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Tillotson HW 37g Inlet Tension Spring
24B-299 Tillotson Inlet Tension Spring
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Tillotson HW 37g Inlet Tension Spring

Part Number: 24B-299
Online Price: $1.79
Tillotson 24B-299 Inlet Tension Spring – 37g Pop-Off Control
Availability: Out of Stock.

Description

Tillotson 24B-299 Inlet Tension Spring – 37g Pop-Off for HW Carburetors


The Tillotson 24B-299 Inlet Tension Spring is an OEM-rated 37-gram pop-off spring designed to precisely control fuel metering in Tillotson HW-series diaphragm carburetors. It regulates when the inlet needle valve opens in response to engine vacuum and diaphragm movement, directly affecting throttle response, low-end acceleration, and fuel delivery consistency.


Whether you're building a performance 2-stroke or fine-tuning a small engine for stability at low speeds, the correct pop-off spring is essential for carburetor function.


Key Features:

  • OEM Part Number: 24B-299
  • Spring Tension Rating: 37 grams
  • Application: Controls pop-off pressure in diaphragm carburetors
  • Position: Sits under the fulcrum arm above the inlet needle
  • Material: Durable, precision-wound spring steel
  • Target Carburetor Series: Optimized for Tillotson HW series


Understanding Pop-Off Pressure:

Pop-off pressure is the point at which the inlet needle lifts off the seat to allow fuel into the metering chamber. This spring’s tension determines how much vacuum is required to trigger that action.

Important:
Pop-off pressure is not the same as fuel pressure.

  • A typical fuel pump only produces ~1 PSI
  • A 37g pop-off spring does not require 37 PSI of fuel pressure to open the needle
  • Instead, diaphragm pulse and vacuum dynamics drive this process


Tuning Notes:

Pop-off pressure is influenced by:

  • Spring weight (like this 37g spring)
  • Fulcrum arm height
  • Shim placement (under the spring for fine adjustment)

Too much pop-off pressure can delay fuel delivery and cause hesitation. Too little can flood the carb and lead to bogging. Always tune for crisp throttle response based on your engine’s needs and operating conditions.


Ideal For:

  • 2-stroke race engine tuners
  • Small engine rebuilds
  • Fine-tuning Tillotson HW-series carburetors
  • Dialing in throttle response for low-speed stability

FAQ

Q1: What does the 24B-299 inlet tension spring do?

A: This OEM 37-gram spring controls pop-off pressure in Tillotson diaphragm carburetors. It regulates when the inlet needle opens, helping control fuel flow at low speeds and idle.


Q2: What is pop-off pressure, and how is it measured?

A: Pop-off pressure is the point at which the carburetor’s inlet needle "pops off" its seat due to diaphragm vacuum. It’s measured using a pressure tester and depends on spring tension, fulcrum height, and needle shape. It is not the same as fuel pressure.


Q3: Is this spring compatible with all Tillotson carburetors?

A: No. The 24B-299 is specifically tuned for HW-series Tillotson carburetors. Using it on a different series may result in poor tuning or fuel delivery issues.


Q4: How can I fine-tune pop-off pressure?

A: You can fine-tune by:

  • Changing the spring (lighter or heavier)
  • Adjusting the fulcrum arm height
  • Adding or removing shims under the spring
    These changes affect how soon the needle opens and how the engine responds to throttle input.


Q5: What happens if pop-off pressure is too high or too low?

A:

  • Too high: The engine may hesitate, bog, or fail to accelerate smoothly.
  • Too low: The engine may flood, run rich, or have inconsistent idle.
    Always aim for clean, crisp throttle response as your benchmark.


Q6: Should I replace the spring during a rebuild?

A: Yes. Replacing worn or mismatched springs during a rebuild ensures consistent fuel delivery and easier tuning. Use fresh diaphragms and verify fulcrum arm height during installation.

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