How To Tell If A Motorcycle Is Running Lean (And How To Fix It) | Motorcycle Habit (2022)

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How To Tell If A Motorcycle Is Running Lean (And How To Fix It) | Motorcycle Habit (1)

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.

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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.

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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.

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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.

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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.

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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.

FAQs

How do you know if your engine is running lean? ›

Here are some of the most common symptoms of how to recognize when your engine is running lean.
  1. Starting Trouble. This symptom shouldn't come as a surprise because vehicles need fuel to start. ...
  2. Decrease In Performance. ...
  3. Clean Spark Plugs. ...
  4. Check Engine Light. ...
  5. Stalling Engine.
16 Feb 2022

How do you know if your bike is running lean or rich? ›

If you can turn your fuel petcock completely off so that fuel does not refill the float bowls, you can also check for lean/rich conditions. Turn off the fuel and ride the bike. If it starts to run worse as the float bowls drain, it is too lean. If running conditions improve, it is too rich.

How can you tell if carburetor too lean or too rich? ›

If the mixture is too lean, it will leave no color, while a rich mixture will cause the fuel ring to become more prominent. Over-rich mixtures will give the plug a sooty appearance.

How do you adjust a running lean carb? ›

Adjust both screws equally and find the right mixture.

Regardless of whether or not the engine is running too rich or too lean, bring it down to a very lean mixture by turning both screws a quarter-turn at a time, counter-clockwise, then slowly bringing them back up to an equal and smooth mixture.

What does a spark plug look like when running lean? ›

If the engine is running to lean, the spark plug can look greyish white. If the engine is running to rich the spark plug will look black, sometimes a glossy black if it's really rich.

Does backfire mean rich or lean? ›

Lean Air/Fuel Mixture

Not only can a rich air/fuel ratio cause a backfire, a mixture that doesn't have enough gasoline can cause a backfire, too. A "lean" mixture is one that doesn't have enough fuel, and too much air.

What happens if an engine runs too lean? ›

A rich-running engine is caused by excess fuel and lack of air, whereas an engine running lean is the exact opposite – the engine lacks enough fuel or there is an excess of air. These conditions can cause trouble down the line, potentially causing knocking or leading to complete engine failure if not resolved.

Will running lean damage spark plugs? ›

The most prominent engine running lean symptoms include lower power output and stalling. It can also lead to trouble starting the engine, issues with the spark plugs and an illuminated Check Engine Light.

Does choke Richen or lean? ›

When you pull the choke it closes the choke butterfly valve or a choke plate, reducing the airflow and richening the mixture. That means no matter how much the throttle valve opens you will not get full airflow, so you won't get full revs.

What is a good indicator of rich or lean conditions? ›

General Rules of Emission Analysis

Remember, CO readings are an indicator of a rich running engine and O2 readings are an indicator of a lean running engine. High HC, high CO, and high O2 at same time indicates a misfire due to excessively rich mixture.

How do I know if my carburetor needs adjusting? ›

The engine should rev smoothly and quickly as soon as you apply throttle. If the vehicle is displaying any sort of sluggish performance or misfires when you apply the throttle, then more adjustments are required.

How do you adjust the carburetor air fuel mixture on a motorcycle? ›

Pilot Air/Fuel Screw Adjustment Explained - Single Carb - Part 1

How do I know if my carburetor is running lean? ›

Q: How Do You Tell if a Carburetor Is Rich or Lean? A: One way to tell for sure is by "reading" the spark plugs. If the plug tip is white, the mixture is lean. If it's brown or black, it's rich.

How do I adjust the idle on my motorcycle carburetor? ›

How To Adjust The Idle Speed On A Dirtbike - YouTube

What is worse running rich or lean? ›

Q: Is it better to run the too lean or rich engines? An engine running slightly rich will give more power, but running lean will cause catastrophic engine damage. Running too rich can also cause severe damage to your engine.

What causes a lean fuel mixture? ›

A lean mixture occurs when there's a higher concentration of air to fuel than there should be. When this occurs, your engine will likely still run but will result in jerking motions within the mechanics of the combustion engine which in turn leads to damage to the engine, such as burned valves.

Does a lean engine run hotter? ›

So, yes, if you lean out your engine, going from 12:1 to around 14:1, your engine will run a bit hotter. But 14:1 is still "rich" by stoichiometric standards. If you lean out past 14.7:1, all the way to something like 17:1, your engine will run cooler again.

