FCR Tuning – The air screw is set to one turn back as a baseline. How much should it be adjusted?

FCR Tuning – The air screw is set to one turn back as a baseline. How much should it be adjusted?

I started writing a blog about the CBX1000, which led to a biker living nearby asking me to explain how to adjust the air screw. My goal is to write 365 blog posts, and I’m doing it for myself. It’s always nice to get feedback from the blog. I’ll be sharing my thoughts in this post, which is aimed at beginners.

When I mention adjustments like "one turn back" or "half a turn back," I’ll use the clock hand as a reference. For instance, 90 degrees is equivalent to a 15-minute adjustment. If it's one turn plus 90 degrees, it would be described as 1.15 minutes back.

So, let’s get started!

How much should you turn back the air screw to notice a difference?

Assuming that the air screw is already at or near the correct position, here’s what to keep in mind:

If the correct adjustment is closer to a minimal setting, small changes like 5 minutes can make a noticeable difference. However, if you have turned back a larger amount, such as 2.00 minutes, the effect of a 15-minute adjustment may be less noticeable.

I believe the percentage of adjustment from the current air screw setting affects this. I’ve changed the slow jets from size 35 to size 45 sequentially. With the 35 size, even slight adjustments to the air screw resulted in significant changes in idling. But with the 45 size, minor adjustments became less clear, making it difficult to determine if the air screw was having an effect or not.

While the engine does idle, opening the air screw too much can result in a lack of power during acceleration.

Should the air screw adjustments be uniform across all cylinders, or not?

For the CBX1000, it doesn’t matter whether they are uniform or not. The tuning manual indicates changes in RPM, which can be difficult to detect with an analog meter. It’s also challenging to judge by sound alone. When I used a digital tachometer for measurement, a change of 5 minutes was noticeable. At that time, the average adjustment was 1.15 minutes.

Does air screw tuning vary between individual tuners?

Does the adjustment of the air screw vary depending on the person making the adjustment?

In a precise sense, there can be differences. However, in reality, the differences should not be significant. This is because the air-fuel ratio for output is between 12.5 and 13.0. The more skilled the tuner, the more precise the air screw adjustment can be.

In the end, what is the correct adjustment for the air screw?

Japan has four distinct seasons. Considering this, I believe adjusting the air screw to around 1.00 turn back during spring and autumn is appropriate. After that, you should make air screw adjustments for about 30 minutes.

The tuning manual suggests changing the slow jet if adjustments exceed 3.00 minutes. Exceeding the appropriate range too much is not ideal. Essentially, you should adjust the slow jet to stabilize around 1.00 minutes return during spring and autumn. This approach makes it easier to adapt to changes in the air-fuel ratio during summer and winter.

Changes in the air-fuel ratio at low throttle openings also need to be considered, and attention should be given to the straight diameter of the jet needle.

Beginner’s Guide to FCR – Adjusting the Air Screw

Simple Adjustment of the Air Screw

Start by turning the air screw back by 1.00 turn, then gradually open it by about 10 minutes at a time. It’s fine if all cylinders are adjusted the same way. Look for a point where the clutch engagement results in poor acceleration.

From that point, turn the screw back by about 15 minutes. If you can achieve smooth acceleration with a good power feel, then this is likely the correct air screw setting.

For multi-cylinder setups where you want to adjust each cylinder individually, turn all the air screws back by 15 minutes and look for the position where the RPM increases. Once you find this position on one cylinder, make a note of it, return to the initial setting, and then check each cylinder individually. If you find the method described in the tuning manual difficult, try this approach. Using a digital tachometer to adjust in 10 RPM increments may also be helpful. It may not be perfectly accurate, but it should be sufficiently close.

FCR Tuning – The air screw is set to one turn back as a baseline. How much should it be adjusted?【summary】

I’ve provided a rough guide on adjusting the air screw, based on the CBX1000. This is my personal method. The approach may vary depending on the engine type and displacement. I’ve outlined my current method for adjusting the air screw.

See you again!

CBX1000 Photo Gallery

I will include a photo of the CBX1000 with velocity stacks from a past post. Generally, I use the velocity stack setup only when continuously adjusting the air screw. After making a rough adjustment, I fine-tune the air screw while installing and removing the I think it is essential to apply oil to the Foam Air Filter Sock to extend the life of the Foam Air Filter Sock.