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.
Does the scope of the FCR tuning manual affect the throttle opening or the valve opening?
I have questions about the scope presented on page 8 of the FCR tuning manual. This page indicates the impact range of tuning. It is unclear whether the X-axis refers to throttle opening or valve opening. The throttle opening and valve opening of the FCR are not directly proportional. Since this text is written by an amateur, there may be mistakes.
I believe it is likely referring to the throttle opening. The reason is that those who have worked with the FCR have not engaged in this type of discussion. However, it is not clearly stated as the throttle opening.
The world has been operating based on throttle opening for over 30 years. If it were based on the throttle valve opening, things might get complicated.
Therefore, I will verify this with two methods:
①. Directly ask the seller, BITO. Asking the seller is the best approach. ②. Verify the acceleration pump discharge timing. Upon reflection, this can be judged visually. Let me summarize the results first: For ①, I received a response confirming that they refer to the throttle opening. They were aware that the throttle opening and valve opening are not proportional.
For ②, an important point is that even when the throttle opening is at 1/2, the valve opening is about 1/4. By visually checking the acceleration pump discharge timing, it provides a reference to determine whether the X-axis in the tuning manual refers to the throttle opening or valve opening. The results indicated that it is represented by the throttle opening. Thus, it seems safe to say that the acceleration pump discharge timing does reference the throttle opening. The acceleration pump push rod does not operate through a linkage arm. If the push rod were to be proportional to the throttle opening, it would be mechanically incorrect.
The notation method for the X-axis of the acceleration pump discharge timing is the same as that for the slow jet. It seems reasonable to say that the tuning manual uses the throttle opening as the X-axis.
The designer or team that wrote the FCR tuning manual has likely already retired or disbanded, leaving no point of contact for inquiries regarding the truth. Ultimately, it appears that everyone is tuning the FCR based on throttle opening. I believe that FCR tuning is based on throttle opening.
In the following sections, I will write my considerations. Since I have summarized the results above, it may not be particularly fruitful.
So, let's get started!
The valve opening as observed from the acceleration pump discharge start timing.
I have attached photos at the bottom for your reference.
①. The width of the tab is about 3mm, equivalent to two 1-yen coins. I'm using this for easy reference in the verification process. ②. The valve opening when the discharge begins is between 1/4 and 1/2. ③. Based on the scope of the tuning manual, the discharge starts between 1/2 and 3/4. ④. These are markers indicating the range of throttle openings. These four points are the foundational details. At point ②, the throttle opening is around 1/2. I captured the video and used screenshots. When the valve opening is at 1/2, the throttle opening is about 3/4. The acceleration pump discharge begins when the throttle opening exceeds 1/2. It seems that the X-axis for the acceleration pump discharge timing in the tuning manual is based on the throttle opening.
When checking the video, you can see that the acceleration pump discharges at about 1/4 of the valve opening. The adjustment of the tab is around 3mm. In terms of throttle opening, it discharges at about 5/8. As a result, it is highly likely that the X-axis range affected by the acceleration pump refers to the throttle opening. It is also highly likely that the tuning manual is written referencing the throttle opening for the other X-axes as well.
The timing of the acceleration pump discharge.
Does the scope of the FCR tuning manual affect the throttle opening or the valve opening?【summary】
When it comes to what you need to be aware of, it’s the throttle opening. I’ve never heard of needing to focus on the valve opening on the carburetor side when tuning the FCR. There are two important points to keep in mind:
①. The throttle opening and valve opening are not directly proportional. ②. There’s no clear announcement specifying that the throttle opening should be referenced. The FCR is designed so that the speed at which the throttle valve opens is not directly proportional, as it's controlled via a linkage arm. The reason why the FCR causes a sudden acceleration could be that it’s adjusted on the carburetor side to turn into a high-throttle unintentionally after half throttle opening.
Based on the above, it’s highly likely that the tuning manual is written with throttle opening as the reference. Therefore, when tuning, make sure to focus on the throttle opening.
If Keihin, the manufacturer, were to announce in the near future that the reference should be valve opening instead of throttle opening, I’d be quite disappointed.
