FCR Tuning – Tuning the main jet at full throttle is fun.

FCR Tuning – Tuning the main jet at full throttle is fun.

Riding a motorcycle at full throttle! It's undeniably fun.
However, there’s something you must always pay attention to when tuning an FCR carburetor: selecting the main jet (MJ).
While it’s not to say that other jets like the slow jet (SJ) or the jet needle (JN) don’t matter, the MJ tends to be somewhat overlooked on the street.
The MJ is responsible for throttle openings of more than 1/2.
In the racing scene, it’s crucial to keep the throttle fully open for as long as possible.
However, in street riding, there are very few moments when you actually go full throttle.
It’s a bit disappointing not to be able to use the more 'racy' aspects of a racing carburetor.
My focus is primarily on street riding.
I spend a lot of time adjusting the air screw (AS) repeatedly or changing the slow jet (SJ) multiple times, which isn’t something you’d spend much time on in race-oriented tuning.
If your focus is on street riding, you can’t neglect tuning for low throttle openings.
In this piece, I’ll guide you through how I go about tuning the MJ.

First and foremost, it’s important to ensure that the throttle openings of 1/2 or less are well-tuned…

Tuning starts with pre-tuning.
From there, you gradually open the throttle, starting with low throttle openings.
This includes idle, 1/16, 1/8, 1/4, 1/2, 3/4, and finally full throttle.
You adjust by changing the jets or turning the screws responsible for each range.
Once the low throttle tuning is in good condition, the next step is changing the main jet (MJ).
It's often said that the MJ mainly affects the throttle at full opening, but I’ve found it also influences the throttle around 1/2 opening.
I usually try out two or three different MJs that seem promising for the range just past 1/2 throttle before starting the full-throttle MJ tuning.

When it comes to choosing the MJ size,
it’s sufficient to skip one size at a time.
For example, 120, 123, 125, 128, 130.
Skipping from 120 to 125 works well enough.

I believe that using an air-fuel ratio (AFR) gauge is effective in determining the right jet at full throttle.
You can feel when the bike isn’t pulling well at full throttle, but it can be difficult to discern whether you need to go leaner or richer with the MJ.
Since you’re going at quite a speed, you can only glance at the AFR gauge briefly, but it’s helpful in determining which way to go with the MJ.
If you don’t have an AFR gauge, start by lowering the MJ one size at a time from what feels like it might be too rich.
Eventually, you’ll be able to identify the optimal MJ.

Now, let’s get into the specifics of how to start the tuning process…

I'll explain using the CBX1000 as an example.
The FCR on the CBX1000 is a small 33mm.
 Even though there are differences between the large and small FCRs on the CB-F series, the results are quite similar.
I use the highway for testing.
I check in 3rd, 4th, and 5th gears.
Each gear is different, but I start from a cruising speed of around 80 km/h and gradually open the throttle to see if it doesn’t become too lean, whether there’s a sense of acceleration, and if the speed continues to increase.

If an air-fuel ratio (AFR) gauge is installed, it shouldn’t suddenly go lean.
As long as it stays around 12.0 or slightly below, you should feel some acceleration.
Keep in mind that the AFR readings can vary depending on how you open the throttle and your speed, so use them only as a reference.
When you open the throttle from a stable AFR, it may momentarily show a rich direction before going lean.
If you plan to check the numbers to some extent, adjust the MJ while observing the AFR during partial throttle and acceleration.

For the CBX1000, it’s not possible to change the MJ many times in one day.
Considering the speed ranges during test rides and the number of cylinders to adjust, three changes is usually my limit.
If I increase the number of MJ changes, my concentration drops, and in that state, good tuning is impossible.
I stop when I feel tired.

With the CBX1000, I change gears from 3rd to 4th to 5th starting around 6000 rpm, looking for the best feeling during acceleration.
I use the MJ that gives the best response.
If you use a leaner MJ (with a smaller number), the acceleration will feel less responsive.

Do you change the main jet with the seasons?

In Japan, FCR tuning is needed three times a year: in spring/autumn, summer, and winter.
This means that tuning is necessary for each season.
Additionally, factors like temperature, humidity, altitude, and air pressure can also affect the tuning.
A certain level of tolerance is required.
For the main jet (MJ), I think around 125 for spring and autumn, 115–125 for summer, and 128–135 for winter.
Of course, other factors like the slow jet (SJ) and jet needle (JN) can also cause changes.

In my case, I don’t have a fixed MJ size for each season.
Since it’s not for racing use, when I adjust the JN, I change the MJ to one that seems suitable for throttle openings above 1/2 and experiment.
 MJ adjustments include full-throttle runs, which makes testing a bit more challenging, and this is why I haven’t nailed down specific sizes for each season.

Using a slightly richer MJ allows for better acceleration and higher top speed when opening the throttle.
 When selecting MJ sizes, I consider that switching from 125 to 135 might change the air-fuel ratio by about 0.8 to 1.0.
If the air-fuel ratio is too lean or too rich and the MJ size is not appropriate, it becomes harder to feel the difference in performance.

FCR Beginner’s Guide – Main Jet Tuning

I believe the most challenging part of tuning the main jet (MJ) is the test runs.
Testing how well the engine performs at full throttle can be quite dangerous in practice.
If you want to be precise, start with a size around 140 and work down to 135, 130, etc., skipping one size at a time until you find the one you prefer.
 For a less meticulous approach, you can adjust the MJ to the size that feels best around 1/2 to 3/4 throttle.

If you have an air-fuel ratio (AFR) gauge installed, you can determine whether to adjust richer or leaner by looking at the current readings, even if you don’t start with a rich AFR.
If the acceleration feels good and the speed is decent, it’s likely you’re close to the best MJ size.

Although subjective, it’s best if MJ tuning can be completed in one day.
However, taking a week to make adjustments and test them gradually might be necessary.Forcing full-throttle runs on a set timeline can lead to accidents.
It’s fine to check the tuning during a highway tour or at times when it seems safe to open the throttle fully in a straight line.

FCR Tuning – Tuning the main jet at full throttle is fun.【summary】

The primary purpose of installing racing carbs like the FCR on a motorcycle is to achieve higher speeds.
Achieving high speeds means opening the throttle fully.
Other reasons include easier tuning, better response, and the availability of parts.
For sport-oriented motorcycles, enjoying full-throttle acceleration with an FCR is crucial.

I’ve recently become more accustomed to tuning the main jet (MJ).
I’m now able to enjoy the tuning process and find it very rewarding when I can feel the changes in the motorcycle with different MJ settings.
 However, pushing too hard for higher speeds can lead to accidents, so please enjoy it within your own safe limits.

Tuning the MJ is one of the highlights of using a racing carburetor.
 While the slow jet and jet needle are part of the process leading up to full throttle, full throttle itself is one of the most important objectives for a racing carburetor.However, achieving full throttle on the street is not always feasible.

Finding safe places where you can reach higher speeds makes tuning even more enjoyable.
I hope you all enjoy tuning your MJ for full throttle within your safe limits! See you again!

CBX1000 Photo Gallery

These are photos from when I went on a solo tour with my CBX1000.