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 – For street use, be sure to check the throttle response at the initial opening.
FCR is a carburetor designed for racing. However, in reality, many users spend most of their time using it on the street. The important thing in street scenes is the start. On the street, there are traffic lights. You repeatedly stop and go. On the circuit, once you start, you typically don't come to a complete stop unless there's a crash. What's important on the street, and the subject of this blog post, is the initial throttle opening. In the case of my CBX1000, I've recently been able to tune it without spending much time. While it isn't about pursuing power, the low throttle openings involve many tuning parts. The related components include the pilot screw, air screw, idle stop screw, synchronization, slow jet, jet needle straight diameter, and throttle cable clearance. I will write about throttle openings ranging from fully closed to about 1/16, 1/8, and 1/4. A throttle opening of 1/8 is used for cruising on my CBX1000. The fully open throttle angle on my CBX1000 is 56 degrees. I also have an air-fuel ratio meter installed. I might write something incorrect, but please use this as a reference. So, let's start tuning!
While slowly engaging the clutch, start moving in this order. The motorcycle is stopped with the engine running. Here are my thoughts on the air-fuel ratio. I believe it should be around 12.9, 12.5, or 12.0 to 12.5. If it is too lean, there is no sense of power when starting, and you have to use the half-clutch more aggressively to get moving. If the air-fuel ratio is around 11 when starting, it is too rich. You need to anticipate the symptoms of the mixture becoming lean the moment air is sucked in when the throttle is suddenly opened. The above three throttle openings also affect the response return when revving the engine. When the air-fuel ratio is lean or synchronization is off, the response return is poor.
Throttle opening at 1/16 is difficult to target and adjust precisely.
In the previous section, I mentioned that having a smooth transition from throttle fully closed ~ throttle opening 1/16 ~ throttle opening 1/8 is ideal for tuning. I find adjusting around 1/16 difficult. This is influenced by the pilot screw and the air screw. The most significant factor is the straight diameter of the jet needle.
I have made a habit of consciously checking when moving the throttle from fully closed to 1/8 opening. For a street motorcycle, engine start-up and idling are crucial sections. It is somewhat unavoidable for the carburetor to become a bit complex.
The tuning manual does not cover idle ports and slow ports. I do not clearly understand how they operate. At the very least, the height of the throttle valve affects fuel intake. For low throttle openings, the combination of the slow jet and the straight diameter of the jet needle is important. How you adjust the screws will determine the tuning outcome.
You can ride without paying much attention to the 1/16 throttle opening, but focusing on tuning it can result in a more manageable motorcycle. It is essential to check that the accelerator pump’s discharge timing is not too early. Finally, synchronizing everything will make for a fun motorcycle to ride around town.
Throttle opening at 1/4
Throttle opening around 1/8 is often used for extended periods. When you twist the throttle to 1/4, a large motorcycle will start to accelerate. If you open the throttle further, the engine will begin to rev up aggressively. The number of clip positions on the jet needle also comes into play. Throttle openings greater than 1/4 become the main throttle openings for sport riding.
Throttle cable clearance
One of the crucial aspects of the FCR is throttle cable clearance. Preferences for throttle cable clearance can vary among riders. The FCR is a mechanically-operated carburetor, and it's generally better to have as little clearance as possible. Direct operation of the carburetor is important. Here are some key points to consider:
①The throttle should not open when the handlebars are turned. ②The throttle should not be in a state where it cannot fully close. ③Some clearance must always be present. Among these, point ③ is especially important. If the clearance is 0 mm, the throttle will likely be unintentionally slightly open. While this is a basic aspect of throttle adjustment, it is a crucial item to check for enjoying your motorcycle with the FCR.
Throttle low openings also require attention to synchronization.
In multi-cylinder motorcycles, balancing each cylinder's vacuum pressure is an essential and inseparable task.
When the temperature is low, the vacuum pressure is pulled toward the lower side. When the temperature is high, it is pulled toward the higher side. Carburetor synchronization needs to be done to the extent that you are satisfied with the results. This is because you can align it by observing the instruments.
As you work more frequently with the FCR, being able to synchronize it yourself will broaden your tuning options. If you don’t have the tools, visual alignment is also acceptable. Perfect synchronization is a matter of personal satisfaction; achieving a result that you are okay with is the goal.
If synchronization is significantly off, the engine’s RPM may not drop properly, which can be dangerous.
FCR Beginners: Focus on Throttle Area Dress-Up
This continues from the previous section.
Taking pride in details like the throttle cables, throttle openings, and grips makes riding your motorcycle a lot more enjoyable. The FCR is an exceptional carburetor for its direct throttle response. Paying attention to the throttle area means you are truly enjoying the FCR. Customizing the grips with Domino products or in your preferred colors is also fun.
Sometimes, grips are secured with stainless steel wire. If you’re concerned about vibrations, try changing the weight of the bar ends. Working on the throttle area will likely make you more aware of other maintenance aspects as well.
The grips on my CBX1000 are over 40 years old. They have a lot of character, and I’m very fond of them. The throttle area is a part of the motorcycle that naturally draws your attention while riding. Also, consider paying attention to the brake master and bolts.
FCR Tuning – For street use, be sure to check the throttle response at the initial opening.【summary】
Throttle low openings can be a challenging aspect of tuning the FCR. In racing scenes, the tuning of low throttle openings might sometimes be done roughly. However, for street use, this tuning is essential. Focus on fine-tuning the initial throttle opening and give it a try.
See you again!
CBX1000 – Photo Gallery
I will be featuring photos of the throttle area from past posts. The master cylinder is a Brembo Corsacorta product. The throttle is an Active TYPE3 model. The grips are secured with wiring.
