If you are into modifying and tuning your motorcycle, then you are definitely no stranger to the terms “running rich” or “running lean”. Both are undesirable but I want to focus on running rich in this article.

TLDR – Consistently running rich can damage the internal engines in a motorcycle. Unburnt fuel from a rich mixture can stick and get deposited into cylinder walls, pistons and exhaust valves. Over time, these deposits can cause wear and tear damages to engines and exhausts. 

I made the TLDR above to be as short as I could but surely you want more details – yes? “What wear and tear damages are you talking about? Be specific please!”

Yes, yes, I got you. Let me start by listing all the engine problems that can occur by running a rich fuel mixture on a motorcycle.

6 Engine Problems When Running Rich On Motorcycle

Remember, the root cause for problems when running rich is always carbon buildup –  unburnt fuel from running a rich mixture spreads out and gets deposited into places like cylinder walls, piston rings and exhausts. 

Each of the problems below is somewhat caused by these carbon buildup. 


1. Cylinder Wash

Cylinder wash occurs when unburnt fuel builds up on the walls of a cylinder and causes the existing engine oil on the walls to dilute. This means movement of pistons are not as smooth anymore and will slowly break down from wear and tear damages. 

This effect can also occur on any other lubricated parts in a motorcycle engine – like piston rings. 

If you are just running a little rich for a quick period before you can fix the rich mixture, then you are fine. Cylinder wash happens over a long period –  where an engine constantly runs on a rich mixture. 

2. Discolored Exhaust Tip

Discolored  exhaust tip occurs when hot unburnt fuel escapes into the exhaust system and comes in contact with hot metal surfaces. This causes your exhaust tip to burn and gets discolored. 

Burned exhaust tips are quite common on a motorcycle that has been running rich for some time – they will not come off by just simply washing them. 

On the other hand, exhaust tips can also get black because they are stained by unburned fuel that is flowing out. This can still be washed off. 


3. Low Engine Compression

Low engine compression can occur when unburned fuel from a rich engine builds up over valves and prevents them from closing properly. This causes a decrease in pressure – which can reduce performance. 

If you don’t already know, a motorcycle engine has two important valves that constantly open and close:

  1. Intake valves: Allow air to enter combustion chamber 
  2. Exhaust valves: Allow exhaust gases to exit combustion chamber

These valves open and close at the right timing and maintain a constant balance of high pressure inside the combustion chamber and low pressure outside.

Carbon buildup from unburned fuel can prevent these valves from closing – causing the pressure inside the combustion chamber to drop. 

This could reduce performance because exhaust gases will remain inside and take away space for new fresh air.

4. Clogged Catalytic Converter

A motorcycle Catalytic Converter (CAT) can get clogged if enough unburned fuel flows through it over a period of time. A clogged CAT can reduce motorcycle performance and becomes hazardous. 

Catalytic converter (CAT) is a device that converts toxic exhaust gases into non-toxic components like Carbon Dioxide and water vapor. All exhaust gases from your motorcycle engine flows through the CAT before they leave the exhaust pipe. 

The opening to a CAT is made of small honeycomb structures – which can get clogged up pretty easily (especially from unburned fuel).

Once clogged up, a CAT becomes highly restrictive – preventing exhaust gases to leave your motorcycle quickly. This means increased back pressure and reduced performance. 

If you don’t know what ‘backpressure’ means, then you should check out my other article where I talk more detail about it – Is Straight Pipe Bad For Motorcycle? 

Motorcycle cat honeycomb

The inside of motorcycle CAT. It gets clogged easily.

5. Faulty Spark Plugs

Motorcycle spark plugs can become faulty if they are coated with carbon build ups from a rich mixture. 

Spark plugs are responsible to ignite air/fuel mixture in the combustion chamber for power. Coated spark plugs means fuel ignition is hindered.

Your motorcycle may not start or have a reduced performance because even less fuel will be ignited by the coated spark plugs. 

Rich spark plug

Carbon buildup on spark plug. The tip is not supposed to be black in color.

6. Bad Fuel Economy

Your motorcycle will have a bad fuel economy when running rich because not all fuel can be utilized at once. 

Most of the unburnt fuel exits the motorcycle through the exhaust while the rest are deposited in engine parts. 

Bad fuel economy is usually the first indication that your engine is running rich.

How To Fix Motorcycle Running Rich (& How Much)

If you know that your engine is running rich, then you don’t have to worry so much because it’s quite easy to fix (although it will cost you some money). 

An engine usually runs rich because of a misconfiguration – more fuel is injected into the combustion chamber than required. 

The most common fix? Configure your engine correctly! Have it inject less fuel than required. Depending on what fuel injection system your motorcycle has, there are different ways to fix this. 


1. Dyno Tune

Dyno tune is where you bring your motorcycle to a workshop and have it tested and tuned by a professional. This involves reprogramming your motorcycle’s computer (ECU)

Aside from just fixing the problem of running rich, with dyno tune, you can also specify your preferences. For example, If you ever want your motorcycle to be more aggressive, you could instruct the tuner to increase throttle response and remove the speed limiter. 

Dyno tune costs about $300 – $500 but you could custom tailor your engine performance depending on what you want. You could even tune your motorcycle for fuel economy – which means less performance but improved mileage. 

Here’s an article where I discuss dyno tune in detail. Is Dyno Tune Worth It For Motorcycle?  

Note that dyno tune only works for more modern motorcycles that use computerized fuel injection systems. Some old school bikes that use a carburetor need another approach. I talk about this next.

2. Rejetting Carburetor 

Rejetting carburetor is where you replace the needles in your carburetor to control the fuel intake. You could go with a smaller needle to restrict the amount of fuel that’s injected – this could be the fix to engine running rich. 

If your motorcycle is still running with an old school carburetor, then this is the only way to fix an engine running rich. 

Carburetor is a mechanical device that controls fuel intake – no computers involved here, so there’s nothing to program. 

More info about carburetors and its needles here. How Much Horsepower Can a Carburetor Add? 

carburetor needles

Carburetor needle in different sizes

3. Remap ECU With Power Commander

Remapping ECU can also be done by purchasing an external tool like Power Commander V and having it overwrite your ECU’s configuration. 

With a Power Commander, you cannot customize as much as you want like a dyno tune, but you can always just download another configuration online and load them up on your motorcycle within minutes. 

Power Commander is simple to use but obviously limits customizability. If you are a beginner or just want to fix your engine running rich problem, a Power Commander can be your best bet. 

power commander

Power Commander V

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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