I am the owner of a Honda motorcycle [CBX1000] who lives in Hiroshima City, Japan. I am posting about FCR carburetor and CBX1000 customization. I also post about things other than cars and motorcycles. We will post information that will make bikers around the world happy.
FCR Tuning – Changing the Straight Diameter of the JN Jet Needle – 2023-05-09
CBX1000 FCR Carburetor Tuning Post
I've been slowly and meticulously working on the tuning since last month. I've dialed in both the idle and the main jet for full throttle. However, I'm concerned about the range affected by the straight diameter of the JN jet needle. This time, I’ll be changing the straight diameter and taking it for a test run.
This is the current situation. I adjusted the idle, which was bothering me, and synchronized the carbs. I decided to change the straight diameter of the jet needle before adjusting the clip position.
As for the clip position, I’d like to keep at least one step in reserve as we head into summer. If the clip steps aren’t enough, I may need to either retune with the 90FXM or choose an even leaner straight diameter, such as the 90FTT. Additionally, I’m somewhat concerned about the acceleration pump. I need to carefully consider whether to reduce the total output or to overhaul it entirely.
That’s all for now. I'll share my detailed thoughts and impressions in the next post. Let's get started!
Here’s my feedback after the test ride.
I'll first discuss the changes made to the jet needle from the previous adjustments up to now.
90FTN #1 ↓ 90FTR #2 ↓ 90FTS #2
The above is the sequence of changes. The clip position has been adjusted from #1 to #2 to account for the effects of the change in the straight diameter. Recently, I've developed a preference for tuning in the rich direction. It seems that when I shift from a too rich and clogged feeling towards leaner settings, it results in an optimal rich state. A slightly rich mixture seems to give a stronger sense of power during acceleration.
Lately, I've felt that the throttle opening is a bit too aggressive. I used to have a 90-degree throttle with BITO's FCR kit, but now it's set to 56 degrees. It's enjoyable because I can fully open the throttle in one go. For those considering installing a quick-turn throttle, I recommend not choosing an excessively large cam. In my case, I introduced the quick-turn throttle after I had gained some experience with tuning on my own. If you start with a strong quick-turn throttle, it might make tuning more difficult.
For reference, with a 44mm cam from Active's quick-turn throttle on a 33mm FCR, the throttle opening is about 56 degrees. I hope this information is helpful for those considering a quick-turn throttle.
As for other impressions, I've been tuning the acceleration pump's discharge timing to be slightly delayed, and I plan to gradually advance it to observe how it affects the riding feel.
For FCR Beginners – If Your Tuning is Mostly Set, Focus on JN and AS for Street Use
This is about a situation where the FCR tuning is mostly set. If you feel that the performance is slightly off or if you want to enjoy small changes, you should adjust the JN (Jet Needle) and AS (Air Screw). These are easily accessible and can be adjusted with ease.
Tuning is all about the air-to-fuel ratio. In summer, as the air becomes lighter, you should lean out the fuel mixture. For example, you might change the main jet from 130 to 120. Between spring and summer, you could try leaning the fuel mixture by changing the straight diameter of the JN from 90FTR to 90FTS. You might also consider adjusting the clip position one step towards the leaner side. Regarding the AS, since it increases the amount of air, try opening it slightly more during the summer. This adjustment is likely to result in a more responsive engine.
For street use, these two areas (JN and AS) should be the primary focus of your tuning efforts. The main jet (MJ) is responsible for full throttle operation but also affects the throttle opening around the halfway point. Changing the MJ from 125 to 120, for example, can improve the overall feel.
AS, JN, and MJ are jets with high accessibility. If you feel the need for some minor maintenance or adjustment, go ahead and make the changes. If you're not satisfied with the results, you can always revert to the previous settings.
On the other hand, changing the SJ (Slow Jet) will require more time and effort as it involves re-tuning. You may need to remove parts like the chamber packing, which increases the risk of overflow or fuel leaks. Therefore, preparation is key. Make sure to have spare chamber packings on hand and perform the work in a situation where you can quickly return to the pit if needed.
When tuning the FCR, it's crucial to compare the current setup with the previous one. If the current setup is worse than the previous one, it proves that the previous tuning was more accurate. Always record your current tuning settings before making any adjustments. I manage this using Google Spreadsheets on my smartphone.
FCR Tuning – Changing the Straight Diameter of the JN Jet Needle – 2023-05-09【summary】
One of my recent themes is to avoid changing the SJ (Slow Jet) too quickly. A single JN (Jet Needle) typically costs around 700 yen. Since the CBX1000 requires six of them, it will cost about 4200 yen in total. So, I don't want to make any mistakes when purchasing them.
Many experienced tuners have accumulated a wealth of information about FCR tuning. If you find someone knowledgeable on social media, they might be kind enough to offer advice. It could be a good idea to consult with others about the direction of your tuning. Changing the JN is one of the crucial aspects of FCR tuning.
Here are the rules I follow when tuning: ・The SJ should provide good engine starting performance, and the AS (Air Screw) return position should fall within the proper range. ・Once set, there's no need for drastic changes. ・The straight diameter of the JN can change the air-fuel ratio by about 0.5 to 1.0. ・Changing the MJ (Main Jet) by around 10 points typically alters the air-fuel ratio by about 1. ・The MJ can significantly affect the riding feel at throttle openings above 1/2. ・Even if you double the size of the jet, the air-fuel ratio doesn’t change by double.
Sometimes, what seems like a minor adjustment can lead to significant changes. At times, it can be difficult to judge the effect at the targeted throttle opening. This blog post focused on FCR tuning with an emphasis on changing the straight diameter.
See you again!
CBX1000 – Photo Gallery
often repeat the same tasks, so there are many similar photos. However, I believe there may be moments in my blog posts that could be useful to readers. I usually include around 10 photos in each post.
If you have any questions about maintenance or customization, not just FCR, feel free to reach out via email. My hobbies are FCR carburetors and motorcycle maintenance, and I run this blog with the hope that it will help promote motorcycling.
If you have any questions, don't hesitate to contact me.
