1. Cold Start (Choke Closed)
When the engine is cold, the Thermostatic Spring inside the housing is in a relaxed, expanded state. Its tension pulls the Thermostat Lever, which rotates the Choke Shaft and closes the Choke Plate at the top of the carburetor.
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Purpose: This restricts airflow, creating a high-vacuum environment that pulls more fuel from the main jets, creating the "rich" mixture needed to start a cold engine.
2. The Heat Source
Once the engine starts, hot air is drawn from a "stove" (a small chamber) on the exhaust manifold. This air travels through the Heat Tube and into the Manifold Heat Passage.
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The Vacuum Connection: The engine's internal vacuum pulls this hot air through the tube and into the choke housing.
3. Opening the Choke (Warming Up)
As the hot air enters the housing, it heats the Thermostatic Spring.
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The Reaction: The bi-metallic spring reacts to the heat by contracting (winding tighter).
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The Movement: This contraction overcomes the initial tension, moving the Piston and Lever Assembly. This rotates the choke shaft back, gradually opening the Choke Plate.
4. The "Choke Unloader" (Piston)
You’ll notice a small Piston inside the assembly. This is often called a "vacuum kick" or "pull-down" piston.
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Function: As soon as the engine fires, the sudden intake vacuum pulls this piston down slightly. This "cracks" the choke plate open just a tiny bit (usually about 1/8" to 1/4") so the engine doesn't immediately flood and stall from a total lack of air.
5. Fully Warmed
Once the engine reaches operating temperature, the spring is fully heated and contracted, holding the choke plate wide open. At this point, the carburetor operates on its normal air-to-fuel ratio.
Common Troubleshooting Tip: If your choke stays closed too long (causing black smoke and rough idling), the Heat Tube might be rusted out or clogged with carbon, preventing hot air from reaching the spring. If it opens too fast, the Thermostatic Spring might be weak or the Choke Housing Plate may be misaligned.
