FCR Tuning – 2023-04-12 – Considering the Jet Needle Strategy

FCR Tuning – 2023-04-12 – Considering the Jet Needle Strategy

I spent a long time on the highway when I went to a motorcycle event with my CBX1000. While riding, I kept an eye on the performance of the FCR. The results were neither particularly good nor bad. Overall, my impression is that it might be running slightly rich.

The current tuning is as follows:

SJ-48
JN-90FTN#1
MJ-125
AS-1.30 minutes return
PS-1.00 minutes return

I feel that the overall setup might be on the rich side, possibly due to the replacement of the lip seals and vacuum release plate. Even at full throttle in 5th gear, it pulls well, but based on past experience, it seems like MJ-120 would be better than MJ-125 at this time of year in April. During actual riding, while MJ-125 gives some extension, the air-fuel ratio feels rich, resulting in less responsive acceleration.

Regarding the combination of the SJ and JN, I feel I want to lean out the clip position by one more notch. The 1/8 throttle range, which is mainly handled by the SJ, doesn’t bother me. However, depending on the JN selection, it might be worth considering changing SJ-48 to SJ-45.

Plan A:
SJ-45
JN-90FXM#2
MJ-120

Plan B:
SJ-48
JN-90FTR#1
MJ-120

I'm currently considering which tuning setup to go with. It's always very enjoyable to think about tuning.

It's been a while since I posted about tuning, so here’s a recap on the JN selection. For the CBX1000, the current default JN is 90FXM. Since it's a specialized JN, you can't choose the straight diameter. However, you can select a clip position that seems to match well with the CBX1000. The T series, like 90FTM, offers a wide selection of straight diameters. If you go with 90FXM, the only way to change the lower range is by changing the SJ. With the T series like 90FTM, you can make adjustments with the straight diameter without frequently changing the SJ. This doesn’t mean you’ll never have to change the SJ, but the ability to select the straight diameter is an important factor.

Should I proceed with tuning based on the SJ or the JN?

Currently, I believe that if I lean out the straight diameter of the JN, it's better to use a slightly larger SJ. With my CBX1000, I’m concerned that the clip position might quickly reach the top notch, so I’m not sure what to do. The 90FXM has some room for adjustment in the clip positions, but I find it very frustrating that I might need to change the SJ with the changing seasons. Therefore, I’ve decided to retune based on the SJ with the 90FXM when summer comes around.

For now, I'll proceed with:
Plan B:
SJ-48
JN-90FTR#1
MJ-120

I'll experiment with different straight diameters and try changing the clip position to 1 or 2 until summer. To be honest, frequently changing the SJ isn’t very suitable for the FCR on the CBX1000, which uses a dual cable system. It’s best to minimize the number of SJ changes during tuning. Personally, I prefer to use a slightly larger SJ and combine it with a slightly lean straight diameter.

FCR Beginner’s Guide – Insights on Selecting the MJ

I realized something while riding on the highway. It’s difficult to accurately check the throttle at full open. The correct MJ should allow the bike to accelerate so strongly that it's almost a bit frightening. During the recent test run with the CBX1000, the acceleration felt just a little bit sluggish. Because the acceleration was poor, it was easy to fully open the throttle without hesitation.

My CBX1000 is equipped with an air-fuel ratio gauge, so I can determine whether the current tuning is leaning towards rich or lean. In the past, MJ tuning sometimes resulted in such strong acceleration that it was actually a bit scary. The MJ in those past tunings was likely very close to the best possible setting. However, during this recent test run, the MJ didn’t feel quite right.

I've come to understand that the previous setup was actually better, so I’ll revert to it. If the bike doesn’t accelerate with that slightly frightening power, then the MJ is likely not at the optimal setting.

FCR Tuning – 2023-04-12 – Considering the Jet Needle Strategy【summary】

The day before tuning my FCR, I predict and imagine what changes to make and how it will turn out. It’s very disappointing when my predictions are off. Usually, I can achieve results close to my expectations. When I don't get the imagined result, it often leads to new discoveries. Every time I tune the FCR, it definitely gets better than before. The reason is that there are clear aspects I’m not satisfied with. That’s natural.

Japan has four distinct seasons, and tuning changes with each season. If you think about it in detail, you might need to adjust the tuning differently every day. One of the joys of FCR and other racing carburetors is the ability to change jets immediately when needed. I’ll find some time this weekend or tomorrow to re-tune.

So, enjoy your FCR life, everyone! See you again!

CBX1000 Photo Gallery

Here are photos of the CBX1000 taken during test rides, including the time I headed to the motorcycle event "Wheels Village 2023." = "SyarinMura2023"