FCR Tuning – Testing 6 Variations of Accelerator Pump Diaphragm – 2023-08-22

FCR Tuning – Testing 6 Variations of Accelerator Pump Diaphragm – 2023-08-22

One of the thrills of installing and riding with an FCR is the accelerator pump. In this session, I tuned with all the diaphragms I have on hand, all on the same day. The six variations tested are combinations of accelerator pump diaphragms from the [Standard], [KLX250], and [KLX400], with discharge timings of [1.0mm] and [0.5mm].

My CBX1000 has two accelerator pumps since it uses a baseline carburetor setup with two units. Every change requires removing and reinstalling the carburetors, which means I've disassembled them quite a few times. I’ve realized this is not something to do in the peak summer heat of 35°C.

Now, let's proceed with a detailed explanation. Let's get started!

List of Accelerator Pump Modification Patterns

[Discharge Timing - 0.5mm]

Accelerator Pump - Standard (Large)
Accelerator Pump - KLX250 (Medium)
Accelerator Pump - KLX400 (Small)
[Discharge Timing - 1.0mm]

Accelerator Pump - Standard (Large)
Accelerator Pump - KLX250 (Medium)
Accelerator Pump - KLX400 (Small)
I chose these variations to evaluate the feeling during rapid acceleration, specifically from around 30 km/h with a quick throttle opening. This roughly corresponds to a throttle position of 1/8 to 3/4.

※The final tuning results are detailed at the end.

Thoughts on Changing the Accelerator Pump Diaphragm

Here are my impressions for each type of accelerator pump diaphragm:

Standard Diaphragm:
・With the standard diaphragm, the air-fuel ratio gauge consistently shows a reading of 10.0 for several seconds, which is too long.
・This setup seems incompatible with the high throttle on my CBX1000.
・When using the standard throttle, it was rich but less noticeable. I believe the choice of accelerator pump diaphragm is significantly influenced by the rider's style.

KLX250 (Medium):
・This diaphragm provides a good balance, not becoming overly rich and proving to be the best option.
・It stood out as the best choice when compared to the other two diaphragms.

KLX400 (Small):
・The discharge amount is clearly too low, making it feel unusable.
・Despite initial biases, I tested various discharge timings.
・Currently, the optimal setup is using the KLX250 (Medium) diaphragm with a discharge timing of 1.0mm.

Thoughts on Changing Discharge Timing

In this test, it seems that the discharge timing had minimal impact. My main concern was to alleviate the issue of the engine running lean when suddenly opening the throttle due to excessive air intake.

Here are my observations:

・0.5mm Timing: The accelerator pump generally starts to take effect around a throttle opening of 1/8. However, in rapid throttle openings, you quickly pass through 1/8, so this timing might not be as effective in addressing the lean condition during sudden acceleration.

・1.0mm Timing: The accelerator pump begins to work closer to a throttle opening of 1/4. Since in rapid throttle openings, the engine passes through both 1/8 and 1/4, this timing seems more effective in preventing the lean condition.

While the discharge timing might not have had a significant effect in this specific test, it's worth noting that if the timing is too early, the fuel discharged might directly hit the throttle valve, which can cause issues. Therefore, I ultimately set it to 1.0mm based on personal preference. This setting helps avoid overly aggressive fueling at 1/8 throttle opening, which could make the bike difficult to ride smoothly in street conditions.

How to Determine if the Accelerator Pump is Rich Using an Air-Fuel Ratio Gauge

For Those Using an Air-Fuel Ratio Gauge:

The FCR carburetor can sometimes show a reading of 10.0 on the gauge immediately upon rapid throttle opening. It is unclear whether this behavior is characteristic of the FCR carburetor itself or the PLX air-fuel ratio gauge.

When the accelerator pump is heavily engaged, the gauge may indicate 10.0 for an extended period. If the 10.0 reading persists for too long, it suggests that the accelerator pump is too strong and adjustments are necessary.

Here are some tips:
・Adjustment: If the 10.0 reading is prolonged, consider adjusting the accelerator pump to reduce its effect. Alternatively, try easing the throttle opening to manage the mixture more smoothly.
・Testing: When the accelerator pump is turned off, the gauge should not show 10.0 for a long time. This can help you determine whether the issue is related to the jets or the accelerator pump.

