If you’re into cars or motorcycles, you know that few sounds in life are as satisfying to hear as a popping exhaust on deceleration.
It’s the sound of power, and it reminds you and those around you of the raw and untamed power of the internal combustion engine in your bike.
Now, while we all love those crackling noises coming from the exhaust, they can in fact be telling you something about the way your engine is operating. To be more specific – they’re telling you that your fuel-to-air mixture is incorrect.
But does popping on deceleration indicate a lean or rich mixture? Let’s take a look.
Also Read: Gas Leaking From Overflow Tube – Lean or Rich?
Popping on Deceleration: Lean or Rich?
Here’s the deal – popping on deceleration can occur from both lean and rich mixture. In fact, it’s almost impossible to tell whether a motorcycle is popping and crackling due to a lean or a rich mixture.
However, the most common reason for popping on deceleration is a lean ATF mixture.
But how do you determine whether the popping on deceleration is caused by a lean or a rich mixture? Well, the absolute best way to do that is by looking at the spark plugs.
Lean spark plugs look white, dry, and crusty, while rich spark plugs look black and sooty.
Lean vs Rich Mixture Symptoms
Apart from looking at your spark plugs, you can also try to determine whether your motorcycle is running lean or rich by looking out for the following symptoms.
Lean Mixture Symptoms
- Low power at low RPMs
- Tan, White, or Clean Spark Plugs
- Poor idling while cold
- Difficult to start
- Runs better with choke when warm
- Pulsing RPMs
- Engine overheating
- Popping on deceleration
Rich mixture symptoms
- Poor throttle response
- Black, Sooty, or Wet Spark Plugs
- Rough Idle when warmed up
- Increased fuel consumption
- Sooty exhaust pipe
- Popping on deceleration
What Causes Exhaust Popping on Deceleration?
The most common cause for popping on deceleration is a lean air-to-fuel mixture, however, a rich mixture can also cause the same crackling noises, although it’s rarer.
A lot of the time, a lean air-to-fuel mixture is the result of an unfortunate vacuum leak, a blown exhaust gasket, or a problem with the carburetor.
But why does a lean mixture cause the engine to create popping and crackling noises from the exhaust?
First, let’s not forget that once the engine is running lean, it’s also running hot and so are the exhaust pipe and the headers.
When you’re riding at 6000 RPM, you’re using up quite a bunch of fuel, and if you suddenly release the throttle, all of that extra fuel is no longer needed and has to go somewhere.
The unburnt fuel then gets pushed out through the exhaust valves and into the very hot exhaust headers, where there’s enough heat and oxygen for the fuel to combust. That combustion of unburnt fuel is what causes the popping sounds.
Is Popping on Deceleration Bad?
Now, after reading that, you’re probably thinking that popping on deceleration is a bad thing, and you would be right.
A bit of popping from the exhaust on deceleration is not that bad for your engine and should not cause any immediate damage, especially if the timing is correct and the engine does not overheat.
However, if left untreated for a long time or if the lean condition gets really bad – your motorcycle’s engine could face catastrophic failure. We’re talking melted pistons and burnt exhaust valves.
While popping and crackling sounds from the exhaust sound great, they do need to be fixed if they’re not intended to be there by the manufacturer.
How To Diagnose Popping on Deceleration
Diagnosing the reason why your motorcycle is popping on deceleration is a pretty straightforward process, but it can be time-consuming.
Diagnosing a Rich Condition
Let’s start with the rich air-to-fuel mixture, more specifically – the carburetor.
1. Float Set Too High
The most common reason for a rich air-to-fuel mixture is a float that’s set too high.
If the float is set too high, it will allow too much fuel into the carburetor bowl, and that fuel will then get pushed into the engine, causing it to run rich.
A quick and easy way to check if the float is set too high is by removing the carburetor bowl and then measuring the float height with Vernier calipers. The exhaust float height specifications are different for each make and model, so make sure to consult your bike’s service manual.
2. Stuck Choke
When was the last time you inspected the choke on your motorcycle? If it’s been a while, then that might be the cause for the rich condition.
