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. 

Here’s an easy guide to help you determine whether your motorcycle, ATV or side-by-side's engine is running rich or lean. 

What is a Stoichiometric Ratio?

This is the happy place between lean and rich. When you have exactly the right amount of air to burn, all the fuel present, and the exact amount of fuel relative to the air available so that complete combustion takes place, that’s the stoichiometric ratio. If you think of this ratio as a line, rich and lean are found on opposite sides of it.

What is a Rich Running Engine?

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. In essence, 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.

What is a Lean Running Engine?

In a lean condition, there’s a higher percentage of air and a lower percentage of fuel than the stoichiometric ratio. Because there’s less fuel, the engine can’t reach full power, and you end up with an engine running hot because the fuel acts as a coolant and lubricant inside the cylinder.

When you’re running your motorcycle or ATV, you can’t see what's going on inside the cylinder, but there are symptoms you can look for to tell if the engine is running rich or lean:

Symptoms of a Rich-Running Engine

• Uneven acceleration
• Need to keep the throttle wide open to get acceleration
• Noticeable engine performance drop-off in warm conditions, better in the cold
• Black exhaust smoke
• Bad fuel efficiency
• Removing the air intake improves engine performance
• Rough idling where exhaust smells like fuel
• Black soot around exhaust pipe opening and on spark plugs

Watch the video above to learn how to inspect and replace a motorcycle spark plug.

Symptoms of a Lean-Running Engine

• Hard to apply quick throttle, or the engine appears to accelerate when you back off the throttle
• Knocking or pinging coming from the engine
• Engine surges or RPMs fluctuate even at a steady partial throttle
• Slow acceleration
• Engine runs worse in the cold, better on warm days
• Removing the air intake hurts engine performance
• White, flaky residue on spark plugs
• Popping or spitting from the carburetor on acceleration, popping and backfires from the exhaust pipe on deceleration.

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