9 Symptoms Of A Bad Motorcycle Carburetor You Shouldn’t Ignore

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It can be very frustrating when your carburetor fails. In my experience, when a motorcycle starts acting up I check the carburetor first because that’s usually the culprit.

Sometimes it can be difficult to know whether or not it’s the carburetor causing the problem. I’ve listed 9 common symptoms of a bad motorcycle carburetor that you can check to see if it really is the reason your motorcycle is running poorly.

Running Lean

The carburetor’s main purpose is to mix the correct amount of fuel and air for the motorcycle to run. When a motorcycle is running lean, that means there’s too much air and not enough fuel in the air/fuel mixture during the combustion process. If your motorcycle is equipped with a carburetor this happens.

Lean motorcycles will have sluggish performance, exhaust pipes that run hotter than usual, which may turn blue, as well as spark plugs that wear faster than normal. This is because excessive air causes the mixture to burn more than it should.

The carburetor is often clogged, causing a less powerful motorcycle engine. When the jets are clogged, that means not enough fuel is being delivered hence the reason there’s too much air in the mixture. Altering the idle screw on your carburetor could be enough to adjust the fuel and air mixture. You can read my other article to learn more about how your motorcycle runs lean.

Running Rich

Sometimes, a motorcycle running full throttle is viewed as the exact opposite of a motorcycle running slow. A motorcycle that is running rich will have too much fuel in its carburetor. This can cause the bike to run poorly.

When there’s too much fuel added and not enough air, you’ll likely notice your motorcycle smelling strongly of fuel, notice a fuel leak (most likely from the carburetor), and an overall poor performance on the motorcycle. Some motorcycles will naturally have a smell of fuel about them, but if you notice an excessive amount of this smell, you’re likely running rich.

There are many ways to make your motorcycle run smoothly. You can adjust the fuel/air screw or fix a stuck needle. (all of which are part of a carburetor). You can click here to read my other article about how to keep your motorcycle running smoothly.


While some motorcycle owners purposely make their motorcycles backfire, it isn’t something that should be happening normally.

Unintentionally, a motorcycle could backfire for many reasons. Two of the most common reasons for this are too much or too little fuel being delivered (lean or sweet) by the carburetor.. If there’s too much fuel present, the combustion process may not be able to burn off all the fuel which will push some of it into the exhaust pipes. The extreme temperatures in the pipes will cause the fuel to combust, giving it the loud pop sound.

With too little fuel there’s way too much air and the fuel has difficulty combusting. Once the exhaust valve is opened, and the hot mixture of fuel and air hits the exhaust header, the fuel ignites causing a loud bang sound. Check out my article for more information about the reasons your motorcycle is not working properly.


The carburetor is likely to be the culprit if your motorcycle is sputtering. Three reasons are possible: A vacuum leak, a fuel leak, or being out-of-tune.

A vacuum leak in the carburetor can cause the motorcycle’s engine to sputter. It causes an interruption of the proper fuel to air ratio, which results in poor performance. Vacuum leaks can be caused by cracked or brittle intake boots. A fuel leak from the carburetor will also disturb the air to fuel ratio because it’s going elsewhere.

Sputtering could also be caused by the carburetor being out of tune which means you’ll need to adjust the air/fuel screw to ensure the right mixture. Click here to read my article on why your motorcycle is sputtering.

A video series I created about motorcycle restoration from start to finish. It includes a 25-minute video detailing how to clean and rebuild carburetors. The series also covers other difficult-to-tackle parts such as electrical and body work. I give dozens of tips and tricks that you won’t find anywhere else online. Click here for more information if you’re interested in viewing multiple videos that will help fix up your bike or if you’re interested in completely building your dream motorcycle!

Motorcycle Won’t Start

There are several reasons why a motorcycle won’t start. When I find myself in this situation, I always resort to looking at the carburetor because it’s usually the problem. I’ve purchased dozens of bikes from bike sellers over the years. “didn’t run.” I was able to get them to run like new 99 percent of the times by just cleaning the carbs.

