SI Vergaser #40: The mystery behind that little corner on some slides for dellorto SI.

Summary

In this detailed video, the speaker explains the intricacies of a carburetor and its components, particularly focusing on the slider's geometry and how even minor adjustments can affect engine performance. He highlights the potential issues caused by the corner on the slider, which can lead to an overly rich mixture and poor throttle response when certain jets are used. The speaker also discusses various carburetor designs, sharing insights on custom modifications and their impact on engine efficiency. The video aims to educate viewers on the importance of proper setup for optimal performance in tuning motors.

Takeaways

• 😀 The speaker emphasizes the importance of the carburetor slide's design, particularly its bottom edge (cutout), in affecting engine performance at low speeds.
• 😀 A pronounced cutout in the carburetor slide can lead to excessive fuel flow and an overly rich fuel mixture, which causes poor performance and rough running at low speeds.
• 😀 The speaker argues that carburetor slides without a cutout allow for better control over the fuel mixture and smoother low-speed operation.
• 😀 Small adjustments to the idle screw can have a significant impact on fuel flow, even with a small change in throttle position.
• 😀 The speaker highlights that even small design changes (such as a millimeter of difference in the slide's geometry) can make a big difference in carburetor performance.
• 😀 Using oversized jets (Nebendüsen) with a carburetor slide that has an improper cutout can result in excessive fuel delivery, leading to poor throttle response.
• 😀 The speaker demonstrates how minimal throttle movement can open the secondary fuel jet (Nebendüse) if the carburetor slide design is not correct, resulting in a rich mixture.
• 😀 The speaker specifically avoids using slides with a cutout at the bottom of the slide to prevent the carburetor from becoming too rich and difficult to control at low speeds.
• 😀 The speaker also mentions experimenting with a prototype slide design, which was too large and ineffective due to an overexaggerated cutout.
• 😀 Precision in carburetor tuning is essential for achieving the correct fuel mixture and throttle response, especially for tuned or modified engines.

Q & A

What specific carburetor component is the main focus of the discussion?

The main focus is the carburetor slide, particularly the lower front edge (cutout) and how its geometry affects fuel delivery from the idle (pilot) circuit.

What is the 'corner' or 'cutout' on the slide, and how large is it typically?

The corner, also called a cutout, is a small recessed section at the lower edge of the slide. It is typically around 1 to 1.5 mm deep.

Why does the presenter consider the cutout problematic?

Because even minimal throttle movement (about 0.5–1 mm) can prematurely expose the idle circuit port, causing excess fuel flow and an overly rich mixture at low speeds.

How does the idle speed adjustment affect the slide position?

Turning the idle speed screw raises the slide slightly. Even a small adjustment creates a gap that can bring the idle circuit port very close to opening, especially if the slide has a cutout.

What happens when the idle circuit opens too early due to the cutout?

The engine receives fuel from the idle jet too quickly and in excessive amounts, leading to over-fueling, rough running, and poor low-speed drivability.

Why do large idle jets worsen the problem?

Large idle jets already deliver more fuel. If the slide cutout allows the idle circuit to activate too early, the mixture becomes excessively rich almost immediately.

What riding symptoms indicate this over-rich condition?

The engine may sputter, hesitate, or ‘blubber’ when cruising at low speeds (e.g., 20–30 km/h in higher gears), especially with small throttle inputs.

In what situation can the cutout be beneficial?

It can help in thermally stressed engines during engine braking and re-acceleration, where a richer mixture may protect the engine from running too lean.

Why did the presenter reject a larger, deeper cutout design?

A very large cutout allowed too much air under the slide, reducing airflow velocity and vacuum at the main jet, which prevented proper fuel draw and caused a loss of engine performance.

What physical limitation prevents making extremely deep cutouts?

The slide material thickness is only about 3.8–4 mm. Making a deeper cutout would compromise structural integrity or simply not be physically possible.

How does airflow velocity relate to fuel draw in the main jet system?

Sufficient airflow velocity creates vacuum (pressure drop) in the main jet stack. If airflow is too unrestricted due to a large cutout, vacuum decreases and fuel is not properly drawn from the main jet.

What is the presenter's overall conclusion regarding slide cutouts for tuned engines?

For smooth and controllable low-speed operation in tuned engines, he prefers slides without the lower cutout, as they provide better throttle control and prevent excessive richness at small throttle openings.