Most motorcycle engines are combustion engines, meaning the compressed fuel and air mix are combusted or ignited to move the piston.
These days, that mix is regulated by an Electronic Fuel Injection or EFI computer via the bike’s ECU.
Before fuel injection became the standard, most motorcycles equipped a part called a carburetor for maintaining and distributing the appropriate amount of air and fuel for proper combustion.
Carbureted motorcycles have a choke lever that can be pulled out to restrict airflow, increasing the amount of fuel in the air-fuel ratio to raise the temperature of the mix.
If your motorcycle starts fine with the choke on but dies when you turn the choke off, it’s likely because your motorcycle’s fuel supply is restricted and the bike is running lean. Here are the most common reasons why that can happen:
1. Motorcycle Carburetor Jets Are Clogged
If the jets of your motorcycle’s carburetor are clogged by contaminated fuel, moisture, rust, or debris, your bike might die when you turn the choke off.
Your carburetor jets distribute fuel into the cylinder’s combustion chamber.
Therefore, the fuel flow is restricted if the jets stop due to the build-up of grime, dirt, or corrosion particles in the fuel.
The result is more room for air to rush into the combustion, causing your air-fuel mix to run lean.
When you pull the choke out on a motorcycle with clogged carb jets, your airflow is restricted, causing the air-fuel ratio to even out for a short while. Once you turn the choke off, the motorcycle’s fuel mix will return to running lean, and the bike can die.
- Remove the carburetor from the bike’s engine.
- Spray the carb down with OEM-suggested carb cleaner, following the instructions on the product.
- The carb cleaner should free the jets of clogs that may restrict fuel flow, causing the bike to run lean and die when you run the choke off.
Carb cleaning is part of standard upkeep on any bike with a carb. Maintaining your carburetor with a general carb service at the intervals outlined in your bike’s manual is an effective way to keep your motorcycle from stalling when the choke is pushed in.
2. Clogged Fuel Filter
As its name implies, the fuel filter cleans the grime, dust, moisture, rust, and other solid matter out of your fuel supply.
While some motorcycles have external fuel filters, many fuel filters live inside the bike’s fuel tank, making it challenging to inspect and clean.
This, unfortunately, makes it a popular piece to neglect to maintain on bikes that aren’t taken into the shop for routine service.
If a motorcycle’s fuel filter is stopped, the fuel flow into the carburetor is restricted, and your motorcycle’s fuel supply will run lean of fuel.
When you turn the choke on, the airflow will be restricted and may closely match the fuel flow, but once you turn the choke off, the increased air intake may cause the bike to stall.
Here are 5 common symptoms of a clogged fuel filter on a motorcycle:
- Decrease in RPMs and overall speed when the throttle is fully opened.
- Even with the choke pulled out, the bike is hard to start when old.
- The cycle takes longer to build acceleration power.
- The fuel pump/gas tank is making unusual sounds.
- Your bike’s engine light is illuminated.
Fuel filter inspection and maintenance are part of standard ownership maintenance. While inspecting the fuel filter may require tank removal, special tools, and more workspace than you can access, it is part of most standard bike services.
Fuel filters need to be cleaned or replaced to maintain the quality and flow of your fuel supply. Running with a dirty fuel filter may cause engine damage down the road.
3. Faulty Fuel Pump
Your motorcycle’s fuel pump is responsible for pumping the fuel through the lines, ensuring it gets through the filter, into the carb, and into the combustion chamber.
A malfunctioning fuel pump will have a negative impact on your motorcycle’s combustion cycle and may cause your bike to stall out when the choke is turned off.
Here are 3 common symptoms of a faulty fuel pump on a motorcycle:
- An unusual whistling sound develops from the bike’s engine.
- Your bike’s check engine light is illuminated.
- You notice a significant lag in power in the high-end gears and the low-end when you’re riding uphill.
Fuel pumps must be inspected periodically as they wear out from typical use. Furthermore, running expired or low oil levels through your motorcycle’s engine can damage your fuel pump.
A worn fuel pump needs to be replaced; engine oil needs to be changed per the service intervals outlined in the owner’s manual to keep the bike from stalling when the choke is turned off.
