FCR Tuning – Carburetor Synchronization – Is the Balance of Your Vacuum Gauge Correct?

FCR Tuning – Carburetor Synchronization – Is the Balance of Your Vacuum Gauge Correct?

I'm going to write a column about the needle's zero calibration on vacuum gauges. To put it simply, it's better to assume that the zero position of the needles on four-bank vacuum gauges is generally miscalibrated.

↓ I've written in detail about this in a past article ↓
You'll be happier if you read the following first.
https://cbx-lalala.com/archives/4473

I use two sets of four-bank vacuum gauges combined together. When I bought the second set, I checked them. Even when new, the zero calibration of the needles was off. You should not assume that a brand-new vacuum gauge is accurate. The above refers to multi-bank vacuum gauges. Even when applying the same vacuum, the needles on the four-bank gauges just won't align. By the way, it's unclear whether they correctly indicate -20 kPa when it's supposed to be -20 kPa.

In carburetor tuning, what's important is that the vacuum levels are consistent, so the balance between each gauge is the most crucial factor. I've written posts on this topic several times before. It's a critical point worth repeating. So, let's dive into the details. Let's startin'!

unfortunate case

Recently in Japan, Japanese Classic Motorcycles have gained popularity. More and more owners are now riding motorcycles with carburetors. Many of them are also attempting to synchronize their carburetors themselves. I have performed carburetor synchronization quite a few times. The CBX1000 has a six-cylinder engine, and it's only natural to want to carry out synchronization properly.

I purchased a vacuum gauge and adjusted the synchronization myself several times. However, no matter how many times I tried, something always felt off. When I switched the gauges between the 1st and 6th cylinders, the readings were incorrect. The balance of the vacuum gauges was not aligned.

This imbalance in the vacuum gauges can lead to a major issue: the person performing the synchronization might not notice and end up with a poorly synchronized carburetor. The operator might think they've achieved perfect synchronization, but in reality, the carburetors are not balanced properly. If the vacuum gauges are not balanced correctly, it's impossible to achieve a perfect adjustment.

In my case, the synchronization appeared perfect based on the readings. Fortunately, because it was a CBX1000, I noticed the issue early on. The reason I caught the problem was that I was using a four-bank vacuum gauge and switched the gauges during the adjustment process.

For motorcycle use, most multi-bank vacuum gauges are four-bank models. The likelihood of noticing the issue with a brand-new gauge is quite low. It's something you'd only realize if someone told you about it. As humans, we tend to have preconceived notions, and we generally don't expect a new multi-bank vacuum gauge to be miscalibrated.

It's rare to come across a motorcycle with perfectly synchronized carburetors. If the synchronization is extremely off, you can usually tell that something isn't right. When the synchronization is correct, the motorcycle has a mysterious pulsating feeling. If the balance is right, you'll feel a distinct rhythmic beat.

For FCR Beginners – Does Synchronization Work Need to Be Perfect?

It depends on how you look at it, but the goal of carburetor synchronization is perfection. The minimum standard is that it should not interfere with riding. If the synchronization will eventually go out of balance and you don’t have the tools for fine adjustments, then roughly adjust it to the point where it doesn't hinder riding. The top priority is riding the motorcycle.

A situation that interferes with riding is when the engine fails to idle down properly. This is when the engine speed doesn't drop as expected when you release the throttle. This happens when the slide valve of one of the cylinders is significantly open. You can make adequate adjustments visually or with a taper gauge, so roughly lower the throttle valve of the affected cylinder.

FCR Tuning – Carburetor Synchronization – Is the Balance of Your Vacuum Gauge Correct?【summary】

When you ask a motorcycle shop to perform carburetor synchronization, inquire about how they carry out the synchronization. If they adjust it using only one meter, they may not be achieving proper synchronization.

If they use a four-bank multi vacuum gauge, ask about the calibration of the zero position of the needles. If they respond that they haven't adjusted it, be cautious. This could lead to incomplete synchronization.

This was about the calibration of the zero position of the needles on a multi-bank vacuum gauge. If you're planning to challenge carburetor synchronization, please pay close attention to this.

See you again.

CBX1000 – Photo Gallery

I will be posting photos related to vacuum gauges. My vacuum gauge consists of two four-bank units that I disassembled and combined into a six-bank gauge.