Photo by Kevin Bidwell from Pexels

If you thought that “running rich” is a good thing (at least in the context of motorcycles, mopeds, and scooters), think again.

It’s actually a bad thing.

But what is it, and what are some common symptoms?

In this blog post, you’ll discover:


  • What “running rich” means (and more importantly, why it’s bad)
  • The concept of Lambda 1
  • 4 “running rich” common symptoms
Prefer to get this information in audio form? Listen to the 30 Minute Motorcycling Podcast episode about running rich:

What is “Lambda 1?”

Lambda 1 is a unit used to measure the air/fuel ratio that goes into the combustion chamber for an internal combustion engine.

Without going into too much-complicated math, it all boils down to this – Lambda 1 refers to a perfect and equal amount of both fuel and air.

If the value is lower than Lambda 1, the mixture contains more air than fuel – known as “running lean”

If the value is higher than Lamda 1, the mixture contains more fuel than air – known as “running rich.”

Essentially, if you’re “running rich,” your bike is burning far too much fuel than it should. So how can you tell if this is the case?

4 Common Symptoms That You’re “Running Rich”

Symptom #1: A foul smell from the exhaust

foul smell

Image by mohamed Hassan from Pixabay

Do you smell rotten eggs? Is it coming from your exhaust? 

That’s the biggest telltale sign of a rich fuel mixture. Since there’s more fuel than the air going into your combustion chamber, there’s more carbon build-up.

And in turn, that means that the tailpipe emissions are stronger.

Symptom #2: Poor engine performance

image of a motorcycle engine

Photo by Rachel Claire from Pexels

Remember that the fuel is just one part of a complete internal combustion process – which consists of:

  1. Fuel
  2. Air
  3. Compression
  4. An electric spark from the spark plug

With the amount of air reduced, you know have an incomplete internal combustion process, and consequently a less effective one.

Symptom #3: Bad fuel economy

As if it wasn’t enough that your engine performance will suffer if you’re “running rich,” your fuel economy will also take a hit. 

If your fuel economy suddenly gets worse, it might be because you’re burning through too much fuel. 

Assuming of course that you’re not revving your bike too hard.

Symptom #4: Carbon build-up on your pistons, valves, and spark plugs

Remember what I said earlier about how running rich causes more carbon build-up? Here are some common areas where you’ll find that build-up. 

If your spark plugs look like they’ve been dipped in tar, that’s another common sign that you’re engine’s “running rich.”

How To Fix an Engine That’s “Running Rich”

Fix #1: Clean or replace the air filter

image of motorcycle air filter

Image by Juergen_G from Pixabay

If there’s not enough air inside the combustion chamber, it might be because not enough air passes through the engine.

If the air filter is clogged up, there’s no air going into the engine, and consequently, there’s no air in your fuel/air mixture – it’s mostly fuel.

Therefore, the most logical solution is to replace or clean your air filter

If you’re someone who’s dying to learn about how to work on your bike, you’ll be happy to hear that replacing an air filter is one of the easiest DIY maintenance tasks you can do.

Fix #2: Check the air to fuel ratio for your carburettor (if you’ve got one)

image of a motorcycle carburettor

The job of the carburettor is to control the amount of fuel from the fuel tank. 

Each carburettor has a screw somewhere that adjusts the fuel and air ratio. It has many names depending on the manufacturer – some common ones include “fuel screw” and “idle screw.”

Regardless of its name, if your bike is running rich, try adjusting this screw so that there’s less fuel flow. As always, check your owner’s manual for more information.

Fix #3: Change the O2 sensor (for fuel injected bikes)

Unlike a carburetted bike, a fuel-injected bike controls the fuel flow from a fuel tank through a series of sensors. 

It’s generally quite reliable, but of course, the weakest link is in the O2 sensor – if it breaks down, now you’ve got a problem.

Not all fuel-injected bikes have O2 sensors, but if your bike has one, it might be what’s causing the rich fuel mixture.

“Running rich” is not a good thing, but by now, you’ll not only know some of the most common symptoms but also how to hopefully fix it.