A stuck or disconnected choke is pretty rare, but it does happen and will cause fuel mixture problems.
The best way to check whether the choke is working properly is by removing the carb and inspecting the choke mechanism for binding, as well as inspecting and lubricating the choke cable.
3. Main Jet Too Large
One of the most overlooked causes of a rich air-to-fuel mixture is a poor carb rejet.
If you’ve recently installed the stock air filter or exhaust back, or changed the main jet to a larger one, you might want to go down in size.
A main jet that’s too big will cause the engine to run rich, popping, and crackling on deceleration.
Diagnosing a Lean Condition
A lean air-to-fuel mixture is almost always caused by a vacuum leak. A small vacuum leak might not be enough to cause the engine to run lean, but a big one will.
1. Torn Intake Boots
Probably the most common vacuum leak location on motorcycles is the rubber intake boots, that connect the carbs to the cylinder head.
Over long periods of time and due to constant high-temperature changes, these rubber intake boots dry out, harden and become brittle. Eventually, they crack or stop sealing properly and cause a significant vacuum leak.
The best way to check whether the rubber intake boots are leaking is by spraying a little bit of brake cleaner on them while the engine is running. If there is a vacuum leak there, then the engine will suck in the brake cleaner which will briefly increase the RPM.
2. Blown Exhaust Header Gaskets
Another common reason for popping on deceleration is blown exhaust gaskets.
If the exhaust header gaskets have gone bad, they will allow oxygen to enter the exhaust stream in between exhaust pulses. When there’s more oxygen in the exhaust, the unburnt fuel is more likely to combust and cause a popping sound on deceleration.
With that said, the exhaust header bolts do get loose over time, and they can cause a popping sound on deceleration as well.
The best way to check whether the exhaust header gaskets or bolts are causing a popping sound on deceleration is by removing the exhaust and inspecting them for leaks.
3. Cracked Vacuum Lines
Last but not least, another pretty common reason for popping on deceleration is cracked or brittle vacuum lines.
These vacuum lines are made of rubber, and over time they can become cracked or brittle due to age, heat, and vibration.
The best way to check whether the vacuum lines are causing a popping sound on deceleration is by inspecting them for cracks and replacing them if necessary.
4. Checking the Spark Plugs
Reading spark plugs has become a lost art, but it can tell you a lot about the health of your engine.
The color of the spark plug can give you an indication of whether the engine is running too lean or too rich.
If the spark plug is white or light gray, it means the engine is running too lean. On the other hand, if the spark plug is black or dark gray, it means the engine is running too rich.
You can also check the condition of the spark plug electrodes. If they are burnt or damaged, it’s an indication that the engine is running too lean.
Lastly, you can also check the color of the ceramic insulation. If it’s white or light gray, it means the engine is running too lean. If it’s black or dark gray, it means the engine is running too rich.
The last thing you want to do is continue to ride your motorcycle with a lean or rich air-to-fuel mixture. Not only will it cause popping on deceleration, but it can also lead to engine damage. If you’re not sure how to properly adjust the carburetor, it’s best to take it to a mechanic or a professional who can do it for you.
Conclusion
Popping and crackling sounds from the exhaust can be caused by either a lean or rich air-to-fuel mixture, but a lean mixture is the most common reason.
If left untreated, popping on deceleration can cause serious damage to your engine, so it’s important to diagnose and fix the problem as soon as possible.
The best way to diagnose popping on deceleration is by checking the spark plugs, intake boots, exhaust header gaskets, and vacuum lines. If any of these are damaged or leaking, it’s likely the cause of the popping sound.
Once you’ve identified the problem, it’s important to fix it sooner rather than later to avoid any potential damage to your engine long term.
Nick Kowalski is a diesel mechanic by trade, but his passion lies in motorcycles.
He’s been riding them since he was just a wee lad, and he loves restoring old Japanese street bikes and ATVs in his spare time.
Nick is an adrenaline junkie at heart, and he loves nothing more than hitting the open road on his motorcycle.