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 – To what extent does the main jet affect the throttle opening range? – 2023-09-19
This is a discussion about changing the main jet. The focus is on how much the main jet change affected throttle positions other than full throttle, based on feel. Recently, I had concerns around the half-throttle opening range. So, I changed the jet needle from:
90FXM#2 → 90FXM#1
I felt it had improved. As the next step, I thought it might be better to make the overall mixture leaner. So, I changed the main jet and went for a test run. The goal wasn’t to achieve a higher top speed, but rather to see if the changes improved the feel of the bike.
In terms of what I did: I tested in this order: MJ-120 → MJ-115 → MJ-110
The result was that I settled on MJ-110. I swapped to MJ-115, but wanted it to be a little leaner, so let's proceed with the assumption of using MJ-110. Since I quickly changed from MJ-115 to MJ-110 after the test run, I don’t have much detailed feedback on MJ-115. My thoughts on MJ-115 were simply: "Let’s try MJ-110."
How does changing the main jet affect throttle openings up to 3/4?
Let’s proceed on the assumption of using MJ-110. According to the tuning manual, there shouldn’t be any impact, but I feel the engine starts slightly better. This is when the engine hasn’t fully warmed up, so it might just be a placebo effect. Even at around half-throttle, the bike responds smoothly without hesitation, and it revs up lightly. There’s a noticeable change in the feel. I’m being a bit rough when it comes to assessing throttle positions, but from half-throttle to about three-quarters throttle, there’s a clear change in the feeling. The tuning manual suggests there should be little impact, but when I switched from MJ-120 to MJ-110, the difference was significant. For reference, changing the main jet by a value of 5 doesn’t even cause the air-fuel ratio to change by 0.5 on a gauge. This is just my subjective impression within what seems to be the optimal range. If the MJ was far outside the optimal range, the numbers might not change as much. It seems necessary to consider that the influence of the MJ may be greater than what’s stated in the tuning manual.
Since overhauling the FCR, this setup feels the best so far. Tuning is a process of choosing the better option as you compare, so if you're properly working on the tuning, the result will always be better or at least as good as the previous setup. After changing the main jet, it seems like the top-end performance improved slightly. However, there is still an issue. At around half-throttle, it feels like there’s a bit more power to be gained. If I shift gears properly, there’s no problem, but when shifting roughly, it feels slightly lacking. So, for the next step, I plan to drop the clip position on the jet needle by one notch and do another test run:
90FXM#1 → 90FXM#2
It might seem like a step backward after changing the main jet, but since it’s on my mind, I want to check it out just to be sure.
For FCR Beginners – Preparations and Considerations for Changing the Main Jet
Preparations for Changing the Main Jet for FCR Beginners
What You'll Need:
・Rags (to catch gasoline spills) ・Gasket for the drain bolt (in case of rare fuel leaks) ・A measuring cup or similar container to catch gasoline, if possible
You should have a good selection of main jet sizes ready, and you can thin out by increments of 5 from a richer starting point. While you can buy individual KEIHIN jets, there are also aftermarket sets available. Personally, I’m not too concerned about using aftermarket products since they’re made with modern technology. Just ensure that if you need four of size 125, they should all come from the same supplier.
When changing the main jet, always visually check for any blockages in the jet hole. Make sure to verify the size of all main jets, as sometimes different sizes can mix in. The main jet is made of brass, and as you get more comfortable, you might accidentally overtighten and strip the threads. I recommend using just your index and thumb to avoid applying too much force.
Testing Method: Even if you can't fully open the throttle, you can judge the performance based on a throttle opening of about 3/4. If it feels good at 3/4, it’s usually fine at full throttle as well. Most riders don’t reach full throttle in normal riding conditions, so if there’s a slight discrepancy, checking at 3/4 will not lead to a major misjudgment at full throttle.
The main jet is easily accessible and doesn’t take much effort to change, but you will need to drain some fuel. It’s a rewarding tuning process, with a low technical barrier. I highly encourage beginners to try this tuning. You can experience significant changes without needing to adjust other tuning components like the air screw or jet needle. Even if things don’t go as planned, you can quickly revert back to the original setup.
FCR Tuning – To what extent does the main jet affect the throttle opening range? – 2023-09-19【summary】
I believe that changing the main jet has an impact on the overall tuning. My CBX1000 is equipped with an air-fuel ratio gauge, and I feel there are changes that encompass the entire system, not just the effects based on throttle positions as described in the tuning manual. The jet needle significantly alters the feel of the motorcycle. As for the pilot screw, there may not be as noticeable changes as suggested in the tuning manual.
In Japan, the time available for riding motorcycles has been decreasing in 2023. I find myself reflecting on how, since overhauling my FCR in the spring, I’ve struggled to finalize the tuning as I write this blog. For those gentlemen and ladies who enjoy tuning their FCRs as a hobby, please take care during your test runs.
