FCR Tuning – Adjusting Accelerator Pump Timing and Air Screw – 2023-06-05

FCR Tuning – Adjusting Accelerator Pump Timing and Air Screw – 2023-06-05

I was concerned about the throttle opening being less than 1/4, so I made some adjustments. It seemed that the amount of opening for the AS (air screw) was insufficient. Adjusting it towards the leaner direction improved things. I experimented with the accelerator pump, turning it off, setting it to 0.6mm, and then to 1.0mm, and also changed the discharge timing. There wasn’t a notable improvement or deterioration. I settled on 1.0mm, which felt the best. If the discharge timing is set too fast, fuel hits the slide valve. I avoid making it extremely fast to prevent that issue.

[Previous Tuning]
AS - 1.15 minuts return
PS - 0.30 minuts return
SJ - 48
JN - 90FTR #2
MJ - 120
Accelerator Pump - 1.4mm
Accelerator Pump - KX250F

[Current Tuning]
AS - 1.25 minuts return
PS - 0.30 minuts return
SJ - 48
JN - 90FTR #2
MJ - 120
Accelerator Pump - 1.0mm
Accelerator Pump - KX250F

I might not have learned anything new from this tuning.

Every time I did tuning before, I would notice something or feel that it had improved compared to the previous time. This time, I felt that there was nothing new to learn. Although there was some improvement over the previous tuning, the changes seemed minimal despite the effort and time spent. It appears that the tuning before the changes was already in a very good condition. As a result, I realized that the tuning before the changes was indeed performing well.

Next Tuning Plan

I’m considering changing the accelerator pump from the current small size to the standard size to adjust the total fuel discharge volume. When the throttle is rapidly opened, the air-fuel ratio shows a significantly lean value. Since the change only affects the duration of fuel discharge, there might not be a noticeable difference.

There was no significant difference with the current size and with the accelerator pump off, so this time I’ll aim for a higher total volume. Depending on the overall tuning, an over-effective accelerator pump can sometimes cause an unpleasant sensation of clogging.

Last year, I tuned the accelerator pump to be quite slow, enriched the mixture around 1/8 throttle opening, and increased the size of the SJ. Currently, I’m concerned about how the engine responds when the throttle is rapidly opened. It sucks in air very abruptly. After showing lean symptoms, the air-fuel ratio settles down.

It’s unclear whether it will improve or worsen compared to the current setup. I’ll visualize the adjustments and their potential effects, and tackle the tuning over the weekend.

Switching the accelerator pump on and off

Adjusting the timing of the accelerator pump discharge alone doesn’t fully turn it on or off. During tuning, the presence or absence of the accelerator pump can make a significant difference. There are also simple methods to turn it off.

I’ll guide you through my method for adjusting the on and off settings of the accelerator pump in the link below. During tuning, even partial positive changes can make it feel like progress is being made. Turning the accelerator pump off or, conversely, increasing the total discharge volume can lead to significant changes. Making extreme adjustments can help in assessing the changes more easily. It’s best not to adjust it too finely during detailed tuning. However, when doing rough tuning, turning it off and checking the effects is acceptable.

↓ The past article opens here ↓
FCR Tuning – An Easy Way to Turn Off the Accelerator Pump

For FCR Beginners: What Is the Purpose of the Accelerator Pump?

When the throttle is rapidly opened and the carburetor inhales a large amount of air, the air-fuel ratio becomes lean. The accelerator pump is a device designed to physically add fuel to correct this lean condition. It’s not just a substitute for the choke.

The power provided by the accelerator pump when it’s working can be quite addictive. If you adjust the accelerator pump's discharge timing to match the lean conditions when the throttle is rapidly opened and the pump is off, you’ll likely find it very satisfying.

While it’s suggested to make adjustments, the default settings are also fine. For users who want to try tuning the FCR themselves, it’s relatively easy. Beginners can start by adjusting this component as one of their initial tuning tasks.

FCR Tuning – Adjusting Accelerator Pump Timing and Air Screw – 2023-06-05【summary】

I focused on adjusting the accelerator pump. The adjustment plate with a discharge timing of about 1.0mm to 1.2mm seems to be the most effective overall. Carburetor tuning is also affected by the climate. Minor imperfections might not be worth worrying about. If you don’t mind, it’s fine to leave the tuning alone year-round.

With my current tuning setup, it’s sufficient to adjust the screw slightly for winter starting performance. Recently, I’ve been tuning in a way that reduces the frequency of time-consuming disassemblies. I’m focusing on adjusting the pilot screw and air screw.

This weekend, I’ll switch the accelerator pump to the standard size and then adjust the discharge 

CBX1000 – Photo Gallery

Photos of the area around the accelerator pump adjustment plate.