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Idle Mixture Screw for Honda GX390 & Big Block Clone Ruixing Carburetors
188F-59 GX390/420 Clone Idle Speed Screw
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Idle Mixture Screw for Honda GX390 & Big Block Clone Ruixing Carburetors

Part Number: 188F-59
Online Price: $1.12
Idle Mixture Screw – Fits GX390 & Clone Engines with Ruixing Carburetors
Availability: In Stock.

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Description

Idle Mixture Screw for Honda GX390 / Predator 420 / Big Block Clone Ruixing Carburetors


Fine-tune your engine’s low-speed performance with this Idle Mixture Screw, designed specifically for Ruixing-style carburetors found on Honda GX390, Predator 420, and other Big Block Clone engines.


This screw adjusts the air-fuel ratio at idle, working in tandem with the idle speed screw to set precise idle RPMs. It’s especially useful for performance tuning, engine swaps, or racing applications where reliable and adjustable low-speed operation is critical.


Unlike newer carburetors restricted by EPA regulations (which often come without an idle mixture screw), this part allows full control over idle fuel delivery, making it a must-have for builders and racers aiming for optimal throttle response and stable idle.


Key Features:

  • Enables precise tuning of idle air-fuel mixture
  • Works with idle speed screw to set engine idle RPMs
  • Compatible with Ruixing carburetors on GX390-style engines
  • Ideal for performance builds and small engine racing
  • Allows use of larger low-speed jets for improved response
  • A performance advantage over EPA-restricted carbs without mixture screws


Compatibility:

  • Honda GX390
  • Predator 420
  • Wildcat 460
  • Other GX390-style horizontal shaft clone engines with Ruixing or similar stock-style carburetors


Note: Some newer stock carburetors do not include idle mixture screws due to emissions regulations. If you're building for performance or racing, ensure your carb has the proper port/thread to accept this screw.

FAQ

Q1: What does the idle mixture screw control?
A: The idle mixture screw controls the air-fuel ratio at idle, allowing you to fine-tune how rich or lean the engine runs at low RPMs. It’s critical for a smooth idle and throttle transition.


Q2: Will this fit all GX390 carburetors?
A: This screw is designed for Ruixing-style stock carburetors, which are common on GX390, Predator 420, and other clone engines. Some modern EPA-compliant carbs may not have threaded ports for this screw.


Q3: How is this different from the idle speed screw?
A: The idle speed screw adjusts how far the throttle plate stays open at idle (RPM control), while the idle mixture screw adjusts how much fuel enters at idle (fuel mixture control).


Q4: Why do some carburetors not include an idle mixture screw?
A: Due to EPA emissions regulations, many newer carburetors come with sealed or non-adjustable idle circuits, which limit tuning flexibility. For racing or modified engines, using a carburetor with an idle mixture screw is recommended.


Q5: Can this help with engine bogging or poor idle?
A: Yes. If your engine hesitates, bogs, or idles roughly, this screw allows you to adjust the fuel delivery to correct lean or rich idle conditions.

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