What are the symptoms of a rich fuel mixture? ›

Seven Signs Your Air and Fuel Mixture Is Too Rich
  • Check Engine Warning. If your vehicle's exhaust has too much gas in it, your check engine light will come on. ...
  • Strange Odors. ...
  • Poor Fuel Economy. ...
  • Engine Performance Problems. ...
  • Emissions Test Failure. ...
  • Engine Idle Trouble. ...
  • Part Damage.
15 Dec 2020

Does backfire damage engine? ›

Backfires and afterfires are worth paying attention to since they can cause engine damage, power loss, and decreased fuel efficiency. There's a variety of factors that can cause your car to backfire, but the most common ones are having a poor air to fuel ratio, a misfiring spark plug, or good old-fashioned bad timing.

What causes popping on deceleration? ›

Decel popping is caused by the detonation of unburned fuel in the exhaust pipe. This happens with high flow exhausts that allow more fresh air to be pulled into the pipe, causing the exhaust temperature to rise and detonate any unburnt fuel.

What causes exhaust pops? ›

What causes a car to pop and bang? “Pops and bangs” are created by unburned fuel in the exhaust system. When excess fuel enters the exhaust system it increases in temperature and ignites inside the exhaust instead of the combustion chamber. The noise can be exaggerated by fitting a decat or full decat exhaust system.

What air fuel ratio is too lean? ›

Generally, the engine is considered to be running too lean when fuel trim is above 10% (with the engine running in closed-loop mode). If fuel trim is below -10%, the engine is typically considered to be running rich.

Does running lean save fuel? ›

Run the engine fuel-lean, that is, use excess air. It is well known that fuel-lean running improves the efficiency. In the old days, under cruising conditions, the engines always ran lean – about 15% excess air -- this was economical.

How lean is too lean? ›

TOO Lean Defined:

Low Body Fat percentage: Under 10% for men and, under 15% for women.

What makes fuel run rich? ›

Running rich only occurs if there is an issue with your engine's combustion process. The cause of this is your engine receiving too much fuel. Conversely, “running lean” refers to your car receiving too much air and not enough fuel. With this in mind, is it better to have your car be running rich vs lean?

Can bad fuel cause a lean condition? ›

Lean conditions are a result of a lack of fuel getting to the engine. So, it's reasonable to assume the trouble may lie with the fuel system. An obstruction in the fuel filter or a failed fuel pump can reduce fuel pressure and hinder fuel delivery.

What does a rich engine sound like? ›

Re: what do rich and lean sound like? A rich condition will result in excessive smoke from the silencer, the plug will often carbon foul and the engine will produce a sputtering/crackling sound. A lean condition will result in slow throttle response, you twist the throttle but the power delivery is lethargic and flat.

Which position is choke on? ›

Typically the “choke” is on when the choke lever is in the up position or when the choke lever is inline with the handle bars on your bike. Choke on means that the choke plate is covering the carb throat.

Does a choke let more air in? ›

The choke is a plate in the carburetor that opens and closes to allow more or less air into the engine. Similar to the throttle, the choke plate rotates from a horizontal to vertical position to open up the passageway and allow more air to get by.

What happens if main jet is too big? ›

(A too-large main jet also mimics this symptom.) - The engine runs hot, knocks, pings and overheats. - The engine surges or hunts when cruising at part-throttle. - Popping or spitting through the carb occurs when the throttle is opened.

How do you make a lean fuel mixture? ›

How To Lean The Mixture - YouTube

How do I know if my motorcycle is running rich? ›

Don't stick your face down by the exhaust , just stand next to the motorcycle while it's running and if it smells really strong of fuel then that means your engine is running rich and you need to adjust your air to fuel ratio.

Which gas is generally considered to be the lean indicator? ›

O2 readings that show up on your five gases analyzer is extra oxygen that was not converted during the combustion process. This is why O2 is an excellent lean indicator. When the engine is running lean, O2 will increase proportionately with the air/fuel ratio.

How do I adjust my idle RPM? ›

Disconnect the idle air adjustment valve by locating and disengaging the electronic connector that holds it to the throttle body. Turn the idle screw to adjust the idle—turn the screw to the left to increase the RPM or to the right to lower it.

How do you adjust a idle screw? ›

Set the idle screw so that the engine is idling at a normal RPM. Take a small screwdriver and start turning one bleeder screw on one side of the carb inward until the idle either drops or rises. If it drops, you are going the wrong way, so turn it the other way until the you hear the engine's idle rise back up again.

Which way do you turn a mixture screw to lean? ›

On most carburetors, turning the mixture screw in (clockwise) leans the mixture, while counterclockwise (out) enriches the mixture. Initially, if the engine stumbles or the vacuum drops when turning the mixture screw in, turn both screws out about a -turn and evaluate the results.

