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.
This post is a continuation of my previous blog where I talked about test riding after tuning the FCR carburetors on my CBX1000.
I've decided to lean out the mixture by changing the MJ (Main Jet). 125 ↓ 120
In my previous post, I mentioned that I was unsure about which JN (Jet Needle) to choose, but after giving it more thought, I realized there's no rush to change the JN right away. So, I decided to start by only changing the MJ. Ideally, it's not recommended to change multiple components at the same time, so I'll be making changes one by one.
However, there are times when I do make simultaneous changes based on past experience. As for the JN, changing it is relatively straightforward, with little hassle. On the other hand, when changing the MJ, you always need to drain the fuel first, so it's easier to decide on the MJ first and then take your time choosing the JN.
According to last year’s records, the MJ of 120 seems to be better for this time of year. From how the bike felt during the ride, 120 also seemed to work better. Whether it's because I replaced the Vacuum Release Plate or due to the season, the current setup feels slightly rich.
After changing the MJ, I'll go for a test ride before making any adjustments to the JN.
Alright then, let's get started!
Let's Startin'!
Does changing the vacuum release plate cause the mixture to become slightly rich?
As mentioned, I have replaced both the vacuum release plate and the lip seals. It is possible that the mixture has become slightly rich due to increased airtightness or similar reasons, but the specific details are unclear. Regardless, it's plausible that changing the repair parts has caused a change in the fuel mixture. Last time, the tuning was done around the end of autumn. It is also normal for the mixture to be slightly rich due to seasonal changes.
The only change is to the MJ. I have already completed a test ride. My impression is that further comparison might be needed. The performance is good, but there may be room for improvement. According to the tuning manual, the MJ mainly affects the full throttle range, but it also has some influence around half throttle. It seems that the symptoms of a slight richness around half throttle, which felt somewhat clogged, have eased up a bit.
In any case, I think it’s necessary to adjust the JN's straight diameter to slightly lean and then experiment with various clip positions.
My personal view on the change in air-fuel ratio at low throttle openings due to the main jet size
My Personal View on the Change in Air-Fuel Ratio at Low Throttle Openings Due to Main Jet Size
For example: ・At full throttle, a partial air-fuel ratio of 12.0 corresponds to an MJ of 120. ・At 13.0, it corresponds to an MJ of 130.
In reality, there are differences in acceleration and partial throttle at full throttle, and since acceleration changes continuously, it's difficult to account for precise changes. However, roughly speaking, if you want to adjust the air-fuel ratio by 1.0, I change the MJ size by 10 units.
You can feel a significant impact on acceleration around throttle openings beyond 1/2. If you change the jet size too drastically, it might result in unintended changes, or it might be interesting to observe the effects. Even if the impact seems minor, it feels like it affects the overall performance.
Without rigorous testing, it's hard to make definitive claims, and since I’m not conducting formal research, I keep it to the level of personal observation for enjoying riding my motorcycle. The tuning manuals suggest that such changes might not have a significant impact.
If you previously completed tuning and notice odd behavior, try changing the MJ size by 5 units and perform a test ride. For fine adjustments, changing the jet size by just one unit should suffice. For large motorcycles primarily used on the street, achieving full throttle around 3/4 open can be dangerous on public roads, so testing might not be feasible.
Usage differs between street and racing applications. Whether what I’m writing is correct is uncertain. What I’ve written is intended for street use. Opportunities for full-throttle riding on highways with large motorcycles are quite rare.
Beginner’s Guide to FCR – Minimize Changes to the SJ (Slow Jet)
Until about a year ago, I felt that tuning required a lot of effort to be worthwhile. As mentioned, I now believe that it's best not to change the SJ (Slow Jet) frequently. Although changing the SJ can significantly improve acceleration and alter the riding feel, once you find an SJ that works well, it's better to stick with it and fine-tune other aspects.
The reason is that changing the SJ involves a high time cost, and there may be alternative adjustments with other jets. Tuning other jets generally involves less effort. By using the SJ as a baseline and focusing on tuning other jets, you can increase the number of tests and refinements.
If possible, fine-tuning with the JN (Jet Needle) and AS (Air Screw) is preferred. When making major tuning changes, referring to past records usually results in fewer issues. Frequent changes to the SJ should be limited to the initial year of tuning or at the start of each season to maximize your riding enjoyment.
FCR Tuning – Main Jet Change – 2023-04-14【summary】
When changing the main jet (MJ), you need to drain the gasoline from the chamber. This is a necessity due to the physical design. Frequent changes might be challenging due to this requirement, although the actual process is relatively simple apart from draining the gasoline.
Considering the design of the FCR carburetor, the MJ is designed in a way that the developers do not explicitly recommend frequent changes. For this tuning session, I only changed the MJ. My impression is that I initially expected improvements with the changes, but I now feel that the original setup might have been better.
Currently, I am considering whether to change the JN (Jet Needle). I will enjoy tuning as time permits.
I wish everyone a fantastic FCR carburetor experience. See you again!
CBX1000 Photo Gallery
Here are some simple photos of changing the main jet on the FCR of the CBX1000. The FCR is removed, and I'm draining the gasoline into a lightweight cup while removing the drain bolt.
Removing the CBX1000-FCRCBX1000 – Main Jet ChangeFCR – JetsCBX1000 – FCR RemovalHONDA CBX1000 – FCR tuningHONDA CBX1000CBX1000 – Main Jet Change
