FCR Carburetor Tuning – Slightly rich or too rich. It’s best to keep it at a slightly rich level.

FCR Carburetor Tuning – Slightly rich or too rich. It’s best to keep it at a slightly rich level.

Tuning the FCR Carburetor on the CBX1000:

My recent trend has been to tune it to a slightly rich setting. The target air-fuel ratio (AFR) is 12.5. Considering the symptom of leaning out when the throttle is opened, I'm aiming for around 12.0. However, precise adjustment isn't always possible.

The ideal state is when the AFR stays within the 12.0 range, and the accelerator pump is doing its job well. When the mixture is too rich, it feels like the engine is bogging down and misfiring, making the motorcycle almost unrideable. If the bike emits black smoke, that's a clear sign of being way too rich.

When the AFR is too rich, it's easy to notice as the bike struggles when accelerating. On the other hand, symptoms of a lean mixture can be harder to detect because the engine revs up but lacks power. Sometimes, when you pull the clutch or shift gears, the bike gains a bit of power, leading to the impression that it's just "okay." This preconception can make lean conditions harder to identify.

Conversely, it's easier to recognize lean conditions during acceleration from a stop, while slightly rich settings are harder to detect. If the AFR is too lean, the bike will have trouble getting off the line properly.

There are certain factors that make low-throttle tuning more challenging: idle ports, slow ports, pilot screws, air screws, straight diameter, synchronization, and throttle cable clearance. All these elements are influenced by slight throttle openings.

Achieving the best possible tuning takes time. However, if the engine starts well and there are no issues with acceleration, there's no need to be overly concerned.

The accelerator pump might be a potential pitfall…

The accelerator pump can have its discharge timing and total discharge (duration) adjusted. When the throttle is opened abruptly, the FCR carburetor may cause the engine to suddenly stumble, and this can be related to the fuel injection from the accelerator pump.

When you open the throttle, the engine draws in a lot more air. Before installing an air-fuel ratio (AFR) gauge, I thought this was just a characteristic of the FCR carburetor. Sometimes, the engine can stall due to too much air being drawn in. If the mixture suddenly becomes too lean, strange symptoms can occur.

For example, when you open the throttle suddenly and see the AFR drop to 10.0, it may indicate that the mixture has become too rich, possibly because the accelerator pump is delivering too much fuel. To address this, you may need to delay the discharge timing or reduce the total discharge amount. Generally, delaying the discharge timing yields more immediate improvements than reducing the total discharge. Alternatively, you could try turning off the accelerator pump completely and see how the engine behaves.

The FCR carburetor is equipped with an accelerator pump. It seems a bit of a waste to turn it off entirely, but if you adjust it properly, you can surely achieve a happy balance and get the most out of your FCR carburetor.

air-fuel ratio at idle

There is some margin of error in air-fuel ratio (AFR) gauges. I've tried several AFR gauges, including AutoGauge and PLX, but I haven't replaced the sensor. The AFR is influenced by the individual differences in the sensors.

For the AFR at idle, I find that an AFR of around 12.9 is ideal. Even without adjusting it while watching the AFR gauge, I usually get a reading close to this value.

At low RPMs, such as when idling or starting off, lean conditions are easy to notice, but symptoms of being too rich are harder to detect.

For FCR Carburetor Beginners – Keep it at a slightly rich setting throughout the year.

It might not be for beginners, but when it comes to tuning, I've recently come to believe that if you set it to a slightly rich level, just enough so that it doesn't hinder your riding, you'll still get a good sense of acceleration and enjoy your ride. The goal of tuning isn't necessarily to make everything perfect, but rather to slightly lean out the rich areas.

The FCR carburetor makes it hard to notice when the mixture is lean while riding. In Japan, the temperature varies greatly depending on the season. By keeping it slightly rich throughout the year, you can enjoy stress-free riding without needing to make extreme tuning adjustments.

FCR Carburetor Tuning – Slightly rich or too rich. It’s best to keep it at a slightly rich level.【summary】

When I first started tuning the FCR carburetor, I thought slightly lean was better. Recently, I’ve come to prefer slightly rich settings for the power feel. The air-fuel ratio I consider slightly rich might actually be the optimal ratio.

I plan to spend a bit more time trying various tunings to find the one that works best. Spring is here, and it’s a great season for riding. This year, I hope to attend events and see various motorcycles.

See you again!

CBX1000 Photo Gallery

I will be featuring photos primarily from past posts showing the synchronization work on the FCR carburetor. If you don't calibrate the needle to zero, it can lead to significant issues. Even at the time of shipment, there's often considerable deviation.