Mercarb TKS

The Mercarb TKS (Turn Key Start) system was designed to eliminate the manual priming and "throttle-pumping" typically required to start a carbureted marine engine. It essentially functions as an automatic enrichment circuit that replaces the traditional mechanical choke plate.

Here is the technical breakdown of how the system operates:

1. The "Choke-less" Design

Unlike a standard carburetor, a TKS model has no choke plate at the top of the horn. Because there is no plate to restrict airflow, the carburetor cannot create the high vacuum needed to pull extra fuel through the main venturi during a cold start. Instead, it uses a completely independent internal passage to deliver the necessary rich fuel-air mixture.

2. The TKS Module (Thermal Wax Actuator)

The heart of the system is the TKS module, which is a small cylindrical solenoid-like component mounted on the side of the carburetor.

• Inside the Module: It contains a heating element and a reservoir of specialized thermal wax.

• The Plunger: A small needle or plunger is attached to this wax reservoir.

3. The Cycle of Operation

The system operates based on the temperature of the wax, controlled by a 12V electrical lead.

• Phase 1: Cold Start (The Open State)

  When the engine is cold, the wax is contracted. This pulls the plunger up and away from its seat. This opens an enrichment passage that allows fuel from the float bowl and air from a bypass port to mix and enter the intake manifold directly below the throttle plates. This provides the "rich" mixture and higher RPM needed for a cold start.

• Phase 2: Warming Up

  As soon as the engine starts, the TKS module receives a constant 12V signal (usually from the alternator or an oil pressure switch). The internal heater begins to melt the wax. As the wax expands, it slowly pushes the plunger downward.

• Phase 3: Normal Operating Temperature (The Closed State)

  After approximately 8 to 10 minutes, the wax has expanded fully, pushing the plunger firmly into its seat. This completely blocks the enrichment passage. The engine now relies solely on the standard idle and main circuits for fuel.

4. Technical Diagnostics: The "Hiss"

One of the most distinct features of a Mercarb TKS is the audible hissing sound.

• During Startup: You should hear a clear sucking or hissing sound coming from the TKS air intake port. This indicates the enrichment circuit is open and working.

• Once Warm: The hissing should stop. If the engine continues to hiss after 10 minutes, the TKS module has likely failed or lost power, causing the engine to run over-rich, soot up spark plugs, and stall at idle.

Summary of Differences