What is the best air-fuel ratio for a carburetor? ›

The ideal chemical air-fuel ratio, in which all the fuel is oxidized, is 14.7:1, that is, 14.7 parts air to 1 part fuel. A “rich” condition means more fuel is being used, an air-fuel ratio less than 14.7, while a “lean” condition means less fuel is being used, an air-fuel ratio higher than 14.7.

How do you adjust air-fuel ratio? ›

How to Adjust Air & Fuel Mixture on a Vehicle's Carburetor - YouTube

How do you adjust the idle screw on a motorcycle? ›

How to Adjust Engine Idling | QuikrCars Do It Yourself Videos

Does pilot jet affect idle? ›

Does the pilot jet affect idle? Yes, the pilot jet definitely affects the idle. You don't need to adjust the idle screw until you have properly adjusted the pilot jet circuit and air screw or fuel screw.

How do you lean out a motorcycle carburetor? ›

Rich or Lean, how to figure it out? Carburetor adjustments - YouTube

How do you lean on a motorcycle? ›

How To Conquer Your FEAR OF LEANING - YouTube

What are the symptoms of a rich fuel mixture? ›

Seven Signs Your Air and Fuel Mixture Is Too Rich
  • Check Engine Warning. If your vehicle's exhaust has too much gas in it, your check engine light will come on. ...
  • Strange Odors. ...
  • Poor Fuel Economy. ...
  • Engine Performance Problems. ...
  • Emissions Test Failure. ...
  • Engine Idle Trouble. ...
  • Part Damage.
15 Dec 2020

Will running lean damage spark plugs? ›

The most prominent engine running lean symptoms include lower power output and stalling. It can also lead to trouble starting the engine, issues with the spark plugs and an illuminated Check Engine Light.

Is Sputtering lean or rich? ›

Re: what do rich and lean sound like? A rich condition will result in excessive smoke from the silencer, the plug will often carbon foul and the engine will produce a sputtering/crackling sound. A lean condition will result in slow throttle response, you twist the throttle but the power delivery is lethargic and flat.

What is a good lean angle on motorcycle? ›

For modern MotoGP bike and riders, it's 65 degrees, which is a full 45 degrees past the 20 degrees that human beings are naturally comfortable with.

How do you practice lean angle on a motorcycle? ›

Afraid to lean your motorcycle? Try this tool - YouTube

How do I improve my motorcycle cornering? ›

For effective cornering, you should be leant forward slightly with relaxed, bent arms. Elbows should be low, in line with the handlebars if possible. Keep a light grip on the handlebars and don't lean on them to support your weight – you may need to gently grip the tank with your legs. Don't let your vision drop.

What happens if an engine runs too lean? ›

A rich-running engine is caused by excess fuel and lack of air, whereas an engine running lean is the exact opposite – the engine lacks enough fuel or there is an excess of air. These conditions can cause trouble down the line, potentially causing knocking or leading to complete engine failure if not resolved.

What is a good indicator of rich or lean conditions? ›

General Rules of Emission Analysis

Remember, CO readings are an indicator of a rich running engine and O2 readings are an indicator of a lean running engine. High HC, high CO, and high O2 at same time indicates a misfire due to excessively rich mixture.

What makes an engine run lean? ›

Your engine runs lean if your air-to-fuel mixture is too light - this means that the fuel in your ignition chamber is being igniting with too much air or too little fuel. Because your engine is running on less fuel than it should, it is running “lean.”

Does running lean save fuel? ›

Run the engine fuel-lean, that is, use excess air. It is well known that fuel-lean running improves the efficiency. In the old days, under cruising conditions, the engines always ran lean – about 15% excess air -- this was economical.

What causes a lean condition? ›

Lean conditions are a result of a lack of fuel getting to the engine. So, it's reasonable to assume the trouble may lie with the fuel system. An obstruction in the fuel filter or a failed fuel pump can reduce fuel pressure and hinder fuel delivery.

Why does my motorcycle pop when I accelerate? ›

The popping sound is made when unburnt fuel is ignited in the exhaust. This is usually caused by your engine running too lean or too rich. This means you either have 1 of 2 things going on: your bike is getting too much air (lean) or too much fuel (rich) in the mix.

What can cause lean fuel mixture? ›

There are five factors that could cause a lean mixture to occur.
  • Fuel System. A malfunctioning fuel system can reduce the amount of fuel coming into the engine, causing a run lean condition. ...
  • Oxygen Sensor. ...
  • Air Mass Flow Sensor. ...
  • Computer Malfunction. ...
  • Air Leaks.

What air fuel ratio is too lean? ›

Generally, the engine is considered to be running too lean when fuel trim is above 10% (with the engine running in closed-loop mode). If fuel trim is below -10%, the engine is typically considered to be running rich.

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