs to be. As for the spark plugs, if they look white and flaky, that's a sign the carburetor is running too lean. 

Rich vs Lean: The Air Filter Test

If you’re not sure whether your powersports vehicle engine is running rich or lean, find an open, clean space where debris won’t get in the motor, then remove the air filter and run the engine. 

Removing the air filter increases the air flow into the engine, so if the motor is lean, it already has too much air for the fuel cylinder and it’s going to run worse. However, if the engine is rich, adding that extra air will help even out the air fuel ratio and improve how it runs. 

Rich vs Lean: Carburetor

If you think your powersports vehicle’s engine is running too rich or lean, you might want to clean the carburetor. The carb may also need to be re-jetted, or it may simply be time to replace it.

Watch the video below to learn about whether you need to clean, rebuild or replace a carburetor.