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 – Jet Needle Clip Position and Accelerator Pump Diaphragm Replacement – 2023-06-10
I was concerned about throttle opening below 1/4, so I made some adjustments. I had planned these adjustments beforehand. There are two aspects:
1. Changing the jet needle clip position. 2. Replacing the accelerator pump diaphragm with the standard discharge amount. This time, I will discuss the above points 1 and 2.
There are important considerations for points 1 and 2:
・ Do not perform these adjustments simultaneously. Doing so makes it difficult to determine the impact of each change. ・ I believe it is better to make accelerator pump adjustments at the end of the tuning process. If about 80% of the tuning is already complete, avoid making simultaneous changes. This is because you won’t be able to compare the results.
If you make both changes at the same time and the bike performs poorly after testing, you’ll need to adjust either 1 or 2 and retest. Without comparison, you won’t know if the tuning is optimal.
Tuning always involves A/B testing. For rough tuning, you can rely on intuition or experience. With tools like an air-fuel ratio meter, a few tuning iterations should suffice. If you’re tuning as a hobby like me, if possible, make another jet change. Repeat A/B testing until you achieve the desired results.
If it’s not for profit, you should spend the time necessary to get satisfactory tuning.
Apologies for the lengthy preamble. Let’s get into the specifics of tuning.
[Current Tuning] AS - 1.40 minutes return JN - 90FTR#1 Accelerator Pump - Standard
In addition to adjusting the jet needle clip position, I have also modified the AS setting. I’ve made these changes more than twice but will omit the details. For the accelerator pump, the standard with a higher discharge volume provided a stronger acceleration feel. The discharge volume controls the duration from the start to the end of the discharge. Depending on the discharge timing and tuning, there were cases where turning off the accelerator pump provided better acceleration.
Since the beginning of this year, I’ve been concerned about the fact that the throttle sometimes doesn’t rev up smoothly when opened quickly. I’m considering whether it’s better to address this through throttle operation or to fine-tune the FCR further. Generally, running the fuel slightly richer seems to help resolve such issues.
Although I’ve changed the accelerator pump’s discharge volume and timing, it hasn’t resulted in the desired outcome. Currently, I’m keeping the AS slightly open. I’m considering future tuning plans, including testing how things change if the AS is significantly closed.
The current setup isn’t necessarily bad; I’m just looking for areas to improve. I’ll continue to refine the tuning with a focus on quick throttle openings.
FCR for Beginners – Tuning is an A/B Test
If you’re not aiming for perfection in FCR tuning, you can stop once you achieve a setup you like after a few rounds of tuning. However, if you want to refine it further, it's essential to keep records and conduct A/B testing.
For instance, when adjusting the AS (air screw):
・If you want to lean it out, gradually open the screw, making changes in increments of 10 minutes. ・You should not stop at just finding “good” settings twice (Good → Good). ・Continue adjusting until you reach a “bad” result (Good → Good → Bad).
When you revert changes, you can adjust in 5-minute increments to check if the change improved or worsened the performance.
When tuning, always keep a record of the date and what adjustments you made. Professional shops are likely to provide excellent tuning with past data and extensive experience, typically achieving an 80% or higher level of tuning quality. However, this comes at a cost, and they must work within the limits of the service fee.
In contrast, DIY tuning allows for infinite time and detailed A/B testing. The more time you invest, the more refined your tuning can become. While tuning is not always purely scientific, individuals can perform more granular A/B testing. Precision is usually superior with professional shops, but both approaches have their advantages.
FCR Tuning – Jet Needle Clip Position and Accelerator Pump Diaphragm Replacement – 2023/06/10【summary】
Sometimes, I find the tuning to be a bit off. It’s related to the quick turn throttle and its rapid opening. The cause seems to be that the quick turn throttle’s angle is too small.
I want to improve the situation without changing to a looser throttle opening, as much as possible with the current setup. While maintenance and customizations are enjoyable, riding the motorcycle should be the priority.
I tend to prioritize tuning too much, so I’d like to focus more on enjoying riding. Please be careful and enjoy your motorcycles as well.
See you again!
CBX1000 – Working photo
My FCR is an older model, so it has two accelerator pump diaphragms. My backpack contains FCR tuning tools, including a vacuum gauge and a sub-tank. I carry enough tools in both my backpack and the side bags on the bike to handle FCR tuning without any issues.
