FCR Tuning – Air Screw Optimization

HONDA CBX1000-FCR-Slow jet replacement FCR
This article can be read in about 10 minutes.

FCR Tuning – Air Screw Optimization

When I first started working with the CBX1000 FCR, I changed the slow jet several times.
When I didn't know how to tune, I just tweaked everything I could get my hands on.
Currently, I'm trying to avoid touching the SJ as much as possible.
Low throttle openings from fully closed to about 1/8
are tuned by combining ① the pilot screw, ② the air screw, ③ the slow jet, and ④ the straight diameter. In addition, there are ⑤ the idle port and ⑥ the slow port. Both are fuel injection ports that the slow jet is responsible for, and although their existence is not mentioned in the tuning manual, you can read them roughly from the diagram.
The hole on the engine side with the throttle valve closed is the idle port. The hole on the funnel side with the throttle valve closed is the slow port.
The fuel injection from the slow port gradually becomes more effective depending on the throttle opening.
In short, tuning is quite complicated up to about 1/8 throttle opening.
This is not for people who don't need to adjust the low throttle opening precisely for racing use.
I don't understand the details or the complete mechanism, but by being aware of the combination of ① to ④ and the role of ⑤ and ⑥, I was able to smoothly tune from fully closed to about 1/8 of the throttle opening.
This blog post will describe how much the AS air screw should be tuned to. It depends on the case, but if it's roughly 0.5 to 2 turns, the balance of each jet is not bad.
You can cheat by opening and closing the air screw to an extreme degree.
However, if you cheat and tune, you will feel something strange somewhere while riding.
If you don't feel any strangeness, it's the best tuning.
Magazines sometimes say that tuning should be done to your own taste, but
the best tuning will be the same no matter who does it.
Tuning done by an individual over a long period of time can involve the effectiveness of the acceleration pump and tuning that takes into account each rider's way of opening the throttle.

Sometimes it is appropriate to turn the air screw back 1/4 turn from the full tightening.

The tuning manual says that if the air screw falls below 1/2 a turn, you should change the slow jet to a richer number. In the case of the CBX1000,
if the slow jet was set to 35, the correct running value was when the screw was turned back about 10 to 15 minutes, in terms of clockwork. It seems that the screw adjustment also has to be quite strict.
On the other hand, if the screw is turned back more than 2 turns, it is difficult to tell the effect unless you move it about 15 minutes, in terms of clockwork. The actual value will change between 1% of a small value and 1% of a large value.
This is only natural.

Ultimately, I think 1.5 is a good benchmark.

First, find the number of the slow jet and pilot screw that seems to give good idling from engine start.
After that, determine the return of the air screw by the combination of the air screw and the straight diameter.
When starting, if the throttle opening is about 1/16, and there is no jerking when starting and starting,
and if there is no stress when driving on the street at about 1/8, then the return of the air screw is appropriate. Since it changes depending on the season, a combination of slow jet and straight diameter with a return of about 1.5 as the standard is good. If you go below 1.0 rotation, the screw adjustment becomes a little severe.
In the end, if the slow jet is too small, you will not feel any change no matter how much you tighten the air screw. Conversely, if you cannot feel any change no matter how much you open the air screw, the number of the slow jet is too large in the rich direction. When you test drive while repeatedly starting and stopping, there is a point where it is best. Recently, I adjust the return of the air screw while watching the RPMs on a digital tachometer. When test driving, it is not that far off from the appropriate return of the air screw.

Air screw return accuracy

Rather than gradually turning the air screw back in 0.5 degree increments, I initially adjust the air screw in 15-minute increments. I turn it back the same for each cylinder.
When I find a spot that seems good, I'll know whether it's due to the heavy or lean, so I tune the air screw back to my liking in 5-minute increments.
After that, I tighten it another 10 to 15 minutes clockwise and then I'm done tuning the air screw.

FCR for Beginners – Easy Air Screw Adjustment

Adjust the air screw back in about 5-minute increments using the hands of a clock. Adjust the throttle opening to about 1/16 to see if it is easy to start. You can adjust the ports as mentioned above once you have your own preferences. First, you need to be aware of the condition to be able to ride.
If the air screw is open too much, the start will be weak and lacking in power.

FCR Tuning – Air Screw Optimization【summary】

My own best tuning method is constantly being updated. My way of thinking and tuning precision are different from last year's to the year before.
Right now I don't care about fuel economy at all, but next year I may become very particular about it.
A state where the overall balance is roughly achieved is close to the best.
I hope you all enjoy your FCR life.
See you again!

CBX1000 Photo Gallery

This article mainly contains photos from previous posts showing the disassembly of the CBX1000 FCR.
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