Use this information to help distinguish between problems related to the jets or the accelerator pump when testing and tuning.

The Accelerator Pump is Affected by Throttle Opening!

With your current setup, using a 56-degree throttle opening angle on your CBX1000 does seem to make the throttle operation more sensitive compared to the 90-degree throttle. The change to a 56-degree throttle, which is similar to what you’d find on Ducati SS models, makes the throttle response much more immediate and precise. This can indeed make the accelerator pump’s behavior more noticeable, especially during rapid throttle openings.

Given that the 56-degree throttle allows for full throttle with a minimal twist, it’s understandable that you’d experience a more pronounced effect from the accelerator pump. It sounds like you’re finding that the sharper throttle angle demands finer tuning to achieve the desired balance in fuel delivery.

For those considering a quick turn throttle or high-speed throttle setup, it’s worth noting the following:

56 Degrees: This angle provides a very quick throttle response, which can be too aggressive for street riding. It requires careful tuning to prevent overly rich conditions or erratic engine performance.

90 Degrees: While it allows for a more gradual throttle application, it doesn’t achieve full throttle in one twist, which might be less immediate but more manageable.

Fine-tuning the accelerator pump to match the 56-degree throttle’s sensitivity will help improve rideability and performance. Keep refining your setup to balance throttle response and engine behavior, and it’ll make a big difference in your riding experience.

Tuning the accelerator pump is a process with lower precision.

"Low precision" in this context means that the tuning process for the accelerator pump is not designed for high precision. Here’s a more detailed explanation:

Mechanism Limitations: The design of the accelerator pump does not allow for very precise adjustments. The system is inherently less accurate in how it delivers fuel.

Discharge Timing: The timing for fuel discharge should be considered as approximate. The values for discharge timing are not exact and should be treated as general guidelines.

Inconsistent Fuel Delivery: The amount of fuel dispensed by the accelerator pump can vary, with instances where fuel may not be delivered at all, or the flow may differ in quantity and intensity.

System Characteristics: Due to its mechanical design, the accelerator pump is a component with inherently low tuning precision. This means adjustments may not be as fine-tuned or consistent as desired.

In summary, tuning the accelerator pump involves working with a system that lacks high precision, leading to variability in performance and requiring a more flexible approach to adjustments.

Current FCR Tuning for the CBX1000

Current Tuning:
AS (Air Screw): 0.50 minutes return
PS (Pilot Screw): 0.45 minutes return
SJ (Slow Jet): 48
JN (Jet Needle): 90FTS#2
MJ (Main Jet): 120
Accelerator Pump Timing: 1.0mm
Accelerator Pump Diaphragm: KLX250

It seems that the mixture might benefit from being slightly richer around 1/4 throttle. I am considering changing the straight diameter to 90FTR#2 as an option.

For FCR Beginners – Make Sure to Adjust the Accelerator Pump Frequently.

Do Not Leave the Accelerator Pump at Its Initial Setting.

Make adjustments to the discharge timing and accelerator pump diaphragm, and experiment with methods such as using paper clips to turn off the accelerator pump to find your preferred setup. Even though this guide is for beginners, making these adjustments will not negatively affect the motorcycle’s performance. The changes are easy to make and clearly noticeable.

↓ Past blog posts can be found here ↓
FCR Tuning – An Easy Way to Turn Off the Accelerator Pump

FCR Tuning – Testing 6 Variations of Accelerator Pump Diaphragm – 2023-08-22【summary】

Initially, I had decided that the KLX250 (Medium) accelerator pump diaphragm was the best choice. However, after changing to the Standard (Large) diaphragm to test variations, I found that it worsened the performance. This is likely due to the compatibility with the current tuning setup. Consequently, I tested all three diaphragm options, including the KLX400 (Small), and found that the KLX250 (Medium) diaphragm works best with the current tuning.

If I hadn't tested the KLX400 (Small), I wouldn't have realized that the KLX250 (Medium) was the optimal choice for the current setup. This reaffirms the importance of keeping detailed records and conducting A/B tests in FCR tuning. This discussion focused on the accelerator pump diaphragm.

See you again!

Reference Photos

Here’s a rough measurement of the accelerator pump diaphragms:
・Standard: Approximately 4.9mm
・KLX250: Approximately 8.65mm
・KLX400: Approximately 9.65mm