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 – When changing the straight diameter of the jet needle, also adjust the clip position.
When you change the straight diameter of the jet needle, if you don't also adjust the clip position, the engine performance can become significantly worse. Here's an example: if you change from 90FTS#2 to 90FTN#2.
Simply adjusting the air screw may not be enough, and the motorcycle might run so poorly that it becomes undrivable. '90FTN#2' means the second clip position from the top when installed in the FCR.
The straight diameter is adjusted in the lean direction through four steps: N, P, Q, R, and S.
*It is likely that there is no 'O' because it could be confused with 'ZERO.'
I have written posts in the past about the straight diameter and clip position, so I will direct you to those as well.
This post is based on a case where I actually performed a test ride.
Continuing from the previous discussion: 90FTS#2 → 90FTQ#2 is not so bad that the bike becomes unrideable.
The impact of FCR tuning is proportional in a parabolic way. The further you are from the optimal size, the less noticeable the effect. While it's difficult to say exactly how many clip positions you should adjust when changing the straight diameter, it seems reasonable to assume that changing the straight diameter by five steps would correspond to roughly one clip position.
Even with the same jet needle, adjusting the clip position by just one step can significantly change the way the motorcycle behaves. It’s almost as if you could try out seven different jet needles with just one.
As for my tuning method, when I want to make adjustments in the range affected by the straight diameter, I change the straight diameter in increments of two. For fine adjustments, I change it by one step at a time.
In this blog post, I want to share the idea that when you change the straight diameter of the jet needle, the appropriate clip position also changes. I'll go into more detail in the following sections.
So let's get started!
A change of five steps in the straight diameter corresponds to about one clip position adjustment.
LEFT 90FTM RIGHT90FTS
This is a comparison between 90FTM and 90FTS. While the measurement is rough, based on the photos, there is a change of about one clip position over five steps. However, the impact isn't limited to just the area affected by the clip position; other factors also come into play, so the effect on performance isn't necessarily confined to the clip position alone.
How much impact does it have on performance?
Changing from 90FTS#2 to 90FTS#3 results in a more noticeable change than you might expect. Even with the same straight diameter, just adjusting the clip position by one step dramatically changes the feel of the bike. Clip position tuning is less about finding the ideal air-fuel ratio and more about finding the riding feel that suits your preference. On the other hand, moving the clip position by just one step with the same straight diameter can sometimes make the bike run so poorly that it becomes undrivable.
The pitfall of focusing too much on the straight diameter.
After pursuing the combination of the slow jet, jet needle straight diameter, and air screw, there have been cases where, after tuning the low throttle opening, riding in the range affected by the clip position resulted in poor performance. Even a single change in the low throttle opening can sometimes have a significant impact. It’s important to develop the habit of revisiting the clip position whenever you change the straight diameter.
It might be helpful when there isn’t enough rise!
Taking the CBX1000 as an example, it seems that the default jet needle has a slightly insufficient range for adjusting the clip position. The current jet needle shipped by BITO is the 90FXM.
The 90FXM is a special-sized jet needle. For other motorcycles with small-body FCRs, the initial value for the jet needle is often 90FTM. Since the rise is a special size,
90FTM#1 = 90FXM#3
is the situation. If you want to secure a similar clip position with the rise of the M size,
90FTX may be the ideal size.
This corresponds to a straight diameter ten steps ahead. The 90FTS seems a bit lacking. If you progress to around 90FTV, you won’t run short on clip positions. However, this means you'll need to select richer jets for the slow and main jets.
There is also a method to increase the rise by changing the taper angle.
You can change the clip position by about two steps without controlling it solely through the straight diameter. Using taper angle F as a reference:
・If you want to raise the clip position above #1, you would use taper G. ・If you want to lower the clip position below #7, you would use taper E. This logic seems sound in theory. However, since this is purely theoretical, the tuning will generally need to be redone as a whole.
LEFT90FTR RIGHT90ETR
Left: 90FTR#4, Right: 90ETR#2. This is a situation where I only measured the thickness of the jet needles. Even after taking a test ride, I couldn't ride properly right away.
FCR for Beginners – Clips can sometimes be purchased at home improvement stores.
Changing the clip position is a tuning adjustment that I highly encourage beginners to try. There are several reasons for this:
・Minimal tools are required ・Recovery is easy ・Changes can be easily enjoyed
It’s also possible to make adjustments on the go. However, there are points to be cautious about. One common issue is losing the E-clip. If the E-clip goes missing, you won't be able to ride. Always carry a spare with you. If you don’t have a spare, you can also purchase one at a home improvement store. A 3.00mm E-clip can be used as a substitute, while the E-clip used in FCRs is 2.5mm.
Next, here are some precautions for when you're working in your garage. You need to be careful not to make a hole-in-one with the engine. Take extra care while working with the carburetor or spark plugs removed. Be very cautious to ensure nothing gets sucked into the engine. If an E-clip ends up lodged in the piston and causes strange noises from the engine, you’ll be in for some tears.
FCR Tuning – When changing the straight diameter of the jet needle, also adjust the clip position.【summary】
Changing the clip position is more about giving your motorcycle a personal touch in terms of feel rather than strictly tuning. There's no need to excessively adjust things like the air screw, and simply making this change can significantly alter the motorcycle's feel.
For street use, the throttle opening is often utilized. If you’re operating the FCR at the extreme positions like [1] or [7], there may be more room for adjustment. In this blog post, I’ve noted my feelings about the clip position being off after changing the straight diameter and taking a test ride.
I’m not measuring everything with precision, but I’m not reporting anything extremely off either. Knowing these aspects will definitely be helpful when tuning. This was a discussion about the clip position changes due to the straight diameter adjustments.
