FCR Tuning – Lean main jet – Changed from 122 to 118 – 2022-04-11

FCR Tuning – Lean main jet – Changed from 122 to 118 – 2022-04-11

I replaced the FCR main jet from 122 to 118. I wanted to make this adjustment without removing the carburetor and tank, but I couldn’t reach them, so I only removed carbs 1, 2, and 3 by slightly shifting them.

The other day on the expressway, while accelerating, the mixture was too rich to measure accurately, around 10.6. When opening the throttle slowly, it was about 12.3. Even though 118 is two sizes smaller than 122, it shouldn’t be too lean.

There is a significant difference between being able to use full throttle or not, even if you never use full throttle. When tuning as a hobby, you want to ensure the setup is usable even at maximum throttle. To be honest, it works well up to about 2/3 of the throttle opening. Attempting to open the throttle fully on a public road is risky and should be done in moderation.

I feel like if I tried it on a public road, it could be quite dangerous. Even during my test drive on the expressway, the AS Uotani SP2's limiter reached its limit fairly quickly.

You should always consider the surrounding traffic situation, check for animals, and drive cautiously when testing at full throttle.

MJ replacement work

The main jet (MJ) can be replaced even with the FCR installed by simply removing the drain bolt at the bottom of the carburetor. The part where gasoline is stored is called the float chamber. However, fuel will leak out during this process.

Work efficiency can be improved by attaching and detaching the carburetor. You can also catch the fuel in a cup to minimize spills. The reason for replacing the MJ with the carburetor still installed is to avoid removing the adjusted throttle wire. Although you can remove the carburetor with the throttle wire attached, you want to avoid putting stress on the throttle wire area. However, replacing the MJ without shifting the carburetor is challenging, so it's best to remove the FCR carburetor by shifting it slightly.

Another thing to keep in mind is that if you roughly remove or tilt the carburetor, it may cause overflow. To prevent this, be cautious and do it gently. Carburetor overflow is often caused by debris trapped in the float valve. Tapping the suspected overflow carb or disassembling it to remove any debris usually resolves the issue. Even if you can't see the debris with a visual inspection, disassembling and checking by hand often fixes the problem. Naturally, excessive tilting can increase the risk of picking up dirt and rust.

I’ll try this out tomorrow or over the weekend and see how it goes.

The time required for the replacement is approximately 15 minutes. There aren’t many tools needed, so it feels like a relatively easy task. As a side note, the front fender line is one of my favorite customizations.

FCR Tuning – Lean main jet – Changed from 122 to 118 – 2022-04-11【summary】

I suspect that changing the main jet (MJ) from 122 to 118 might not make a significant difference. I feel that checking the air-fuel ratio meter will show how much change has occurred. However, you won’t know for sure until you try it, so if you're interested, you’ll need to replace it and test it out.

This is a maintenance issue not limited to the CBX1000. When replacing the MJ, it’s important to avoid overtightening. Since the MJ is made of brass, overtightening can strip the threads, which would be very unfortunate. I’ve had a thread stripped once, but since I wasn’t on the go at the time, it wasn’t a big issue. However, it would be quite troublesome to strip the threads without a spare MJ while on the road.

If you replace the MJ, you can recover by using a different size, but the biggest problem is dealing with broken screws while driving with the FCR installed. This can be hard to notice and may take some time to diagnose if something seems off. Please be very careful.

Based on my test runs, there’s a possibility that the accelerator pump tuning might be too high when the throttle is fully open. It might be a good idea to reset the accelerator pump diaphragm to a smaller setting and then try fully opening the throttle. Ideally, I’d like to keep the accelerator pump as high as possible while achieving a satisfactory tuning, but this will depend on actual riding conditions.

I plan to research ways to delay the discharge timing of the accelerator pump and reduce the total discharge amount when the throttle is fully open. In any case, I’ll return it to the rental garage tomorrow and measure the fuel consumption before doing so.

Well then! Have a good FCR life!

See you again!