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Tillotson HR Idle Mixture Screw
43-991 Tillotson HR Idle Mixture Screw
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Tillotson HR Idle Mixture Screw

Part Number: 43-991
Online Price: $9.99
43-991 Tillotson HR Low-Speed / Idle Mixture Screw
Availability: In Stock.

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Description

43-991 Tillotson HR Low Speed / Idle Mixture Screw – OEM Positive Stop Type


The 43-991 Tillotson Low-Speed Mixture Screw (also referred to as an idle mixture screw) is an OEM component designed specifically for Tillotson HR-series carburetors. Despite its name, this screw does more than control idle—it regulates fuel delivery to the low-speed and midrange circuits, making it a vital part of tuning overall engine performance.



Key Functions:

  • Controls fuel flow through the low-speed and midrange circuits
  • Crucial for proper acceleration and throttle response
  • Not solely for idle—it influences overall low-to-mid engine performance
  • Used primarily in 2-cycle and 4-cycle racing engines with Tillotson HR carbs


Tuning Note for 4-Cycle Engines:

Tillotson HR carburetors were originally designed for 2-cycle engines, which do idle well. On 4-cycle applications (especially with methanol), these carbs tend to run rich at idle, creating hesitation or dead spots.


To resolve this, avoid tuning the low-speed screw solely for idle quality. Instead, tune it for clean throttle response and midrange performance. Setting the screw too lean can cause the engine to run flat or erratically.


Specifications:

  • Material: Steel
  • Part Number: 43-991
  • Also Known As: Low-speed mixture screw, idle screw, HR carburetor tuning screw


Fits the Following Tillotson Carburetors:

  • HR-181B
  • HR-184A
  • HR-190A
  • HR-191C
  • HR-191G
  • HR-196A

Always check your carburetor model before ordering to ensure compatibility.

FAQ

What does the 43-991 Tillotson mixture screw control?

This screw controls the low-speed and midrange fuel circuits of Tillotson HR carburetors. It affects throttle response, acceleration, and overall low-end performance—not just idle.


Is this the same as an idle screw?

It’s commonly called an idle mixture screw, but it does more. On most HR carbs, it influences the entire low to midrange fuel curve, especially important on racing engines.


Will this work with methanol-fueled engines?

Yes, but caution is required. HR carbs on methanol-fueled 4-cycle engines tend to run rich at idle. Tune this screw for throttle response, not idle smoothness, to avoid bogging or hesitation.


What carburetors does the 43-991 screw fit?

This screw fits the following Tillotson HR carburetors:

  • HR-181B
  • HR-184A
  • HR-190A
  • HR-191C
  • HR-191G
  • HR-196A

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