FCR-SJ Slow Jet – FCR Carburetor Installation

FCR-SJ Slow Jet – FCR Carburetor Installation

I set up the FCR-SJ slow jet after removing the FCR carburetor. I will post a photo of the FCR once it is installed. Here is the tuning process I followed:

[Tuning]

・SJ (Slow Jet): 48 → 40 → Test run → 48
・Jet Needle: 90FTR #2
・MJ (Main Jet): 118

I changed the slow jet from rich to lean, reducing it from 48 to 40. My aim was to test whether adjusting the pilot screw (PS) of just one cylinder affects the engine speed. By making the SJ lighter, the PS should only move in the direction of richer fuel.

As a result, there were no significant changes observed. The verification method involved setting the PS to PS1.0 on all cylinders and adjusting it from fully closed to 3.0, with increments of about 15 minutes. I tested the SJ with both 40 and 48.

When adjusting the air screw (AS), I refer to the air-fuel ratio (A/F) meter. Generally, I prioritize and adjust the following three points:

1. Starting Ability: Ensure the engine can start when the clutch engages.
2. Starting Condition: Aim for good starting performance even when cold, and maintain idle stability.
3. Air-Fuel Ratio: Ensure the air-fuel ratio is as expected up to 1/8 throttle opening.

Given my current tuning skills, changing the return number for each cylinder individually seems impractical. Although I verified with only one cylinder, testing all three cylinders simultaneously might yield different results. Ultimately, adjusting the PS on just one cylinder makes it difficult to notice changes when idling.

FCR Installation – Preparation

Adjust the throttle cable, idle stop screw, and acceleration pump as needed. After making adjustments to the throttle cable and idle stop screw, be sure to synchronize the carburetor.

The FCR's accelerator pump is adjusted based on the width of the notch, as specified in the manual. However, measuring and matching it in 0.1 mm increments is challenging due to the difficulty in measuring the shape accurately. Achieving high precision with the float oil level and the accelerator pump can be complex. This raises the question of whether a carburetor can truly be considered a precision device.

Even if the vacuum gauge is new, the needle might be malfunctioning. Always calibrate the needle before use. Secure the rubber hose attachment port with an insulation lock to ensure it is secure. Before starting the engine, check that the vacuum gauge needle does not move easily. Be aware that the vacuum gauge itself may be damaged.

When using a two-pronged throttle cable, ensure full closure is confirmed with the top screw. The goal is to have both prongs start opening simultaneously. For full opening, adjust the throttle cable so that it opens fully at the same time. Make sure to adjust the play on the throttle side. After placing the fuel tank back on, turn the handlebar left and right to check if it gets caught.

A two-pronged throttle cable has a significant disadvantage when it comes to opening and closing the throttle valve. The benefit of having two prongs is minimal, as the strength of both throttle cables needs to be identical. While dividing the cables might improve work efficiency, it can also create inefficiencies due to the division. I have not used an FCR with a single cable system, but I believe it is generally better to avoid a two-pronged throttle cable setup.

Impressions from the test run

I felt that the SJ48 was better. I don't drive that fast, so my goal was to verify the setup. However, I also had to change the SJ. I keep a history of tunings so I can restore them if needed. If the throttle opening is low or the speed is low, you can use a screw like the AS. Even if the tuning becomes unmanageable, I can quickly revert to the original settings.

The SJ has a significant impact on the screws. Its role is to determine the range of the screw. If the SJ is outside the screw's range, it should be replaced. This is my impression after the test run. I wanted to measure the power and air-fuel ratio during acceleration and respond effectively to sudden throttle openings.

I used a larger SJ, but it wasn't to my current preference, so I changed it back. A richer air-fuel ratio does not necessarily mean worse fuel efficiency.

FCR-SJ Slow Jet – FCR Carburetor Installation【summary】

The FCR is a highly versatile retrofitted carburetor that can be used across different car models. While there are some aspects that could be improved, which might affect maintainability, it’s a flexible option. I wonder if V-type engines face similar challenges. One of the fundamental concepts of racing carburetors, including the FCR, is their ease of setup.

I think one reason I enjoy tuning the FCR is because of the air/fuel ratio meter. Professionals might make adjustments based on their feel, focusing on the driving experience, but non-professionals often worry about the air-fuel ratio being too lean and potentially damaging the engine. There’s a level of uncertainty about whether the tuning is correct. Air-fuel ratio meters are relatively affordable and can be a valuable tool. Not using one, especially when tuning a racing carburetor like the FCR, feels like missing out on an essential part of the process.

Even if you have a shop do the tuning for you, you might not be able to tell if the adjustments are good or bad. Some people install air/fuel ratio meters purely for aesthetic reasons, but I believe it's important to enjoy the tuning process itself. Nowadays, beginners can find valuable information online, read the FCR tuning manual, and watch tutorials on YouTube. In the past, before widespread internet access, information was scarce, and consulting experts was often necessary.

As an amateur who is just starting out, I hope my perspective is helpful.

Well then! See you again!