To CR or FCR?

I recently purchased one of Jim Schmidt's 33mm Keihin CR Smoothbore carb kits. I think it was the last one he had in stock. He has now replaced this kit with one using 33mm Keihin FCR flat slide carbs. At first, I was a bit bummed because I had just missed out on the FCR upgrade. Then, I let AI do a comparison for me, and I think I will be happier with the CRs.

This is what Claude had to say:

Need a performance comparison between Keihin CR smooth bore carburetors and Keihin FCR flat slide carburetors in a motorcycle application
6:47 PM


I'll provide a comprehensive comparison of these two Keihin carburetor designs for motorcycle applications.

Keihin CR vs FCR Carburetors: Performance Comparison​

Keihin CR (Smooth Bore) Carburetors

The CR series features a round slide design with a smooth, venturi-shaped bore. Key characteristics include:

• Throttle response: Progressive and linear, with smoother power delivery throughout the rev range
• Low-end performance: Excellent low to mid-range torque with strong vacuum signal at lower RPMs
• Tuning: More forgiving and easier to tune for street use; wider tolerance for jetting errors
• Air velocity: Higher air velocity through the venturi creates better atomization at partial throttle
• Applications: Well-suited for street bikes, touring, and applications requiring predictable, linear power delivery

Keihin FCR (Flat Slide) Carburetors

The FCR uses a flat slide design that creates less obstruction to airflow. Key characteristics include:

• Throttle response: Extremely crisp and immediate, with sharper throttle feedback
• Top-end performance: Superior high-RPM airflow and horsepower potential due to less flow restriction
• Tuning: More sensitive to jetting changes; requires more precise tuning but offers greater adjustability
• Air velocity: Lower restriction at wide-open throttle, though can sacrifice some low-end signal
• Applications: Popular for racing, track days, and performance-oriented street applications

Head-to-Head Performance Differences

Power delivery: FCRs typically show 2-5% more peak horsepower on the same engine, while CRs often provide slightly better torque in the 25-75% throttle range.

Throttle feel: FCRs offer more direct connection between wrist and rear wheel; CRs provide more refinement and are less twitchy in traffic.

Fuel economy: CRs generally achieve slightly better fuel economy in real-world riding due to superior atomization at cruise.

Maintenance: Both are high-quality, but FCRs have more adjustment options (accelerator pump, leak jet, etc.) which adds complexity.

Which Should You Choose?

Choose CR carburetors if you prioritize street manners, smooth power delivery, easier tuning, or ride frequently in varied conditions.

Choose FCR carburetors if you want maximum performance potential, race/track use, aggressive throttle response, or are willing to invest time in precise tuning.

Many riders find the FCR's performance advantages most noticeable on the track, while the CR's refinement is more appreciated during daily street riding.

Who is Claude?

AI was in its infancy not too long ago. It is getting very good at data collection. I don't use it for choosing parts myself. I'm delusional enough to think I know what will work.

FCR carburetors actually do very well across the board on the street if the intake manifolds, head work, cam, and exhaust are a good match. Amazing acceleration coming out of turns in the right gear. They probably get as good or better fuel milage than the CRs. I regularly get around 67mpg with the 35mm FCRs. I firmly believe based on use that the FCR accelerator pump sucking up tons of fuel is a myth or the results of poor tuner skills. I think you made the right choice with the CRs for your use.