FCR Tuning – Adjusting the Jets Slightly Leaner for Summer

FCR Tuning – Adjusting the Jets Slightly Leaner for Summer

hen it comes to carburetor tuning, keywords like "rich" and "lean" are often used. However, these terms can be quite vague.

For example: "The air-fuel ratio becomes richer in the summer, so change the jets to a leaner setting."

This expression might be a bit difficult for beginners to understand.

Basically, when "rich" or "lean" is used as the subject, it refers to the air-fuel ratio.

Considering 12.5 to 13.0 as the ideal range, 12.0 would be considered rich, while 14.0 would be considered lean. These numbers are based on my current subjective view.

The air-fuel ratio is the ratio of the weight of air to the weight of fuel. For instance, if the ratio is 1:12, it means that 12 grams of fuel are mixed with 1 gram of air, and this mixture is supplied to the engine. In summer, the weight of air per cubic centimeter becomes lighter. Naturally, the air-fuel ratio shifts toward the rich side. Since the amount of fuel supplied by the jet doesn’t change with temperature, this is only natural. While the volume of fuel also changes with temperature, it’s not as significant as the change in air.

This is an important factor to consider when tuning the FCR carburetor. I’ll also introduce a simple method for summer tuning. If your motorcycle is tuned in spring or fall and runs well all year round, it’s likely that it’s tuned slightly rich in the summer. Generally, when the mixture is on the rich side, there are fewer issues with drivability. However, if it’s excessively rich, the bike may not run properly within the range controlled by the jet needle clip position. Even if you notice the smell of fuel during idling or starting, it’s not something you need to worry about too much.

Performing a Quick Tuning for Summer

Move the JN clip position one step towards the lean side. Adjust the air screw (AS) by turning it about 30 minutes (half a turn) towards the lean side. Replace the main jet (MJ) with one that is about two sizes leaner (e.g., from 120 to 115). This kind of tuning on July 1st should not cause any issues.

For the AS, the key is to lean it out just enough so that the bike still starts easily. The AS has a significant impact on throttle openings up to 1/4. Even if the bike starts easily, if it feels lacking in power when revving up, try turning the AS back towards the rich side. An air-fuel ratio closer to 12.5 tends to give a stronger sense of power, particularly above 1/8 throttle opening.

When you open the throttle, the air-fuel ratio temporarily shifts towards the lean side. In some cases, it may be beneficial to pre-tune it to around 11.5. Depending on how you open the throttle, the sense of acceleration should increase.

FCR carburetors are analog devices, so they don't always yield the exact results you expect. However, due to the characteristics of the FCR, even slightly off-tune settings often still allow the engine to rev smoothly.

For FCR Beginners – Is It Okay to Tune in Spring or Summer and Use the Same Settings All Year?

Start by tuning in the spring or fall, then fine-tune the air screw (AS) again in the winter, and you should be able to ride smoothly all year round. In my experience, the AS primarily affects the bike's launch. If the AS is adjusted too far towards the lean side, you might experience a lack of power during takeoff.

If you’re not a fan of carburetor tuning, you don't necessarily have to adjust the screw; the bike might just run a bit less smoothly. However, even a half-turn of the AS could make a noticeable difference and bring you more satisfaction. Personally, I didn’t do any tuning for the first 15 years after purchasing my FCR, even though my bike was stored for long periods. It still ran without any issues.

If you notice a lack of power when starting off in the summer, try turning the AS towards the rich side. If your bike starts and idles well in winter, your tuning is probably good for year-round use. It may become slightly richer in the summer, but this shouldn’t significantly affect the power or performance.

FCR Tuning – Adjusting the Jets Slightly Leaner for Summer【summary】

I haven’t yet found what I consider the perfect condition, so I’m still experimenting with various setups. During the tuning process, I often discover new things or gain fresh insights. Here are a few examples:

・When using a dual throttle cable, there's a slight lag between the cables when opening the throttle.
・When adjusting the AS for each cylinder, a digital tachometer comes in handy.
・When the throttle is suddenly opened, the air-fuel ratio can momentarily read very rich,even with the accelerator pump turned off.
・If the accelerator pump's discharge timing is too fast, the fuel can directly hit the throttle valve.

Tuning is enjoyable because there’s always something new to notice. Keeping an open mind and being receptive to new discoveries can lead to better results. As you go through more tuning sessions and test rides, you’ll start to recognize the symptoms of a too-rich or too-lean mixture. One particularly noticeable change is when you deliberately adjust the JN clip position to an excessively rich setting. The bike won’t run properly, but this can serve as a valuable reference for your own understanding.

This blog post was a seasonal update. Lately, there’s been a lot of rain. Riding a motorcycle is enjoyable until about mid-July. Please be mindful of heatstroke and enjoy your motorcycle life. See you again!

CBX1000 – Photo Gallery

Here are some recent photos of the CBX1000. Since I carry tuning tools in the side bags, I can do some on-the-spot tuning if needed.