FCR Tuning – To What Extent Should Carburetor Synchronization Be Done?

FCR Tuning – To What Extent Should Carburetor Synchronization Be Done?

How Far Should Carburetor Synchronization Be Done?

Many people are likely wondering to what extent they should synchronize their carburetors.
Vacuum gauges have graduations.
Is it acceptable for the reading to be off by one graduation?
Does even the width of the black line on the gauge count as an error?
If you keep thinking about it, there’s no end.

The other day, I got the answer.
Here’s what I was told:

"Vacuum-operated carburetors should be perfect. For FCRs, visual alignment is good enough."

This came straight from Mr.Bito of BITO R&D, so there's no doubt!
This is a valuable opinion from one of Japan’s legends.
This might also be a response considering the requirements, the use, or time constraints.
For vacuum-operated carburetors, he said they should be perfect.
Perfect synchronization means doing it as perfectly as one can manage.

For FCRs, my interpretation is that even if the synchronization is slightly off, it’s not something to worry about as long as it doesn't greatly affect the ride.
That said, there’s no harm in achieving perfect synchronization as far as possible with FCRs as well.
I personally believe that synchronization work on FCRs should be done as perfectly as possible.

Now, let me share my own thoughts on synchronization precision.
"If there’s no problem with how the engine returns to idle, that’s good enough."
Even if there’s a slight difference in the gauge reading, it doesn’t affect the ride.
However, issues with how the engine returns to idle do affect performance.

In the next section, I will focus on what is meant by "visual alignment."
Since this is a blog about FCRs, I won’t be discussing vacuum-operated carburetors much.

So, let’s get started!

Adjusting FCR synchronization by visual inspection.

When I say 'visual inspection,' it doesn’t mean I’m aligning it solely by sight. There are two main methods I usually consider:

1. Using a taper gauge
2. Inserting something like a toothpick and observing the moment they drop simultaneously
If the carburetor is removed, I can even align it by eye, based on the way light passes through. I first perform method 1, then fine-tune the synchronization with a vacuum gauge. When time allows, I also check synchronization with the throttle slightly open. After adjusting throttle cable tension and the height of the throttle stop screw, I recheck synchronization.

As for method 2, inserting a toothpick into the floating valve and adjusting so they drop at the same time when opening the throttle ensures that the heights are matched. The toothpick, being wooden and moderately soft, is ideal for this, as it won’t damage metal components.

One advantage of adjusting synchronization without a vacuum gauge is that you don’t need to start the engine. Since I always use a vacuum gauge, I can’t be certain about the accuracy of this approach. The intake vacuum is affected by more than just the height of each cylinder’s throttle valve. After synchronizing, it’s also necessary to check the air screw again.

Symptoms of Carburetor Synchronization Issues

When performing a throttle blip (revving), the RPM drop is slow. If this symptom worsens, it can be dangerous because the RPM won’t decrease even when you release the throttle while riding. In my experience, when the engine temperature is low, the cylinder with a lower throttle valve height has a stronger influence. Conversely, when the temperature is high, the cylinder with a higher throttle valve height has a stronger influence. If you notice a slow RPM drop during real-world riding, adjusting the carburetor synchronization can be one solution.

Adjusting one cylinder will affect the others as well.

This is about observing the movement of the vacuum gauge's negative pressure. When adjusting synchronization, touching one cylinder affects the others as well. For example, on the CBX1000, adjusting cylinder 1 has a strong influence on cylinder 6. Overall, the RPM can fluctuate, rising or falling. Factors like piston positioning and the engine temperature during the work also have an impact.

Benefits of Achieving Perfect Synchronization with FCRs

Perfect synchronization is defined as achieving a level of precision that satisfies your own standards, where you feel you can't do any better. The benefit of achieving perfect synchronization is the personal satisfaction it brings. This leads to the topic of this blog post: that a moderate level of synchronization for FCRs is sufficient. As long as the synchronization is not so off that it affects performance while riding, it’s acceptable for it to be imprecise. Ultimately, it’s a matter of personal satisfaction. Outside of the danger zone where the RPM drop becomes sluggish, there are no significant issues at all.

Time Required for Synchronization Work

In the case of my CBX1000, if I'm starting from an unbalanced state, the synchronization work takes about 30 minutes. If I've recently adjusted the synchronization, it usually takes less than 10 minutes.

Is a fan necessary?

I mostly use a fan. If it's just for a short time or for a quick check, I may not use a fan. Idling for a long time won't damage the engine, but it’s not comfortable to idle for extended periods without airflow. If possible, I think it’s better to have a fan on. Additionally, applying heat to engine oil can lead to degradation. Therefore, I recommend using a fan while working whenever possible.

Important Precautions When Synchronizing Using a Multi-Vacuum Gauge

There is a mandatory procedure when using a multi-vacuum gauge: calibrating the needle balance.

When I bought a brand-new multi-vacuum gauge and tested it, the needle balance was off by one graduation.
Performing synchronization without calibrating the needle balance is entirely meaningless.
You can't verify if you're correctly measuring vacuum pressure without calibration.
At the very least, the goal is to balance the vacuum pressure across all cylinders.
As long as the needle balance is correct, you can achieve the purpose of adjusting the synchronization.

I use a syringe-type pump and a vinyl tube to adjust the vacuum balance.
As for the specific adjustment method, you can calibrate by pushing or pulling on the needle's pivot.
The picture shows a six-gauge setup, but I’m using a four-gauge multi-vacuum gauge with two additional gauges added.

For FCR Beginners – Synchronization is Not Important for a FCR.

When writing for beginners, the straightforward advice is to say, "You don't need to worry about synchronization."
A common assumption for beginners is that they don’t have all the necessary tools.
In some cases, motorcycles require modifications to extract vacuum pressure for the first synchronization attempt.
The first challenge beginners face is preparation.

FCR Tuning – To What Extent Should Carburetor Synchronization Be Done?【summary】

Until now, I have been performing FCR synchronization with what I consider to be a perfect setup.
Mr. Bito mentioned that visual alignment is sufficient.
In my case, I tune FCRs as a hobby, so I have all the time in the world.
On the other hand, when I’m pressed for time or feeling physically or mentally drained, I can approach the adjustments more casually, thinking it’s okay to skip some steps.
I have previously written articles about FCR synchronization.
I also include information about the necessary tools, the addition of vacuum extraction nipples, needle calibration, and more.
For those who are about to take on synchronization adjustments, please make good use of the search function on the site.
See you again!