How to Tell if a Motorcycle is Running Lean and How to Fix it

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It can be difficult to diagnose motorcycle engine problems. This is especially true if your motorcycle has a carburetor.

Motorbikes are easier than other vehicles and can show us some signs of trouble. If you are familiar with the signs to look out for, a motorcycle that is leaning can be identified.

How do you know if your 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 be slow or sluggish and may not start as quickly as it should.

It isn’t difficult for me to keep a motorcycle moving smoothly. 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.

Diagnose An Engine Running Lean

Today, most vehicles can run on gasoline or diesel fuel. Motorcycles are not an exception to this rule.

A motorcycle should have 14.7 parts gasoline to 1 part air, or 14.7:1. Your motorcycle’s engine will run less efficiently if it has more than 14.7 percent air. This means that the engine is receiving more air than it requires and not getting enough fuel. The mixture will burn hotter due to the excess air.

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.

A motorcycle that is running rough will not only be slow but also feel sluggish on 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.

Excessive heat can lead to damage to your 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 pipes’ extreme heat causes metal to oxidize, which can cause it to take on different colors. You can read my article to find out why exhaust pipes from motorcycles turn blue.

Also, you can check the condition of your spark plugs. You will see some wear at the end of a spark plug if your motorcycle is running well. 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 Running Lean

Fuel delivery system can be the cause of a motorcycle running lean. 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. You can also cause them to clog if you use ethanol-containing gas.

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. In order to deliver the correct amount of air to the mixture, it is important to have clean passages.

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 your motorcycle is fuel-injected, you almost always have an on-board computer that you can plug in and read the fuel mixture.

If you have a carburetor, fixing a lean issue is actually quite simple. There are screws at the sides of all carburetors. One of the screws controls mixture air-to-fuel ratio. The amount of gas that the carburetor can deliver is determined by whether it’s turned in or out. 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). This will allow you to test the adjustment on your motorcycle before trying any other repairs. 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 yourselfespecially 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. You can read my article to learn how to clean your carburetor on a motorcycle.

If you have a fuel injector, you’ll need to take your motorcycle into the shop to have it fixed. It is possible to be frustrated and difficult with fuel injectors, which are an electrical component of your bike. It’s best if that’s left for the professionals to fix.

Is it bad to have a motorcycle 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 could cause damage to your engine’s piston rings and spark plugs.

This is because of the heat that a small engine can emit, which can cause serious damage to essential parts on a motorcycle. Although spark plugs may appear clean, excessive heat can warp their metal tips and cause them to expand or bend further.

Extreme heat can cause piston rings to expand, and even cause engine seize-up. Because the engine heat causes oil to change, and then start to degrade, this can also impact the oil. Degraded engine oil becomes less effective and won’t properly lubricate the engine.

Difference between Running Lean and Running Rich

Sometimes, similar symptoms can make it difficult to determine if a motorcycle’s engine is running lean. They both deal with the wrong fuel/air ratio.

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. Gas droplets may be visible at 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 noticed that a motorcycle with a low engine will have a cleaner spark plug. A motorcycle with a rich engine will have a covered tip that looks more soiled and dirty.

Similar Question

What causes a bike to sputter and why? Most motorcycles sputter due to problems with their carburetors, such as a fuel leak or vacuum leak. Another reason a motorcycle sputters is corroded, cracked, or clogged spark plugs and spark plug wires. For more information, see my article.

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