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 – If the pilot screw is set to 1.00 turns out, it may not be adjusted sufficiently.
Regarding FCR Tuning, the initial values for the screws are typically set to an air screw (hereafter referred to as AS) of 1.00 and a pilot screw (hereafter referred to as PS) of 1.00. This is also mentioned in the tuning manual. The topic of this blog post is hinted at by the title, which focuses on adjusting the PS. I will share the conclusion first: even if you believe that the tuning is optimal at PS 1.0, there may still be an additional boost in power available.
There are tuners who do not adjust the PS from 1.00 during FCR tuning, and some even recommend this approach. I, too, had periods of confusion regarding the PS. At times, I would fully close the PS or keep it fixed at 1.00 while changing the slow jet (hereafter referred to as SJ) sequentially from 35 to 55. I experimented with various throttle openings during idling and other conditions that the PS would affect. Recently, I realized something important: keeping the PS fixed at 1.00 is definitely not advisable.
There’s a reason for this. The AS overlaps with the influence range of the PS, meaning that adjusting the AS to a more optimal position may be possible. If you have a preconceived notion about PS being fixed at 1.00, I encourage you to read until the end.
Now, let’s get started!
AS and PS overlap, but they are distinct entities.
The image shows the influence range of AS and PS from the tuning manual. You should be able to see that there are overlapping areas between AS and PS. It is stated that the PS only has an effect when the throttle is fully closed to 1/16 or less.
FCR-AS renge
FCR-PS renge
You can achieve very good tuning by keeping the PS fixed at 1.00 while adjusting the AS and changing the SJ. The AS is an easily accessible screw, and you can tune the throttle at low openings using only the AS. However, when tuning with the PS fixed at 1.00, it can lead to some peculiar situations.
①I wanted to achieve the best condition at a throttle opening of around 1/8, so I opened the AS. ②The idling became unstable. ③I returned the AS to its original position.
In summary, adjusting only the AS means that you can't achieve the best idling and the best throttle opening at 1/8 simultaneously. If you don't mind, you can sacrifice one for the other. It’s also acceptable to adjust the AS somewhat to find a middle ground. Tuning by opening the AS while keeping the PS fixed at 1.00 can result in a very good tuning condition for idling and starting.
After tuning at PS 1.00, try opening the AS further. This will make the idling and very low throttle openings lean. However, if you open the PS, the idling area will become rich, allowing you to balance out the adjustments made by opening the AS.
By fixing the PS at 1.00, you may be limiting the potential to gain power by opening the AS. While it may be possible to balance things out by changing the SJ or jet needle, adjusting the PS is overwhelmingly more efficient. Adjusting the AS significantly changes the air-fuel ratio in the area managed by the PS. Recently, I’ve come to believe that adjusting the PS after adjusting the AS for final tuning around idling is essential.
In other words, fixing the PS at 1.00 means that you are not utilizing the opportunity to open the AS for adjustments during tuning. There are two reasons why FCR tuning is often done with the PS fixed at 1.00:
①The throttle opening does not correlate proportionally. ②It’s hard to notice the effects of adjusting the PS.
These two points are the reasons for this approach. I will elaborate on this in the next section.
Adjusting the PS makes it difficult to notice the response.
Adjusting the PS can significantly improve cold-start performance and make the power feel more responsive during acceleration. However, it can be difficult to notice the effects unless you make substantial adjustments. In my experience, you need to turn it about 1.5 times the amount of the AS to feel a difference. If the SJ is too rich, the engine can idle smoothly even if the PS is fully closed. The behavior of the engine can be very different when it’s cold versus when it’s warmed up. These factors contribute to the difficulty in adjusting the PS.
To easily notice changes, it helps to understand the amount you need to turn the PS and consider how it affects the motorcycle. In my case with the CBX1000, I have an air-fuel ratio gauge, which allows me to see the changes numerically. Often, you need to adjust all six cylinders to notice significant changes. With the AS, however, you can typically feel the difference after adjusting just two cylinders.
Even on multi-cylinder motorcycles, such as four-cylinder models, accessing the PS can be challenging. Depending on the motorcycle type, you might need to remove the FCR to adjust the PS. This accessibility issue is another reason why adjusting the PS can be difficult. In my CBX1000, the carburetor pilot screw adjustment tool does not fit as is, so I've had to grind the back and sides to make it work.
The best way to adjust the PS is to start the engine while observing the air-fuel ratio gauge.
Ultimately, when it comes to the PS, it can be difficult to gauge the specifics just by sound or the feeling while riding. The best way to adjust the PS is to start the engine and observe the air-fuel ratio gauge while making adjustments. The readings can vary significantly between cold starts and after warming up. Air-cooled engines tend to show a wider range of changes compared to water-cooled engines due to valve clearance considerations. Before I installed the air-fuel ratio gauge, I used to test changes by feel based on how easily the engine started when cold, but I could only check it once a day. By the way, even with an air-fuel ratio of 15 or 13, it can sometimes be hard to tell just from idling. If I hadn't installed the air-fuel ratio gauge, I would have likely treated the PS adjustments much more casually.
HONDA CBX1000 air fuel ratio sensor-Cold engine start
HONDA CBX1000 air fuel ratio sensor-After warming up
FCR Beginner’s Guide – Screw Adjustment
The best option is to install an air-fuel ratio meter. Especially for beginners, using good tools and equipment can greatly enhance efficiency. However, not many beginners can easily attach an air-fuel ratio meter. If you keep just two points in mind, you'll be able to adjust the screws with about 80% accuracy:
・A position that seems to improve cold starting ・A position that makes it easy to take off
If you can meet these two criteria, the screw adjustment should be fine. Fine-tuning adjustments can't be done accurately without an air-fuel ratio meter. Even in the best possible condition, you can't confirm your current state without one. This applies to both beginners and veterans alike.
FCR Tuning – If the pilot screw is set to 1.00 turns out, it may not be adjusted sufficiently.【summary】
In my case, I used to adjust the pilot screw (PS) somewhat intuitively. However, after realizing that I still had room to open the air screw (AS) further, my thinking changed. Now, I tune by fully opening the AS to the point where it becomes difficult to start, and then I adjust the PS from there. In practice, I sometimes feel that both cold starting and performance at about 1/8 throttle are quite good.
My thoughts have evolved between last year and this year. As I gain more tuning experience, I might discover different insights next year. This blog post was about the PS.
