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 – Adjusting throttle openings below 1/4 with the air screw, slow jet, and jet needle straight diameter – 2023-06-11
I’m going to write about the air screw, slow jet, and jet needle straight diameter. This discussion will focus on throttle openings up to about 1/4.
During the previous tuning session, I was concerned about the hesitation when quickly opening the throttle. I managed to resolve this issue, so I’ll share the details here. The solution was to adjust the air screw towards a richer setting.
While this did improve the situation, I noticed that with rough throttle work, the power feels slightly lacking when the throttle opens beyond 1/4. However, with smooth throttle operation, the acceleration is excellent. It might be more of an operational issue than a setting issue.
The characteristic of a push-pull carburetor is that the rider adjusts the throttle as they ride. On my CBX1000, the throttle fully opens at 56 degrees. The fact that the throttle can be fully opened in one go is quite appealing.
This is why the current setup demands a delicate throttle touch. There’s still room for further refinement. In this session, I’ll write about the lower range settings, including the air screw, based on my own experience.
I only adjusted the air screw. The blog title mentions the slow jet and jet needle straight diameter. I will write about that in the following sections.
Speed Range After Takeoff – About 1/8 to 1/4 Throttle
The slow jet, straight diameter of the jet needle, and the air screw determine this range. When the throttle is quickly opened, the acceleration pump also comes into play. I see the acceleration pump as the final adjustment during rapid throttle openings. However, I’ll write this without considering the acceleration pump.
The pilot screw mainly corrects the mixture when the throttle is fully closed. Changing either the slow jet or the straight diameter of the jet needle affects throttle openings below 1/2, below 1/4, and below 1/8.
The slow jet and the straight diameter of the jet needle are closely linked, almost as if they function as one unit. If you change one, there’s a good chance you’ll need to change the other as well. Additionally, when you replace the slow jet or the straight diameter of the jet needle, the air screw will need to be readjusted.
How to Adjust the Air Screw
I believe the adjustment should consider both the power feel during acceleration and when opening the throttle. The air screw should be adjusted from rich to lean. The limit for opening the air screw is up to the point where the engine starts properly. From there, adjust the air screw by turning it in or out to find the right position where the acceleration feel is optimal around 1/4 throttle opening.
Accelerator pump adjustment
Tuning with the acceleration pump always active is one approach. I view the acceleration pump as a correction for lean conditions during rapid throttle openings. For fine adjustments, you can leave the acceleration pump as is and make your tuning adjustments. For significant changes, turn off the acceleration pump, make the tuning adjustments, and then turn the acceleration pump back on to fine-tune.
Turning the acceleration pump on or off often results in noticeable changes, both positive and negative. Even when tuning slightly rich with the acceleration pump off, there can still be a sudden increase in acceleration feel.
For FCR Beginners – The Limits of Air Screw Adjustment
If you open the air screw too much, starting the engine may become difficult. The point where starting the engine remains reliable is the limit for how much you can open the air screw. The tuning manual suggests turning it back about 15 minutes after the RPM increases or making adjustments for each cylinder individually.
The correct setting is where the bike starts properly and the throttle feel up to about 1/4 throttle opening is good after test riding.
The tuning manual advises adjusting the air screw for each cylinder separately. However, I adjust the air screw to the same setting for all cylinders. I have once used a digital tachometer to adjust each cylinder individually, and there didn’t seem to be much difference between them.
As my tuning precision improves, my perception might change. On the CBX1000, adjustments for each cylinder showed minimal differences.
FCR Tuning – Adjusting throttle openings below 1/4 with the air screw, slow jet, and jet needle straight diameter – 2023-06-11【summary】
In circuits and other racing environments, you may occasionally see in magazines or on the web that adjusting low throttle openings is unnecessary. However, for general users, there’s no harm in thoroughly adjusting low throttle openings. The only cost is the time spent on the adjustment.
In the racing world, it’s considered correct to allocate that time to tuning the main riding aspects. If a rough adjustment (about 80% of the optimal setting) for low throttle openings is sufficient, it doesn’t take much time.
For those tuning FCRs as a hobby, spending time on these adjustments is worthwhile. You can use as much time as needed to be satisfied with the low throttle opening settings.
