“Running rich” or “running lean” – these terms are very common in the bikers’ world. Some say you are better off running a little rich, while others say running lean is better. So which one is it?

After this article you will know the difference between running rich and lean in detail. What they are, why do you want any of them and what exactly should you be running. 

TLDR – running just a little lean could improve fuel economy and give extra power. However, run too lean and you risk engine failure because the engine runs too hot. 

Whereas running rich can waste fuel and increase pollution but will not damage the engine.  You ideally want to run at the perfect ratio of 14.7:1. But if you have to choose one, then go for the rich. Running lean could be destructive. 

Satisfied with the answer? I hope not – because I have much more things to talk about. Let’s begin with the difference in running rich and lean.

Motorcycle Running Rich Vs Lean: THE DIFFERENCE

Just a quick one – before we talk about the difference, let’s first understand how a motorcycle engine generates power.

A motorcycle engine is a combustion engine – which means it uses a mixture of air and fuel to create combustion and generate power. 

For a great combustion, the ratio of air and fuel must be perfect – 14.7:1 to be exact. That means 14.7 Kg of air is required to burn off 1 Kg of fuel. 

Every motorcycle engine is configured from factory to correctly inject the right amount of fuel based on the incoming air. This ensures great and powerful combustion – which in turn gives you the most power with a great MPG. 

Running rich means that you have excess fuel in the combustion chamber as compared to air – this could be a 10:1 air/fuel ratio.  This results in fuel that cannot be burned completely. 

Running lean means that you have excess air in the combustion chamber as compared to fuel – this could be a 17:1 air/fuel ratio. There’s too much air and not enough fuel to burn. 

Both are not ideal and one is much more dangerous than the other. Which one is it? Let’s talk about the consequences next.

Motorcycle Running Rich Vs Lean: CONSEQUENCES

Consequences of Motorcycle Running Rich 

Motorcycles that’s running rich will have a horrible fuel efficiency (MPG). The unburnt fuel from the combustion chamber will escape into the environment through the exhaust. 

Some symptoms / consequences when motorcycle is running rich:

  1. Horrible fuel efficiency. The excess and unburnt fuel are escaping out of your motorcycle without being used! Your miles per gallon will quickly tank and it should be noticeable. 
  2. Black smoke coming out from exhaust tips. This happens when your engine is running extremely rich. But could also be caused by other factors like burning engine oil
  3. Black exhaust tip. This is usually caused by the carbon buildup from the escaping unburnt fuel. 
  4. Smell of unburned fuel. If your bike always smells of unburned fuel after you ride it, then that could also indicate an engine running rich. 
  5. Black spark plugs. A rich engine will always can the tip of your spark plugs to go black. This is usually the easiest way to spot a rich engine.
  6. Fails to start. Engine that’s running to rich may fail to start

Consequences of Motorcycle Running Lean

Motorcycles that’s running lean will overheat quickly and potentially break internal engine parts like the pistons and exhaust (nothing works well when it’s too hot). 

Why does a lean engine overheat? Good question – The theory suggests that combustion will burn way hotter when there’s too much air. 

Also, particles from fuel could help in cooling down temperature because they absorb heat. When there’s not enough fuel, you will also lose it’s cooling effect. 

Running lean is dangerous, uncomfortable and destructive. If you have to choose either to run lean or rich, then you are better off running rich. 

Yes, you will have a horrible gas mileage and you will annoy the riders behind you. But it’s at least not destructive. 

Some symptoms / consequences when motorcycle is running lean:

  1. Engine overheats. Your motorcycle engine will overheat quickly when it’s running lean. Causing you to be extremely uncomfortable when riding it. 
  2. Exhaust changes cooler to orange / blue. If you see your exhaust turning color into orange or blue-ish, that’s an indication that your engine runs extremely hot. You have to get the engine checked out asap.
  3. Fails to start. Engine that’s running too lean may fail to start.

