Understanding Carburetors
What is a Motorcycle Carburetor?
The motorcycle carburetor is the heart of the engine’s fuel system. Its main function is to mix fuel (usually gasoline) and air in the correct ratio and then deliver this mixture to the engine’s combustion chamber. The carburetor controls the fuel-air mixture ratio. This lets the engine work efficiently.
A carburetor works on the Venturi effect. When air passes through a narrow channel (the Venturi tube), its speed increases and pressure drops. This drop in pressure draws fuel into the airflow, forming a combustible mixture.
Today, many motorcycle models use electronic fuel injection systems to replace carburetors. But, some older or lower-displacement models still use carburetors.
The Role of a Motorcycle Carburetor
- Mixing Fuel and Air: The carburetor mixes fuel and air at a specific ratio. This gives the engine a suitable mixture.
- Controlling the Mixture Ratio: The carburetor controls the fuel-air ratio. This lets the engine work efficiently.
- The carburetor delivers the mixed fuel and air to the engine's combustion chamber.
Are All Motorcycle Carburetors the same?
The answer is no. Due to tech advances and concerns about carbon emissions, many motorcycles have switched from carburetors to electronic fuel injection. So, it's essential to identify whether your motorcycle uses a carburetor or an electronic fuel injection system.
This does not mean that all carburetors are the same. Motorcycle carburetors fall into three types: slide, constant velocity, and butterfly.
- Slide Carburetors: These have a throttle slide that moves up and down inside the bore, controlled by the throttle cable. When the throttle is opened, the slide rises, allowing more air in. Fuel mixes with air based on the position of the slide and the jetting.
- Constant Velocity Carburetors: They use a diaphragm connected to the slide. It controls airflow based on engine vacuum, not direct throttle input. As engine RPMs increase, the vacuum raises the slide, allowing more air and fuel in. This setup provides a smoother throttle response.
- Butterfly Carburetors: These use a butterfly valve to regulate the airflow through the carburetor. When the throttle is opened, the butterfly valve rotates, allowing more air and fuel through. The valve position directly affects airflow and the fuel mixture.
Slide carburetors are now more common in the market. This includes Keihin's PE, PZ, PWK, PWN, and FCR carburetors.
Does a Bigger Carburetor Mean More Power?
A carburetor's main job is to deliver a fuel-air mix to the engine's combustion chamber. This mix powers the engine through combustion. So, some people mistakenly think that a larger carburetor means more power. They believe that more fuel-air mix will create a bigger explosion.
This is a misconception, as it overlooks whether the engine can handle an overly rich fuel-air mixture. Simply put, an engine cannot accept an unlimited amount of mixture; the right size depends on the displacement and design of the engine.
Here, the term “suitable carburetor” refers to a range rather than a fixed size. Outside of this range, if the size is too small, it will result in insufficient power and sluggish acceleration. If the size is too large, the engine may not handle the overly rich mixture, leading to stalling or difficulty starting.
However, even within the “suitable range,” sizes can vary slightly—larger, smaller, or just right. In most cases, the size of the stock carburetor is adjusted by technicians to ensure optimal engine feedback from idle to full throttle.
If you don't need a power upgrade and just want to revive your motorcycle, buy a carburetor that matches the original. It should work out of the box.
For more power, you can select a larger carburetor within the "suitable range." It may require adjustments to the jets, needle, and fuel-air mixture screws. The downside is a possible reduction in low-end torque and increased fuel consumption.
Few people opt for a smaller carburetor unless by mistake, as everyone generally aims to maximize the engine's potential. A smaller carburetor’s drawback is insufficient power at high RPM or full throttle.
How to Determine the Right Carburetor Size
OEM Carburetors
For OEM parts, select the OEM carburetor using the motorcycle's year, brand, and model. This is less ideal for older motorcycles. Stock may be low, and OEM parts are often very expensive. This can dampen the excitement of reviving an old bike.
However, if price isn't a concern, it's very convenient. Almost no adjustment is needed; just buy and install it out of the box.
Aftermarket Carburetor
- Carburetor Type: Slide-type carburetors are best for power upgrades. They have the best throttle response.
- Performance Ranking by Model:
- Four-Stroke Engines: FCR > PWK > PE > PZ = VM
- Two-Stroke Engines: PWM = TM > PWK > PE = VM
Pay particular attention to whether the carburetor’s intake side has a round or flange inlet. If the larger carburetor lacks a flange inlet, you may need to replace the intake manifold for compatibility. Larger carburetors are usually round.
Also, note if the throttle cable is on top or on the side. When upgrading, if the cable position differs, ensure there’s enough room for the new cable arrangement.
Determine the Carburetor Intake and Outlet Diameters
Check the stock carburetor's intake inner diameter. This will set the range for jetting the engine.
Next, measure the outlet diameter; the manifold and rubber boot typically allow for ±2mm tolerance. If the carburetor's outlet size doesn't match, replace the rubber boot or manifold. The same type and size usually have consistent outlet dimensions.
Determine the Carburetor Air Filter Side Diameter
When upgrading or replacing with a different carburetor, pay special attention to the air filter side diameter. Aftermarket carburetors often have larger air filter inlets. This boosts airflow and power.
If you still want to use the airbox, you can buy an air filter adapter. It will fit the airbox manifold if the aftermarket carburetor's air filter diameter is within ±2 mm.
Or, consider installing a cone-shaped air filter to further increase air flow.
Confirm Carburetor Clearance
Ensure the carburetor fits properly, especially if switching to a cable-pull type from a side-pull type. For ATV models, mismatches are common.
A carburetor that’s too wide may prevent proper installation, making it impossible to attach the air filter to the airbox manifold.
The position of the idle change screw is another consideration; for compact frames, the screw may be obstructed. In such cases, you may need to install an angled manifold or choose a different carburetor model.
Choosing by Engine Displacement
For Chinese horizontal engines (usually flange-type inlets):
- 110cc: 16mm
- 125cc: 20mm
- 140cc: 22mm
- 150cc: 24mm
- 190cc: 26mm
For Chinese four-stroke vertical engines:
- 125cc: 22mm
- 150cc: 24mm
- 200cc: 26mm
- 250cc: 28mm
- 300cc: 32mm
For Honda, Yamaha, Suzuki, Kawasaki, KTM, and Husqvarna, don't choose by displacement alone. Their engines vary greatly in carburetor needs. It’s best to confirm the original carburetor size.
Conclusion
The best approach to choosing a carburetor size is to follow the size of the original carburetor. When selecting a different type, consider whether there’s enough space and compatibility with the manifold.
Different carburetor types vary in throttle slides, circuits, and power jets. These differences affect power and throttle response times.
If you have further questions, feel free to leave a comment below.
