I am the owner of a Honda motorcycle [CBX1000] who lives in Hiroshima City, Japan. I am posting about FCR carburetor and CBX1000 customization. I also post about things other than cars and motorcycles. We will post information that will make bikers around the world happy.
FCR Tuning – Adjust the air screw to be slightly lean within the possible range, then adjust the accelerator pump – 2023-05-27
I adjusted the air screw’s return setting and also changed the start position of the accelerator pump’s discharge. It’s been a while since I worked on the FCR. After a light morning ride with the usual 9F rider, I did the tuning. When I do a test run solo, I tend to ride in a way that suits me, so doing a test ride with two bikes before tuning is really helpful.
FCR tuning is a process of finding and improving areas of concern. Having someone ride with me serves as a valuable reference for tuning. In summer, as the temperature rises, I adjust the jets to be slightly lean overall. This applies mainly to throttle openings between 1/8 to 1/2. If your focus is on racing, you might not need to worry too much about throttle openings below 1/8. However, for street riding, throttle openings below 1/8 are frequently used. When properly tuned, it makes the motorcycle experience incredibly enjoyable. Minor adjustments can lead to significant improvements.
This time, I intended to focus on adjusting the air screw. I didn’t like the rich symptoms that occurred when the throttle was suddenly opened. So, I adjusted the air screw to be leaner. If the acceleration became too lean, I planned to compensate by advancing the accelerator pump discharge timing. The minimum requirement was that there should be a sense of power when starting off.
I adjusted the AS from a rich to a leaner setting. When making a quick adjustment before a test run, I set the air-fuel ratio between 12.9 and 13.5. This time, the lower range felt a bit rich, so I adjusted the AS to lean it out as much as possible. I tightened the PS because the idle seemed a bit rich, adjusting it after the engine warmed up. If starting becomes difficult when cold, the PS will need to be opened to a richer setting.
As a result, the throttle showed signs of being lean when suddenly opened, so I advanced the accelerator pump timing. It’s not perfect yet; I’ll continue testing further.
Adjustment of Accelerator Pump Discharge Timing
I changed it from 2.4mm to 1.4mm.
Here’s my thinking: The accelerator pump exists because it’s necessary for racing carburetors. However, there are times when tuning without the accelerator pump results in significantly better performance. The way each rider uses their motorcycle is also a major factor.
Until now, I had been tuning in a way that minimized the influence of the accelerator pump. This time, I adjusted the throttle so that it showed a lean condition when suddenly opened, as if there were no accelerator pump. From there, I advanced the accelerator pump discharge timing for further adjustments.
As the next theme, I plan to advance the discharge timing of the accelerator pump a bit more and observe the results.
Total Discharge Amount of the Accelerator Pump
The FCR's accelerator pump allows for the adjustment of the total amount of fuel supplied by replacing the diaphragm, similar to adjusting the size of a water gun’s tank. However, you can’t control the force of the spray. What is controlled is how long the fuel is discharged after the throttle opening passes the discharge start timing.
If the air-fuel ratio gauge shows 10 for about 2 seconds, rather than instantly, after suddenly opening the throttle, it may indicate that the total discharge amount is too high. After suddenly opening the throttle, the moment the air-fuel ratio gauge begins to shift from 10.0 towards lean, the bike starts to accelerate rapidly. The duration for which the air-fuel ratio shows 10.0 varies depending on the total discharge amount of the accelerator pump.
Is it better to have an accelerator pump, or not?
There are times when tuning without an accelerator pump can produce excellent results. However, comparing this with tuning done with an accelerator pump hasn’t been straightforward. It’s possible to achieve a tuning setup that relies on not having an accelerator pump, which can yield a great feeling, but it requires careful attention to throttle opening and other factors.
Recently, I’ve been enjoying tuning with the assumption that the accelerator pump is present. Last year, I focused less on the importance of the accelerator pump in my tuning. When tuning without relying on the accelerator pump, the setup tends to be richer, which can result in poor performance with gentle throttle operation.
The accelerator pump is extremely useful when opening the throttle abruptly. Currently, I’m experimenting with controlling the accelerator pump to refine the tuning further.
For FCR Beginners – Considering the Mechanism of the Accelerator Pump, It’s Not Suitable for Fine Adjustments.
Remove the FCR from the motorcycle and take a look at the accelerator pump. It’s not designed for precise adjustments. The fuel supply amount varies slightly whether you open the throttle slowly or abruptly. The amount of fuel supplied also changes depending on the timing when you close the throttle and then open it again. The size of the spray nozzle is also constant.
Compared to tuning adjustments made in 1/100th units like with jet needles, the accelerator pump is not suited for high-precision tuning. Adjustments to the accelerator pump discharge timing can be made quite roughly without issues, and the changes are significant.
FCR Tuning – Adjust the air screw to be slightly lean within the possible range, then adjust the accelerator pump – 2023-05-27【summary】
The idea is to open the air screw to the limit where it’s just bearable and make up for the lean condition with the accelerator pump’s discharge timing. I’ve set the accelerator pump to 1.4mm, but according to the tuning manual, it starts to take effect at 1.2mm with a 1/4 throttle opening, so testing with 1mm or 0.8mm is also necessary.
There is a discrepancy between the discharge timing stated in the tuning manual and the actual timing. I’m aiming for 1.4mm with the adjustment plate, but depending on how it’s measured, it can easily shift by 0.1mm. It’s necessary to mix real-world testing with various trials.
Adjusting the accelerator pump discharge timing is quite simple. I encourage you to give it a try. See you again!
CBX1000 – Photo Gallery
This is the FCR tuning for the CBX1000. The air screw (AS) is easily accessible, so if you're interested, why not try adjusting it yourself?
FCR acceleration pump timing thickness gaugeHONDA CBX1000-FCR Air ScrewHONDA CBX1000-FCR Air ScrewHONDA CBX1000 FCR Foam Air Filter SockHONDA CBX1000HONDA CB900FHONDA CBX1000
