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 Slow Jets, Adjusting Air Screw and Pilot Screw – 2023-11-09
It’s important to make adjustments to the FCR as soon as you notice something off. I’m considering adjusting the throttle low opening screw range and the air-fuel ratio at wide open throttle to be about 1.0 richer. I changed the Slow Jets (SJ) and Main Jets (MJ). This year, I decided not to change the SJ.
48 → 45 → 48 → 45 → 48 However, I have been adjusting them repeatedly in a short period. Currently, my CBX1000 has a special jet number of 90FXM#2 for the JN. Since I can’t choose the straight diameter, I need to adjust using MJ or SJ. If I align it with standard stock sizes, I’m considering using a thinner straight diameter around 90FTS and making the SJ larger, like 52, based on past records. When I switched from 90FTS#1 to 90FTM#1, the performance became very poor.
I consider the clip position to be a separate matter for each straight diameter and tune accordingly. Here’s my current tuning:
For summer: Clip Position - #1 MJ - 110 I think this setup will suffice without adjusting the SJ since the screw adjustment range is adequate. I felt that I would have to make significant screw adjustments between summer and winter, but it seems that changing the MJ and clip position will resolve seasonal variations.
For winter: Clip Position - #2 MJ - 120
That's about it. It's hard to say we’re in winter yet. I will write about the tuning trials as well. So let’s get started!
Selecting the Main Jet After Riding on the Toll Expressway
I recently rode on the Toll Expressway. It seemed that the air-fuel ratio at wide open throttle didn’t quite reach 14.0. Since the air-fuel ratio during full-throttle runs is hard to read, it’s difficult to determine if it’s completely accurate. However, it felt like I had good acceleration even with the current setup. I’d like to richen the mixture by about 1.0 and re-check, so I’m changing the main jet from 110 to 120.
It’s generally not a good idea to change jets all at once, but for the slow jet (SJ), it’s just a matter of adjusting the screw position. I'm not experimenting extensively with tuning this time, so I'm making this change as an extra measure. I believe the main jet (MJ) affects the overall performance, not just at wide open throttle. I haven't had a chance to test it on the Toll Expressway after the change yet, but it feels decent on the back roads.
I have changed the slow jet (SJ) multiple times.
I found that the slow jet (SJ) size of 48 worked well during the summer. Although I was constantly changing the jet needle (JN), the air screw (AS) was generally around 1.30 minutes return. Ideally, I would only adjust the screw year-round, so I tried setting it to 45, anticipating that I would tighten the AS to about 1.00 during the winter. However, I didn’t feel it gave a good riding experience, so I ended up in a situation where I continuously changed between the 45 and 48 jets until I was satisfied.
Currently, I’m focusing on the combination of SJ-48 and JN-90FXM, adjusting the clip position, main jet (MJ), and the screw to hopefully make it work for 2024. My riding style and tuning preferences might have changed from last year to this year. I want to maintain a stance of changing the SJ as little as possible.
Redo the synchronization.
When changing the slow jet (SJ), it involves removing and reattaching the carburetor, so I redo the synchronization. I also redo the synchronization when I change the tension of the throttle wire, adjust the idle stop screw, or excessively move the screws. Even with the same amount of adjustment across all cylinders, the negative pressure can change, affecting the synchronization. That said, I might just be being overly cautious. Some tuners say that visual inspection is sufficient.
コンプリートCBX1000を見てチューニングに対する感覚が変化した
Previously, I found it unclear what aspects of my tuning were performing well. I thought I was achieving the best possible tuning in that moment. However, I couldn't verify whether that was truly producing power effectively. I started to wonder if I could refine my tuning a bit more.
By test riding a complete machine, I began to get a sense of what kind of power should be achievable. It allowed me to gauge, even if just a little, whether my tuning was on the right track. There’s no end to motorcycle tuning. If you don’t establish a certain upper limit and workable settings, you can easily lose track of time.
It’s not wise to hastily set a ceiling for your tuning just because you rode a complete machine for a short while. The experience of having ridden a complete machine can significantly change your perception.
I encourage everyone to take the opportunity to check the tuning of the same motorcycle if possible. You are sure to discover something new.
For FCR Beginners – Get into the habit of making adjustments within reach and take notes.
As a beginner with FCRs, adjusting the carburetor screws can be intimidating. There are several reasons for this:
①. Not having the necessary tools. ②. The fear of not being able to revert the adjustments. These two points are the most common concerns.
Regarding point ①, you can only gradually gather the required tools. You’ll need to research which tools are necessary for the adjustments.
The real issue lies with point ②. When it comes to resetting the screws, being off by about five minutes is generally acceptable, and having them all set to the same position isn’t usually a problem. The real concern arises when disassembling the carburetor, as it can damage gaskets and O-rings.
When you touch it for the first time, be especially cautious about damaging gaskets, O-rings, or fuel hoses. It’s advisable to either prepare these parts in advance or make adjustments in a safe environment where you can leave the motorcycle immobile for a few days.
In terms of tuning, it’s almost impossible to reach a point of no return. The main causes of trouble are often forgetting to tighten bolts, incorrect assembly of gaskets, or contamination from dirt.
Please ensure you research the areas you plan to adjust and confirm what tools will be needed. Items that are not commonly sold and have a high chance of damage include chamber gaskets and the clips for the jet needle (JN).
FCR Tuning – Changing Slow Jets, Adjusting Air Screw and Pilot Screw – 2023-11-09【summary】
Changing the slow jet (SJ) alters the appropriate positions of the air screw (AS) and pilot screw (PS). As the amount of fuel sent by the SJ changes, the correct positions for the AS and PS also need to be adjusted accordingly.
At idle, including when fully closed, the low throttle opening is tuned through a combination of various jets. If you change one component, it will affect the low throttle range, but adjusting another component can bring it back to a state similar to before. While you can make the throttle opening at 1/8 appear the same as before changing the jets, you can’t achieve a completely identical state when considering other openings like 1/16 or 1/4; there will be some differences in feeling.
If you look at the FCR tuning manual, you’ll see that there are no tuning points with the exact same influence range.
So, this was an account of the many changes made to the SJ. As winter approaches, it will soon become a tough season for riding motorcycles. The end of riding for 2023 is also approaching.
Please be cautious of accidents and machine troubles, everyone.
