How To Tell If A Motorcycle Is Running Lean (And How To Fix It)


When it comes to diagnosing motorcycle engine issues, it can often be hard to find the exact culprit to the problem. This is especially present if the motorcycle has a carburetor.

Luckily, motorcycles are a lot more simple compared to other vehicles so they can give us a few easy signs that indicate specific problems. A motorcycle running lean is one of those issues that can be easily identified if you know what to look for.

How can you tell if a motorcycle is running lean? If a motorcycle is running lean, that means there’s too much air and not enough fuel in the air/fuel mixture in the combustion process. The motorcycle will likely idle and/or run sluggish, the engine and exhaust pipes may become hotter than normal, the exhaust pipes may start to turn blue, and the spark plugs will have little wear on them.

Having a motorcycle running lean is not anything foreign to me. I’ve come upon this problem several times myself and this is what I’ve learned about how to identify it as well as how to fix the issue.

Diagnosing An Engine Running Lean

Most vehicles today run on either diesel fuel or gasoline. There needs to be the correct amount of air and fuel added to the engine for the vehicle to work properly and motorcycles are no exception to this rule.

The perfect ratio of gas to air that a motorcycle should be receiving is 14.7 parts air to 1 parts gas or 14.7:1. If you have more than 14.7 parts air, then your motorcycle engine will run lean, meaning the engine is getting more air than it needs and is not getting enough fuel. The excessive air will cause the mixture to burn a lot hotter than normal.

The first sign you’ll have of a motorcycle running lean is if you are having a rough idle. If you start the motorcycle and it has a difficult time staying on by itself or you notice a rough sputter while it’s in a stationary position, this is a good indication of a lean mixture. However, this is also a symptom of a rich mixture so look at the other signs accompanied with a rough idle if that is a problem you’re having.

Alongside running with a rough idle, a motorcycle running lean will also act a little sluggish when you take it out for a ride. The engine isn’t receiving enough fuel to make the motorcycle run efficiently.

You’ll also notice that if an engine is running lean, the engine and exhaust pipes will become very hot, even hotter than normal. You may see some smoke coming from the engine, the exhaust pipes glowing orange from the heat, and/or it may be unbearable to be around the engine while riding because it’s so hot.

It is possible for your engine to overheat while it’s running lean because the air and fuel mixture is burning at a higher temperature. If you have a temperature gauge, look at it closely while you’re taking a normal ride down the road and you suspect a lean issue.

The excessive heat can actually start taking a physical toll on the exhaust pipes. It’s pretty normal for your exhaust pipes to become a little discolored over time, but if you notice they’ve turned either a yellow or bluish color in a short amount of time, your engine may be running lean. The extreme heat going through the pipes causes the metal to oxidize and therefore turns colors. See my article here to learn more about why motorcycle exhaust pipes turn blue.

You can also try looking at your spark plugs and their condition. A motorcycle running well will have a little bit of wear on the end of the spark plug. A motorcycle running lean will have little or no wear on the spark plug because it’s not igniting near as much fuel as it should be.

How To Fix The Issue Of Running Lean

When a motorcycle runs leans, the likely culprit is the fuel delivery system. Specifically, it’s probably the carburetor that’s causing the problem. It is the carburetor’s job to deliver the appropriate amount of fuel and air to combust inside the combustion chamber of the engine and will often manifest any problems it’s having by the way the motorcycle is running.

In my experience, I have found that the number one reason a motorcycle isn’t getting the right amount of fuel is because of clogged jets inside the carburetor. This is a problem that happens too often because those jets are so small and carburetors are susceptible to dirt particles if they aren’t taken care of well enough. They can also get clogged by using gas with ethanol in it.

An engine can also get excessive amounts of air due to either using aftermarket air filters or there’s a vacuum leak somewhere in the system. The process of delivering the right air to the air/fuel mixture requires to have air tight passages so the fuel delivery system can produce the right ratios.

Fuel injectors can also become clogged and cause a motorcycle to run lean though it’s not as common. Some newer motorcycles have diagnostic tools that you can plug into it’s computer. If the motorcycle is fuel injected, it is almost guaranteed that there will be an on board computer that you can plug into and read the fuel ratio.

Fixing a lean issue can actually be pretty easy if you have a carburetor. All carburetors have some screws on the side of it. One of those screws controls the air to fuel ratio mixture. Turning it in or out controls how much gas the carburetor will deliver. Usually, if you screw it out, you’ll get more gas and if you screw it in you’ll get less. In this case, you’ll want to screw it out slightly to increase the gas flow.

Look for your specific model on YouTube and you’ll be able to easily find exactly where that screw is and how much you’ll need to turn it (every motorcycle is different). Try this first before any other fixes and see how your motorcycle reacts to the adjustment. If you’ve adjusted it a few times and it doesn’t seem to be helping, there are likely issues within the carburetor you need to address.

At this point it’s best to clean your carburetor. It’s possible to successfully clean your carburetor yourself, especially if it’s just clogged jets you’re dealing with. Again, look in your owner’s manual or online about how to clean your specific carburetor. See my article here to learn how to clean a carburetor without removing it from the motorcycle.

If you have a fuel injector, you’ll need to take your motorcycle into the shop to have it fixed. Fuel injectors are an electrical component of your motorcycle and dealing with the electronics can be frustrating and difficult. It’s best if that’s left for the professionals to fix.

Is It Bad For A Motorcycle To Run Lean?

Riding a motorcycle that’s running lean can seem rough for both you and the motorcycle. The obvious signs it portrays isn’t all that pleasant to witness and you may wonder if it’s bad for your motorcycle to continue to run this way.

It’s actually unhealthy for your motorcycle to be running lean, especially if it runs lean for a long time. It can potentially be harmful to your spark plugs, piston rings, and the oil inside the engine.

The extreme heat that a lean engine emits is what can cause damage to these essential components on a motorcycle. Though a spark plug may look clean, the extreme heat can warp the metal tip on it causing the gap to expand and bend further than it should.

The extreme heat inside the engine can also cause the piston rings to expand and in extreme cases, cause the engine to seize up altogether. This also affects the oil inside the engine because the heat causes the oil to physically change and start to degrade. Degraded engine oil becomes less effective and won’t properly lubricate the engine.

Difference Between Running Lean And Running Rich

It can often be confusing when deciding whether or not a motorcycle engine is running lean or running rich because they can sometimes give similar symptoms. They both also deal with the incorrect amount of air and fuel ratio given to the engine.

A lean engine means there’s too much air and not enough fuel whereas running rich means there’s too much fuel and not enough air. The biggest difference between the two is that running rich isn’t near as detrimental for your motorcycle as it is to run it lean.

You’ll also notice that an engine running rich is usually accompanied by the obvious smell of gas because of the extra amount of fuel that’s being delivered to the engine. You may also notice some gas droplets spewing out the ends of your exhaust pipes. A motorcycle running lean won’t have excessive gas like this and likely won’t give off as strong of a gas smell.

If you take a look at the spark plugs, you’ll notice an obvious difference. We noted that a motorcycle running lean will have a clean spark plug. A motorcycle running rich will have a much dirtier and soot-looking covered tip.

Related Question

What causes a motorcycle to sputter? The most common reasons a motorcycle sputters are carburetor issues such as a vacuum leak, fuel leak, or tuning issues. Other culprits could include corroded or cracked spark plugs or spark plug wires, a faulty ignition coil, a clogged air filter, or engine timing issues. See my article here for more information.

Recent Content