Make sure to also read our article about reasons a motorcycle dies when slowing down.
4. Fracture in the Fuel Lines; Fuel Leaks
If your motorcycle’s fuel lines develop breaks, cracks, fractures, or ruptures, your bike will develop a fuel leak.
The fuel leak will hinder the bike’s ignition process, as the fuel supply will dwindle, and the pressure change of the leaks in the fuel lines impacts the fuel pump’s performance.
Furthermore, if the fracture is significant enough, it will allow air to rush into your fuel supply against the leaking fuel, causing the air-fuel ratio to run increasingly lean. Eventually, your bike may only start with the choke pulled, and your motorcycle will stall when you shut it off.
Here are the 5 most common symptoms of a motorcycle that has a fracture in the fuel lines:
- Fuel leaking from the fuel tank into the engine or dripping onto the ground when parked.
- Your motorcycle’s check k engine light is illuminated.
- Your motorcycle’s engine performance is inconsistent.
- The engine loses power.
- Your motorcycle exhaust produces thick black smoke/smells of burning fuel.
You can read this article for more information on reasons a motorcycle blows white or black smoke.
Furthermore, fuel leaking from worn or damaged seals and o-rings can cause a bike to run lean and stall out unless the choke is on.
Inspect the fuel tank’s cap and the various seals and 0-rings within the fuel and air intake systems, looking for lack of integrity and fuel residue indicating the potential for leaks.
Any leaking part that can’t be tightened needs to be replaced asap to restore your normal fuel flow.
Cracked or ruptured fuel lines need to be replaced. Some riders opt to install protective fuel line covers that prevent the rubber line from getting brittle and cracking.
5. Blocked Fuel Lines
Your motorcycle’s fuel lines are the pathway for the fuel to travel from the gas tank to your motorcycle’s carburetor.
Due to a failed fuel filter, internal corrosion, a broken seal, or a crack in the line, the presence of solids in your bike’s narrow fuel lines can block the fuel flow, causing the motorcycle to run lean and die when you shut your choke off.
Here are the 3 most common symptoms of blocked fuel lines on a motorcycle:
- Abrupt dips in power and engine performance.
- Inconsistent and rough idling.
- Erratic fluctuations in engine revs; RPMs go up and down while idling.
Fuel lines can also get pinched or tangled with other cables. Start by inspecting the fuel lines for pinches and tangles and replacing the damaged pipe to restore fuel flow to its spec capacity.
If you suspect a clogged fuel line is the issue, empty your fuel lines into a safe container from your fuel lines by sticking a pin in your carb jet’s hole and opening the petcock drain screw.
If the clog is particularly hard, you may have to flush a fuel additive like Seafoam through your fuel lines and carb, following the instructions on the product.
If flushing the fuel lines doesn’t work, you may need to rebuild your carb.
Please also check out this article about reasons a motorcycle smells like burning.
6. Faulty Fuel Pressure Regulator
Your fuel pressure regulator hooks up to your motorcycle’s fuel tank to govern the fuel flow pressure and regulate how much fuel enters your carburetor.
Like all other motorcycle components, heat, vibration, and the wear and tear of general motorcycle riding can wear out the fuel pressure regulator.
Once worn, the pressure wavers in its ability to govern the fuel pressure as it fails to keep up with the flow.
The lack of fuel pressure caused by a faulty fuel pressure regulator can result in a motorcycle that dies when the choke lever is pushed in or turned off.
- If you suspect a faulty fuel pressure regulator is why your motorcycle stalls when the choke is turned off, you’ll have to remove the regulator from the fuel tank to inspect it.
- Once the fuel pressure regulator is removed from the fuel tank, turn the ignition switch on while applying pressure to the pressure regulator’s adjustment valve.
- Turn the ignition switch and see if the adjustment valve turns smoothly, with little force.
- If the fuel pressure regulator’s adjustment valve doesn’t turn quickly, it may be faulty, and thus the reason your motorcycle stalls when you shit off the choke.
Defective fuel pressure regulators must be replaced to restore proper fuel flow and prevent your bike from running lean and stalling.
Restricted fuel flow increases airflow, which causes overheating within the engine. Rising engine temperatures can cause irreparable engine damage, including a seized motor.