16620-Z8D-305 Auto-Choke Thermowax Assembly

Key Features

• Precision Thermal Activation: Utilizing a heat-sensitive wax element, this assembly automatically regulates the choke plate position based on engine temperature, eliminating the need for manual choking.
• Optimized Fuel Economy: Ensures the choke is fully opened as soon as the engine reaches operating temperature, preventing a "rich" fuel condition and reducing spark plug fouling.
• Seamless Cold Starts: Provides the exact amount of air restriction needed for a reliable first-pull start, then gradually retracts the plunger as the engine block warms up.
• High-Durability Brass Housing: The wax element is encased in a corrosion-resistant brass cylinder designed for rapid heat transfer and long-term structural integrity in high-vibration environments.
• Direct OEM Integration: Engineered to factory specifications for Honda's GCV series, ensuring the plunger stroke and tension perfectly match the original auto-choke linkage.

Replaces / Cross-Reference Part Numbers

• 16620-Z8D-305
• 16620-Z8D-842
• 16620-Z8D-841
  

Compatibility & Fitment

This Thermowax assembly is the primary component of the Honda "Auto-Choke" system used on premium walk-behind mowers. Key compatible models include:

• Honda HRX Series Mowers: HRX217, HRX217K1, HRX217K2, HRX217K3, HRX217K4.
• Honda HRR Series Mowers: HRR216, HRR216K5, HRR216K6, HRR216K7, HRR216K8, HRR216K9.
• Honda GCV Engines: GCV160, GCV190, GCV160LA0, GCV190LA.
• Note: Specifically designed for engines featuring the automatic choke system located near the exhaust manifold; not used on manual choke models.

When to Replace

• Hard Starting When Cold: The plunger fails to extend, leaving the choke plate open and making the engine nearly impossible to start without manual intervention.
• Engine "Hunting" or Surging: The Thermowax is partially seized, preventing the choke from fully opening or closing, leading to inconsistent RPMs.
• Black Smoke / Sooty Spark Plug: The wax element has failed in the "cold" position, keeping the choke closed even after the engine is hot, causing a rich air-fuel mixture.
• Engine Dies After Starting: The assembly retracts the plunger too quickly or too slowly, causing the engine to stall during the transition from cold-start to operating temperature.
• Visible Plunger Seizure: The metal pin is stuck in place and does not move when the brass body is heated, indicating a internal wax failure.

Installation Tips

• Handle with Care: Do not drop or strike the brass cylinder. The wax element inside is precision-calibrated and internal damage can render the auto-choke system inoperable.
• Check the Linkage: Before installing, ensure the choke linkage and springs are not bent or rusted. A sticking linkage will often mimic a failed Thermowax assembly.
• Clean the Mounting Port: Use a wire brush or cloth to clean the area on the engine block where the Thermowax sits. Proper contact is essential for the wax to "sense" the engine's heat.
• The Hot Water Test: You can verify the new part's function by placing the brass tip in hot water; you should see the plunger slowly extend. As it cools, it should retract.
• Avoid Lubricants: Do not apply heavy grease to the plunger pin, as it can attract dust and debris, eventually causing the mechanism to bind. A light spray of dry silicone lubricant is acceptable if needed.