If you have a motorcycle that won’t start, the carburetor may be dirty and will need a good cleaning. This could mean that your motorcycle’s jets are blocked with deposits. It is possible to clog your jets by using the wrong fuel or not having a fuel filter. Also, if you let your motorcycle sit for long periods without properly cleaning it, this can lead to blocked jets.

I suggest doing a thorough clean by taking the carburetor off to clean it, but it’s also possible to clean it without removing it (see my article here to learn how to do that). Click here to read my article on why your bike may not start.

Acceleration Caused a Loss of Power

The carburetor plays a major role in acceleration on a motorcycle. The carburetor is controlled by the throttle cable. This opens and closes butterfly valves, which feed air into the fuel/air mixture in the carburetor.

When power is lost during acceleration, it’s likely there is some issue within the carburetor. You could have clogged or broken jets, a leaking carburetor spring or intake boot vacuum leak, or a backwards-placed spring. The usual good carb cleaning should do the trick, however if that doesn’t fix the issue take a look at the springs and play around with the throttle to make sure it’s properly opening those valves.

Click here to read my article on why your motorcycle might lose power when you accelerate.

Idling high

A high idle on a motorcycle is an issue that can cause a slew of other problems if it isn’t taken care of promptly. High idling can wear out the engine faster and burn off more fuel than you’re willing to pay for. Again, it is possible that the problem lies within the carburetor.

A motorcycle that is idle high indicates too much air/fuel mixture. Essentially the combustion reaction continues to occur more frequently when the motorcycle isn’t going fast, or really going at all. This is typically caused by the idle screw being too high on the carburetor. It is possible to fix the problem by simply turning a few knobs.

A bad throttle spring can cause a high idle. This spring is pulled when you twist the throttle. This is used when the throttle is released to bring it back into idle.

When this spring malfunctions, the butterfly valves don’t close all the way which lets excessive amounts of air into the carburetor causing the combustion process to occur more than needed. This can be easily identified by visually inspecting the spring. An unsatisfactory spring will make your bike look stretched. Click here for more information about why a motorcycle idles too high.

Gas Leaking

Gas leaks can be detected easily, thanks to their simplicity. You can detect a gas leak by looking at your motorcycle or smelling a stronger gas odor. Leakages of gas can occur anywhere in the fuel system.

If your gas leaks, you should inspect the fuel system, particularly the carburetor. Leakage in the carburetor could indicate that the gasket on the top of the carburetor bowl may be worn or brittle. That carburetor bowl is what holds the prepared gas for combustion and with it sitting there next to a faulty gasket, it’ll easily leak out.

It is easy to fix, but can take some time. You’ll need to order a new gasket for your carburetor and take it apart to replace the faulty one. Although carburetors are unique to each motorcycle, you can find online instructions for how to remove your carburetor. Check out my other article to find out why your motorcycle could be leaking gas.

Hissing sound

Hissing is always an unpleasant sound no matter where it’s coming from. It’s not only annoying if you notice a hissing sound from your bike, but it could be an underlying problem with your carburetor. A hissing sound is typically from the carburetor. This almost always indicates a vacuum leak.

Carburetors operate by applying a certain pressure to create the right mixture of fuel and air for combustion. When there’s a vacuum leak (meaning if there’s a hole in the fuel system somewhere), the carburetor can’t do it’s job right. This is like sucking water out of a straw. If there’s a hole in it somewhere above the water, you can’t really suck it up effectively.

Vacuum leaks can occur at many locations and can be caused in many ways. Wearing intake boots, brittle gaskets and loose screws are all possible causes. Because of this, it can be a little overwhelming to find the exact location of the vacuum leak if the hissing isn’t obvious to find.

Use a can or extension nozzle carb cleaner to spray the small areas of the carburetor. This will help you pinpoint the exact location. If you notice the motorcycle idle improving after a certain spray on the carburetor, you’ve found the location of the leak. This is because the leak will suck the cleaner/WD-40 and combust the gas, increasing the air/fuel ratio.

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