What Is The Cause Of My Motorcycle Engine Running Rich?

What is the cause of my motorcycle engine running rich? Anything from a broken butterfly valve to a stuck float can lead to a rich engine.

You may have heard the term quite often. When your engine is running rich, it means there’s too much fuel going into the combustion chambers instead of enough air. This can lead to all kinds of problems such as overheating, bad gas mileage, and an intoxicating gas smell.

So, is it possible to fix it? Yes. And many solutions don’t even require professional aid. Here’s all you need to know about a rich motorcycle engine and how you can fix it today.

What Is the Cause of My Motorcycle Engine Running Rich?

cause of Motorcycle Engine Running Rich

What causes your engine to run rich? Here are a few possible reasons:

1.    Jet Is Too Large:

If the jet is too large, it will hold more fuel, resulting in more fuel consumption. This is what will lead to your engine running rich.

2.    High Float Level:

The float is what controls how much fuel gets into the jet. If the float level is too high, the jet could overfill.

3.    Stuck Choke:

How much air is getting into the fuel? If you have a working choke valve, it will decide that for you. However, if the choke is stuck, then too much air may be getting into the fuel. The choke can get stuck or remain open when you’re turning the handlebars.

4.    Stuck Float:

A stuck float will result in too much fuel entering the jet and getting into the venturi. If your airbox has a bit of fuel in it, then you will know your float is stuck. Similarly, a float with a standard petcock will also result in the same thing. However, this time, the fuel will overflow even when the bike is idle.

Rich Symptoms in Motorcycle Carburetors:

Symptoms of rich in Carburetors

Now that you know what the cause of your motorcycle engine running rich is, here’s how to diagnose it:

1.    Reduced Power Output:

If you notice your engine isn’t providing as much acceleration and power as it used to, you could be running rich.

2.    Lower Gas Mileage:

If you find that the range you can get from your bike is not the same as before, then your engine is rich.

3.    Black Spark Plugs:

Spark plugs are your windows to the engine. If your spark plug is black, that means your engine is running rich.

4.    Works Better with More Oxygen:

Remove the air filter from your engine and check if the vehicle is running better. If that’s the case, your engine is running rich indeed.

Likewise, if your engine runs worse at higher altitudes (due to the lack of oxygen), you eventually know your engine is running rich.

5.    Unburned Fuel Smell:

Although unlikely, if your engine is running too rich, you may sense a constant unburned fuel odor. The odor will be stronger near the exhaust.

6.    Exhaust is Black:

If your exhaust is black and covered with a black sooty material that can be wiped off, you most likely have a rich engine. This is due to excessive carbon buildup and is not normal.

7.    Black Smoke:

Carbon doesn’t just deposit on the exhaust. Black smoke is what causes your exhaust to turn black. If your notice the smoke from your exhaust turning black, your engine is running rich.

8.    Rough Idle:

Finally, if your engine is idling rough, you have a rich engine. Rough idling means that your bike shakes and sputters when idle and doesn’t seem to be stable.

6 Ways to Fix A Motorcycle Engine Running Rich:

With all that out of the way, how can you fix a rich engine on a motorcycle? Here are 6 solutions:

1.    Adjust the Air/Fuel Screw:

Adjust the Fuel Screw

If you suspect your motorcycle is running rich, the first thing to check is the fuel screw. Now, it’s a bit tricky since every manufacturer calls it by a different name. Fuel screw, air-to-fuel screw, idle screw – you get the gist. But you can easily locate it on your carburetor.

Fixing the screw isn’t easy, however. That’s because each bike has a different procedure. Have a look at the manufacturer’s manual to find out how to tighten the screw to the right position. No, it won’t be easy as simple screwing it down. In some cases, you may need to screw it out several times before setting it back in.

2.    Fix Stuck Floats:

An engine float works like the float in your toilet. It allows a certain amount of fuel to flow through. However, if the float is stuck in the open position, it will overfill the jet and cause your engine to run rich.

First, you need to access the floats. Find the 3-4 screws near the bottom of the carburetor bowl and unscrew them. Then lift the bowl up to find the floats inside. Before you proceed, it’s a good idea to memorize where the floats were before you meddle with them. Or perhaps take a picture.

Push the pin sticking out of the float such that the float falls free. Next, spray carburetor cleaner inside the carburetor bowl. Also, clean the floats themselves and the little stoppers on them, along with the carburetor’s holes where they get seated.

Then, put everything back together and give your bike a test ride.

3.    Clean Out the Air Filter:

Clean motorcycle Air Filter

The next most obvious solution is to clean out the air filter. If the filter gets clogged with dirt and debris, it will restrict airflow and lead to a rich engine.

The thing is, it’s not that easy to clean the air filter. While superficial dirt can be readily washed off, dirt stuck inside the little foam pockets will be harder to remove.

It would be much easier to buy a new filter. They don’t cost a lot. And they’ll work much better than your old filter.

4.    Replace the Oxygen Sensor:

If you have a newer bike that is fuel-injected, then ignore everything we just said. Your problem is entirely different from what we discussed. In most cases, your oxygen sensor will be the problem.

Oxygen sensors degrade over time and quite rapidly so since they’re so close to the combustion chambers. In this case, there isn’t much you can do other than replace them. You can do this yourself by following the manufactures’ guidelines or hire a professional for it.

5.    Fix Stuck Butterfly Valves:

Fix Stuck Butterfly Valves

Butterfly valves are those small disc-shaped valves that open and close when you throttle your bike. These are what control the flow of fuel into and out of the carburetor. If the valve is left permanently open (mainly due to a broken spring), you have a rich engine.

The only way to test this is to look at the valves and see if they open and close properly as you twist the throttle. Remember to do this only when the engine is turned off. Remove the air filter and peer through the ports. If the valve doesn’t open and closes at the same rate as you’re twisting the throttle, then you need to replace it.

6.    Sticking Needle:

The needle is a slim needle-like object that moves up and down as you turn the throttle on your bike. It lifts up and down to allow for more or less airflow respectively as you throttle your bike. Many factors can cause your needle to get perpetually stuck in the open position, such as an idle bike or a dirty carburetor.

Remove the air filter from the carburetor and locate the needle. Turn the engine off and twist the throttle. If the needle isn’t moving usually, then you have a problem. Clean the carburetor and the needle thoroughly before setting everything back into place.

Lean Vs. Rich Motorcycle Engine:

You’ve probably heard the words “rich” and “lean” too often, but what do they mean? You see, your engine doesn’t just take fuel and burn it. It mixes it with air to get the perfect fuel-to-air ratio for efficient combustion.

A motorcycle running too “rich” means this mixture has too much fuel and not enough air. A “lean” running engine means that the mixture has too much air than fuel.

There are some stark differences between the two. A lean engine doesn’t show any apparent signs that a rich engine does, such as a black spark plug or gas smell. Instead, your spark plug will make bicomplete clean, and there will be no smell.

Despite this, a lean engine is worse than a rich engine. Hence, if, after reading this guide, you think your engine isn’t running rich, you’re not done just yet. Your engine could always be running too lean, which is no better.


Know you know what is the cause of my motorcycle engine running rich. Motorcycle engines that are running too rich are quite common, especially among the newbies. While not as destructive as a lean engine or seized engine, they’re still not negligible.

What causes an engine to run rich? A rich engine that has too much fuel will run rich. This can be caused by many things, such as broken butterfly valves, stuck needles, floats, chokes, and even a clogged filter.

A rich engine is a disaster. But good thing you now know all about fixing it. It’s time to put your knowledge to the test and fix that carburetor for good!