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.

I fitted a pair of 33mm CRs to my 920 years ago & have been very happy with them. I think you will love them, as they stay in tune & require no real maintenance to speak of. To my mind they also look more in keeping on our old bangers than FCRs.

Yakatak said:

.................... Then, I let AI do a comparison for me ...................

Click to expand...

Gotta laugh I thought you said " I'll let AL " do a comparison ... puzzled me for a minute until I read further down the page.

Interesting topic, thanks

The trouble is AI trawls the internet for data… so imagine what it would come up with if you searched ‘Seeley Norton yoke offset’ or… ‘are drum brakes safe’ or…

NPeteN said:

Who is Claude?

Click to expand...

Claude is a chat-type artificial intelligence bot.

I think you are deluded if you think you can get the best performance out of petrol using a normal carburetor. In the 1970s, every different model of the same Japanese motorcycle had different mid-range jetting. It is probably impossible to find out what was going on in their design departments. Normally, when you tune a carburetor, the adjustment of mid-throttle is in steps by raising and lowering the carb needles. The difference in ajustment between the steps is too large - if you achieve the best power position for acceleration, you would be lucky. In the combustion process, three things are in balance - compression ratio, ignition timing and jetting. The normal way to tune is to use fixed ignition timing and compression ratio, and adjust the jetting. However jetting is adjusted step wise, and ignition timing is adjusted on a continuous basis. The guys who use dynos and get the mixture right by using an oxygen measurement of exhaust gas, are in a better position to get the best performance by moving the timing, while the bike is running on the dyno. Using methanol for racing is a cheat - it gives more power by hiding-up the tuning errors. Petrol has one-third higher calorific value than methanol - it should give more power. Heat is a problem - methanol runs cooler. A really well-tuned road bike might be a pain in the bum - weather affects performance. Standard jetting in a road bike is probably less weather-affected.
I have been thinking about programmable Boyer ignitions - I do not know what their software is like or what the outputs are on a dyno - but a laptop computer can probably be used to accept dyno outputs and adjust the advance curve in a Boyer. We would need a throttle position sensor.