FCR Tuning – Having an air-fuel ratio gauge is useful for adjusting the pilot screw.

FCR Tuning – Having an air-fuel ratio gauge is useful for adjusting the pilot screw.

Do you all have an air-fuel ratio gauge installed?
When tuning the FCR carburetor, having an air-fuel ratio gauge is quite handy.
Even without one, once you get used to tuning, you can manage it just by real-world riding.
For mid-to-high-speed settings, you can sufficiently adjust just by feel.
In fact, it might be better not to get too caught up in the numbers.
However, it can sometimes be difficult to judge whether the low-speed throttle range is running well or not.
Especially for idling, it's hard to tell unless you pay attention to the smell of gasoline or how well it starts.
Lately, I've been adjusting the pilot screw (PS) and the air screw (AS).
This time, the main focus is on the PS adjustment.
The PS can sometimes be tricky to adjust, especially on multi-cylinder motorcycles.
Using a carburetor pilot screw adjustment tool makes it more convenient.
Based on the readings from the air-fuel ratio gauge, even a minor adjustment of about 0.30 turns back can result in a change of about 0.5 in the air-fuel ratio, or sometimes no response at all, making it hard to judge.
Even with real-world riding, the effects are only noticeable at engine startup and when taking off from a stop.
As for starting the engine, since I use the accelerator pump, it becomes even harder to gauge.
That’s why this time I want to emphasize that having an air-fuel ratio gauge makes PS adjustments much easier.
So, let’s get started!

Tuning by setting the pilot screw to a fixed position of 1.00 minutes return.

In FCR tuning, many people proceed by fixing the pilot screw (PS) at 1.00 minutes return.
This is likely because it's difficult to detect any noticeable changes.
By adjusting the air screw (AS), you can easily feel the difference in startup and takeoff performance.
Once you adjust the AS, the engine's starting and takeoff can be optimized, making it unnecessary to adjust the PS.
If the AS adjustment range is exceeded, you can change the slow jet (SJ), so there's no need to touch the PS.
You can achieve the intended tuning without adjusting the PS because it simply isn't necessary.
However, in some cases, adjusting the PS can help shorten the tuning time.

In the end, as long as the engine idles well, starts easily, and you feel sufficient power when engaging the clutch during takeoff, the PS is set correctly.
Even with the PS fully closed, the engine will still idle.
It will also idle if you turn it back as far as 3.00 minutes return.
Whether it’s running well or poorly is hard to judge because the engine is running either way.
Sometimes, the air might smell like gasoline if it’s running too rich, but that's not a major clue.
Adjusting the PS to anything other than 1.00 minutes return without proper thought feels uncomfortable mentally.
Using an air-fuel ratio gauge helps to resolve that discomfort and allows you to achieve a higher level of tuning.

Let's move on to the next section.

Adjust the pilot screw (PS) after adjusting the air screw (AS).

Using an air-fuel ratio gauge allows for the following:

You opened the air screw (AS) significantly toward the lean direction.
→
There is no sense of power during takeoff.
→
Adjust with the pilot screw (PS).
→
Performance improved.

After tuning with the PS fixed, you can visually adjust the air-fuel ratio at idle.
The PS only affects the area around idle.
The AS, on the other hand, influences the range from idle to about 1/4 throttle opening.
When tuning, there are times when adjusting with just the AS reaches its limit, but using an air-fuel ratio gauge makes resolving this issue much easier.
Even without an air-fuel ratio gauge, you can still make adjustments, but it’s difficult to notice the effects of the PS based on human perception alone.
Tuning while visually checking the air-fuel ratio by numbers greatly contributes to reducing the time required and improving accuracy.

The pilot screw (PS) has little to no effect on the air screw (AS).

When you adjust the air screw (AS), it overlaps with the range affected by the pilot screw (PS), which can disrupt the PS tuning range.
On the other hand, the range where the PS overlaps with the AS is only around idle.
Therefore, after tuning the AS, the best way to adjust the idle area is with the PS.

I have set the air-fuel ratio of my CBX1000 to 12.9 during takeoff.

When tuning my CBX1000, I find that an air-fuel ratio of 12.9 feels optimal during takeoff.
The air-fuel ratio at idle differs depending on the oil temperature and the gear position, whether it's in gear [1] or neutral [N].
Idle performance can also vary slightly based on the RPM and voltage.
Some engineers suggest that the air-fuel ratio at idle should exceed 14.
By keeping it slightly lean at idle and opening the throttle just a bit during takeoff, you can achieve a smooth launch.
There are suitable air-fuel ratios depending on oil temperature and engine characteristics.

FCR Beginners – How to Tune the Pilot Screw (PS)

Fix the pilot screw (PS) at 1.00 minutes return. if takeoff is easy, you’re good.
If there’s a lack of power and takeoff is difficult, open the PS by 0.30 turns in the richer direction.
If you need to lean it out, you can change the slow jet (SJ) to a leaner size, and if you feel the need to enrich it, you can open the PS.
If the idle is too rich, it may cause difficulties with restarting the engine.
You can also find the appropriate tuning for takeoff using the air screw (AS).
In fact, the AS adjustments are often more straightforward.
Typically, the position where you feel power during takeoff corresponds to the correct AS setting.
The PS mainly affects the idle range, while the AS influences up to about 1/4 throttle opening.
Please adjust the screws while considering the interaction between the AS and PS.
If the SJ is set too rich, it may make it difficult to gauge the effect of the PS.
While it’s easy to notice if the low throttle is too lean, it can be harder to tell if it’s too rich, so keep adjusting until you’re satisfied.
If you get stuck, restarting from the initial settings won’t damage your motorcycle.
Some motorcycles require a carburetor pilot screw adjustment tool to make adjustments.
You might be able to adjust it but have trouble knowing how much you’ve turned it.
If the adjustment is challenging, either use the carburetor pilot screw adjustment tool or find a way to make the work easier.
I believe beginners should use good tools.
When attempting to adjust the PS, please purchase a carburetor pilot screw adjustment tool.
If it doesn’t fit your motorcycle, you might be able to modify the sides or back to make it work.
I have used a tool on my CBX1000 after trimming the sides.
There’s no issue using it on other motorcycles.
However, a drawback of modifying the tool is that it may be difficult to resell it when you no longer need it.

FCR Tuning – Having an air-fuel ratio gauge is useful for adjusting the pilot screw.【summary】

Having an air-fuel ratio gauge is very convenient because it allows you to visually adjust the air-fuel ratio at idle.
It’s also essential to have a carburetor pilot screw adjustment tool that fits your motorcycle.
With this tool, you can adjust the PS without having to remove the FCR carburetor.
Adjusting it every time after starting the engine isn’t very efficient.
Turning the PS while the engine is running makes it easier to see the changes.
The changes in air-fuel ratio from adjusting the PS can be hard to notice visually.
However, if you turn the PS while the engine is running, you can quickly see how much the air-fuel ratio changes with the gauge.
For the CBX1000, you can notice a change with about 0.15 minutes return of the PS.
Adjusting three or more cylinders will start to show changes in idle and air-fuel ratio.
If the SJ is too far from the correct size, it can be hard to notice changes even when turning the PS.
When the SJ is large, the engine can still idle with the PS fully closed.
Depending on the combination of the AS and SJ, it may also be difficult to gauge the effect of the PS.
In these situations, having an air-fuel ratio gauge allows you to quickly determine whether the PS is functioning correctly.
See you again!

Various Related Photos

These are related photos of the air-fuel ratio gauge and carburetor pilot screw adjustment tool.