Why Running Rich Is Better Than Lean

If we have to choose one between running rich or running lean – what would it be? I see this question quite often. I don’t know why we have to choose either but let’s answer it in detail anyway – because why not?

Running rich is definitely better than running lean because it won’t destroy your engine. If you still remember the symptoms of running rich, they are mostly things to do with unburned fuel. 

Your exhaust tip turns black, your bike starts smelling and you have reduced gas mileage. Bad things but not the end of the world. Your bike will still run and your engine won’t be damaged in the long run.

Running lean can get extremely destructive. Nothing mechanical works well under extreme heat. When running lean, your engine gets hot very quickly – as a result you can cause wear and tear damages at the least and engine failure at the worst. 

Your engine also won’t idle properly and can even lose performance. It’s ironic because enthusiasts tend to accidentally run lean because they want more airflow for performance.

Now that you know the differences, let’s talk about how you could end up running rich or lean.

Causes & Fixes of Motorcycle Running Rich

1. Malfunctioning sensors

Your motorcycle (especially the modern ones) uses a bunch of sensors to determine how much fuel to inject. Malfunction on any of these sensors can cause you to run rich. O2 sensor, Airflow sensor, temperature sensor, etc.

The fix: Replace those broken sensors immediately! They should not cost that much and can be installed rather quickly. 

2. Clogged Air Filter

Filthy air filters can become very restrictive – they don’t allow air to pass through. This causes the air/fuel mixture to go rich. Same amount of fuel is injected but significantly reduced air. 

The fix: Clean or replace your air filter. You can also consider using a performance air filter from companies like K&N.

They don’t cost that much and are reusable (you can wash them and don’t have to replace them). Performance air filters are usually advertised to improve performance but don’t expect any significant gain. 

Use them if you want your air filters to be reusable and last long. Performance air filters only help improve performance if you paired with other performance mods like an exhaust. 

Motorcycle performance air filter

Performance Air Filter

3. Faulty Tuning

Enthusiasts could tune their motorcycles incorrectly or download the wrong configuration – causing them to run rich. 

The fix: Simply re-tune them to its previous values if you have a backup. Otherwise, you need to download other available configurations and install them. 

Even better if you are working with a custom flash tuner. These tuners can tune your motorcycle according to what you want: removing rev limiter, more throttle response, you name it.

Causes & Fixes of Motorcycle Running Lean

1. Performance Mods Without Tuning

Installing performance mod without proper tuning is the number one reason for running lean. Performance mods like full system exhausts and air filters work by allowing more air into the combustion chamber.

If you pair the extra air with extra fuel by tuning, you can get a more powerful combustion (more power) However, if you only provide extra air but no extra fuel, you are likely going to run lean. 

The fix: When installing significant performance mods like a full system exhaust, make sure to tune your engine! 

Use a Power Commander if your motorcycle equips a fuel injector. Or rejet your carburetor if your motorcycle equips a carburetor. 

More info about tuning on my other article Do You Have To Tune Motorcycle After Installing Exhaust?

2. Malfunctioning sensors

Similar to running rich, malfunctioning O2 sensors or air flow sensors could cause your engine to run lean. 

These sensors control how much fuel is injected into the combustion chamber. If they malfunction and cause a reduction in fuel injection, then you will surely run lean. 

The fix: Identify those malfunctioning sensors and replace them.


3. Faulty Tuning 

Again, similar to running rich, your motorcycle can run if you incorrectly tuned it. With tuning, you can practically control how your motorcycle engine behaves. 

With an incorrect tuning, you could instruct your motorcycle to not reduce fuel injection – which would cause you to run lean. 

The fix: Re-tune your motorcycle into it’s previous values. Or simply download other popular configurations and install them.

Ifandi S.

Ifandi S.

Passionate about everything mechanical. Ifandi has been involved with motorcycles and cars since the old days - in his family's auto parts shop. Want to keep in touch? Scream "STRAIGHT PIPEEEEE" at the top of your lungs and Ifandi will show up.

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