This is the bowl vent and should be attached via a hose to the PCV.
WHAT DOES THE BOWL VENT DO
The float bowl vent on a Rochester Monojet carburetor serves several crucial functions:
- Atmospheric Pressure Equilibrium: The primary purpose of the vent is to maintain atmospheric pressure inside the float bowl. This is essential for accurate and consistent fuel metering. If the float bowl were sealed, changes in fuel volume (due to consumption by the engine) would create a vacuum or pressure inside the bowl, which could interfere with the proper flow of fuel through the carburetor's jets and circuits.
- Fuel Vapor Escape: On hot days, especially after shutting off the engine, fuel within the carburetor can vaporize. The vent allows these vapors to escape, preventing excessive pressure buildup within the float bowl.
- Overflow: In case the float or needle and seat malfunction and allow too much fuel into the float bowl, the vent provides an exit route for the excess fuel. This can help prevent flooding, although in some cases, it might also mean fuel spilling out of the carburetor.
- Anti-Percolation: Some carburetors have vents equipped with a bimetallic coil valve. When the engine is turned off and the engine bay temperature rises, this valve closes the vent. This helps prevent fuel from boiling (percolating) in the carburetor, which can lead to hard starts and rough idling when the engine is restarted.
However, it's worth noting:
- The vent should remain unobstructed and clean to function correctly. If it's clogged or obstructed, it can lead to fuel delivery problems and erratic engine behavior.
- On some vehicles, especially newer models (relative to the age of the Monojet), the float bowl vent might be routed to an evaporative emissions system to capture and recycle fuel vapors, helping reduce emissions.
Always ensure that the venting system is in good working order when servicing or diagnosing issues with a Rochester Monojet or any other carburetor. Proper ventilation is crucial for optimal carburetor function.