The throttle opening and throttle valve opening of the FCR carburetor are not directly proportional.

The throttle opening and throttle valve opening of the FCR carburetor are not directly proportional.

Even when the throttle of the FCR is opened halfway (1/2), the throttle valve on the FCR side does not open up to halfway. This is depicted in a graph on page 3 of the FCR tuning manual. Since it's not described in words, it might be hard to understand.

Knowing this could change how you approach operating the throttle on an FCR. For this blog post, I'll first write out the explanation in text, and I’ll also include some photos as needed. As a premise, I'll refer to the throttle on the handlebar side as the "throttle opening" and the one on the FCR side as the "throttle valve opening."

The FCR tuning manual focuses primarily on throttle opening as a reference. However, the relationship between the throttle opening and throttle valve opening is not proportional. Up to a throttle opening of 1/2, the movement of the throttle valve is minimal. When the throttle opening is at 1/2, the throttle valve opens about 1/4. After that, as the throttle is opened further, the throttle valve opens rapidly. When the throttle is fully opened, the throttle valve on the FCR side feels like it opens a little beyond full open.

In short, the "throttle opening" ≠ "throttle valve opening."

This is what I will cover in this blog post.

Alright, let's get started!

【Verification】The throttle and throttle valve are not directly proportional.

Let's take a look at the relationship between throttle opening and throttle valve opening. When the throttle is fully closed and fully opened, the state is as follows:

Throttle Opening = Throttle Valve Opening

However, please note:
①When the throttle is fully closed, the throttle valve is still slightly open.
②When the throttle is fully open, the throttle valve goes slightly beyond full open, by about 2mm at the top.

Throttle Opening Angle – close

Considering throttle free play and the idle stop screw, the throttle valve is slightly open to maintain idle.

Throttle Opening Angle – 1/4

Even when the throttle is opened about 1/4, the throttle valve does not open to 1/4. At first glance, this might seem like measurement errors or inaccuracies in throttle operation.

Throttle Opening Angle – 1/2

Clearly, the throttle valve opening is different from the throttle opening at 1/2 throttle.

Throttle Opening Angle – 3/4

I'm taking the photos by myself. The measurements are a bit rough. At 3/4 throttle opening, the throttle valve opening is about 1/2. When opening the throttle from this position, the throttle valve opens suddenly.

Throttle Valve Opening Angle – Full Open

When the throttle is fully open, it can be said that the throttle valve is also fully open. However, the throttle valve does intrude into the information by about 2 mm.

Physical reasons why the throttle opening and throttle valve opening are not directly proportional.

It seems that the way the throttle valve opens is determined by the shape of the arch section of the link arm mechanism. The graph might not be entirely accurate but is generally correct. I created it by inputting values for the throttle opening and throttle valve opening. It's more of an illustrative concept. The settings manual on page 3 should be accurate, but it does not include numerical values. Please use this as a reference.

Consideration of the reasons why the throttle opening is not directly proportional in the design philosophy of the FCR.

This is purely a consideration. I imagine that racing carburetors like the FCR were designed with the idea that having a gradual opening at low throttle would make them easier to handle. There may have been improvements over the predecessor CRS carburetor as well. It could have been a part of the designer's conceptual approach.

The FCR features a smooth opening at low throttle. To balance this, the throttle valve opens rapidly beyond about 3/4 throttle. There may be some disadvantage to having a direct proportional relationship between the throttle and the throttle valve in terms of carburetor performance. The way the FCR’s throttle valve opens might be the appropriate design for a carburetor.

It has been over 30 years since the FCR was released, and it has not been a major topic of discussion. There doesn’t seem to be an option available for a 1:1 proportional throttle valve opening. This suggests that the design philosophy was indeed exceptional. Since this is my own consideration, it remains in the realm of speculation. The exact reasons would need to be confirmed by the designer.

Which should be tuned: the throttle opening or the throttle valve opening?

There is a puzzling part on page 8 of the tuning manual. It concerns the range of influence of the jet. Is it indicating the throttle opening or the throttle valve opening? Since it's not clearly stated, I believe it refers to the throttle opening. When I called the distributor BITO, they confirmed that tuning is done based on the throttle opening.

Upon closely examining the tuning manual, it seems that the timing of the accelerator pump discharge suggests it is based on the throttle opening. I actually tested this, and since the accelerator pump operates directly without a link arm, it moves in accordance with the throttle. The jet needle crosses the 1/2 throttle opening. This might be why the curve of the graph appears unusual.

For beginners with FCRs: Tune the carburetor without considering that the throttle and the throttle valve do not have a direct proportional relationship.

Just keep this as a piece of trivia. At low throttle openings, the operation is smoother. Looking at the graph, around 1/4 throttle opening, even slight throttle adjustments cause the throttle valve to open. In practice, it seems that the throttle valve opens abruptly around 1/2 throttle. When tuning, it is fine to refer to the throttle opening. There are people who have trusted this graph for tuning over the past 30 years. If it could be proven that there is an issue, it might become big news worldwide.

The throttle opening and throttle valve opening of the FCR carburetor are not directly proportional.【summary】

I still have a slight doubt. It concerns whether to refer to [throttle opening] or [throttle valve opening] for tuning.

There are three possibilities regarding the influence range graph:
① graph that refers to throttle opening for tuning.
② The graph is incorrect.
③ The graph correctly considers both throttle opening and throttle valve opening.
④ A graph that refers to throttle valve opening for tuning.
I am currently tuning based on possibility ①.

I found it very interesting to realize that the throttle and throttle valve do not move in direct proportion. This is a topic where if you focus too much, it may seem never-ending. If you don’t focus on it, you can continue to enjoy tuning the FCR as usual. Even if you pay attention to it, you can still proceed with FCR tuning.

One question remains. I hope everyone enjoys their FCR experience. See you again!