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 – I bought an acceleration pump diaphragm so I’ll compare the sizes
I bought an acceleration pump diaphragm for the KX400 to tune the FCR of the CBX1000. As of November 2022, the price should be 1,892 yen for two pieces. I don't remember exactly because I bought them together from another source. I should mention in advance that I only purchased them, and did not compare them after installation. Please use this as a reference before purchasing the Kawasaki FCR diaphragm. The offset sizes of the acceleration pumps I own are: ・FCR standard 0mm ・4mm for KX400 ・3mm for KX250 About. The total discharge volume of the acceleration pump changes depending on the offset height. This time, I will write a column-like account of what I have learned about acceleration pumps. [Part number] @ Replacing the diaphragm ・FCR standard (large) 1201-055-2102 (keihin) ・Medium for KX250 43028-0001 (kawasaki) ・Small for KX400 43028-0006 (kawasaki)
It may be better to think of the acceleration pump discharge volume in terms of time.
As the offset of the accelerator pump increases, the accelerator pump diaphragm stops moving, and the amount of fuel discharged decreases. It should be easy to imagine up to this point. Next, instead of thinking of the amount increasing or decreasing, you can also think of it as the discharge time. The size of the accelerator pump's nozzle does not change, so the discharge time changes, and it appears that fuel is being added. You can see this by actually twisting the throttle. It's as you see it. The way fuel is sucked in is different when the engine is running and when the engine is stopped. You can see it by actually looking at it. The total amount discharged is also important. The timing at which the discharge starts is also important. It seems that the fuel is not being discharged according to the throttle opening as written in the FCR tuning manual. This is a problem with the accelerator pump mechanism, so you need to check it visually. The straight diameter and other things are manufactured with an accuracy of 0.01 mm. In contrast, I think that the adjustment of the accelerator pump is a mechanism with low precision.
Impressions after comparing accelerator pump diaphragms
We compared the acceleration pump diaphragms of the KX250 and KX400. The offset size only differs by about 1mm, but it feels like the diaphragms are quite different. A difference of 1mm in an industrial product is significant. In fact, changing the straight diameter of the jet needle by just one size changes the air-fuel ratio by nearly 1.0. In terms of clip stages, one clip stage is a difference of about 1mm. The riding feeling changes so much that it feels like a completely different thing. It feels like there are also fairly big changes in the acceleration pump diaphragm, so we can expect good results.
Tuning the accelerator pump
In my opinion, I don't think an accelerator pump is necessary in the first place. The acceleration pump can make the engine run slower. In fact, I think it's fine to turn the accelerator pump off during tuning. When tuning is complete and the engine is slow when the throttle is opened suddenly or during acceleration, I think it's a good idea to tune the accelerator pump's discharge timing as a seasoning. It's normal to have an accelerator pump, so you can start tuning without touching it at the initial tunings. The effect of the accelerator pump varies greatly depending on how you open the throttle and whether you use quick throttle or low throttle. If you use quick throttle and your throttle is rough, it's better to set the accelerator pump's discharge timing very strictly. If the throttle is fully open at about 90 degrees and the throttle is low, the ride will be mild even if the total discharge volume is large. In short, I think the flavor of the accelerator pump changes greatly depending on the rider's operation. Rather than saying that the total discharge volume of the accelerator pump is large or small depending on the motorcycle, the accelerator pump should be tuned according to the rider's preference and riding style. For riders who gently open the throttle all the way, a large accelerator pump discharge volume is fine. For riders who open the throttle roughly, a smaller total discharge volume is easier to ride. I think of adjusting the total discharge volume by replacing the accelerator pump diaphragm as a matter of time. This means how many seconds the fuel is discharged. I like the feeling of acceleration when I open the throttle roughly, so I would like the accelerator pump total discharge volume to be small. That's my current preference. My preferences may change in a year.
Be careful not to discharge the acceleration pump too early
As I wrote in another article, if the acceleration pump discharge timing is too early, the fuel will hit the throttle valve directly. It is unclear whether that is a good thing or a bad thing. It is a part that gets hot and will atomize quickly, so it may not be an issue. If you actually look at the fuel injection port of the FCR acceleration pump with your own eyes, you will see that the fuel hits it.
For FCR beginners – See how the accelerator pump works
Remove the Velocity Stack if possible. Then visually observe the operation of the accelerator pump. With the engine running, try racing (idling) to see how it is sucked in. Check how it is expelled when you open the engine slowly. See at what throttle opening it is expelled and for how many seconds. You don't need to take notes or make a video, just see how it is moving. With a little effort, try using a paperclip to turn off the accelerator pump. Check the above and tune it, or even if you don't tune it, it should change your awareness of how to ride a motorcycle, even if just a little.
FCR tuning – I bought an acceleration pump diaphragm so I’ll compare the sizes【summary】
As mentioned above, when deciding on an accelerator pump, it may be better to decide based on whether it suits your riding style and your own feeling rather than a comparative test. Throttle opening and riding style are quite different for each individual. Also, if you have an air-fuel ratio meter, the accelerator pump is working too much at the moment when the air-fuel ratio becomes extremely rich and bumpy. In such cases, turning off the accelerator pump and test driving can make the motorcycle run much better. My current tuning of the CBX1000 is set so that the accelerator pump starts working at 3/8 throttle opening and a rich jet is used at low throttle opening. The air-fuel ratio meter shows extremely rich or lean for a moment, but it gives a satisfactory acceleration feeling. I think that the accelerator pump is only necessary when the engine becomes lean when it is opened suddenly. Tuning with an accelerator pump should be quite different from tuning without one. The FCR has a mechanism called an accelerator pump. I would like to deepen my knowledge so that I can make effective use of the functions that can be used. I'm sure you've all had trouble tuning your accelerator pump, but I hope you'll try out different things and find the tuning that satisfies you. See you again!
*A photo of the accelerator pump is posted at the bottom.
CBX1000 Photo Gallery
The accelerator pump diaphragm has an image with text and an explanation. Please check it out for reference.
