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Tillotson HR Idle Speed Screw
15B-338 Tillotson HR Idle Speed Screw
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Tillotson HR Idle Speed Screw

Part Number: 15B-338
Online Price: $2.69
15B-338 OEM Tillotson HR Idle Speed Screw
Availability: In Stock.

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Description

15B-338 Tillotson OEM HR Idle Speed Screw


Throttle Stop Screw for HR-Series Carburetors | Ideal for Race Applications | Prevents Throttle Damage

The 15B-338 Tillotson Idle Speed Screw is a genuine OEM component designed to position the throttle shutter (butterfly valve) slightly open, allowing the engine to idle at a set RPM. It threads into the carburetor body and acts as a mechanical stop for the throttle plate.


This specific screw is made for Tillotson HR-series carburetors and is essential for setting or limiting throttle closure — especially important in racing environments where engine protection and consistent throttle positioning are critical.


Key Features:

  • OEM Part #15B-338
  • Controls throttle shutter opening for idle setting
  • Prevents throttle plate from fully closing
  • Helps avoid throttle shutter or carb body damage
  • Fits select HR-series Tillotson carburetors
  • Precision-machined for a proper fit and function

Racing Application Note:

For 4-cycle racing applications, idling is not recommended. Allowing the engine to idle often results in an overly rich air/fuel mixture, which can hurt performance and throttle response. Instead, many builders use the idle speed screw only as a throttle stop to:

  • Protect the throttle shutter from slamming shut
  • Prevent wear or damage to the carburetor body


Fitment Compatibility:

The 15B-338 Idle Speed Screw is compatible with the following HR-series Tillotson carburetors:

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

Always verify your carburetor model before ordering.


Ideal For:

  • 4-cycle racing engines
  • HR Tillotson carburetor rebuilds
  • Replacing stripped, missing, or damaged idle screws
  • Throttle safety and control adjustments

FAQ

Q1: What does the 15B-338 Tillotson Idle Speed Screw do?
A: It sets the throttle shutter (butterfly valve) position to control the engine’s idle speed or act as a mechanical stop to prevent throttle closure damage.


Q2: Is this screw required for 4-cycle racing engines?
A: Not necessarily for idle purposes — in 4-cycle racing, idle is often disabled. However, the screw is still used to prevent the throttle plate from slamming shut and damaging the carb body.


Q3: Which carburetors does this screw fit?
A: It fits Tillotson HR-series models, including HR-181B, HR-184A, HR-190A, HR-191C, HR-191G, and HR-196A.


Q4: Can this screw adjust idle RPM directly?
A: Yes, in standard applications it adjusts idle RPM by positioning the throttle plate. In racing setups, it's commonly used just as a throttle stop.


Q5: Can I use thread locker when installing this screw?
A: It's not required, but a low-strength (blue) thread locker can be used if the screw tends to vibrate loose under high RPM conditions.

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