Carburettor compensating jet: why is its petrol flow nearly constant across speed? Context: A compensating (air-bleed/emulsion) jet is used to correct mixture at higher speeds/suctions. Choose the best explanation.

Concept
A compensating jet uses an emulsion/air-bleed arrangement (and a separate well vent) that causes the effective head and/or discharge pathway to change with suction. As air velocity and venturi depression rise, more air is bled into the emulsion, reducing the net fuel head and thus limiting the increase of fuel flow. The result is an approximately constant fuel delivery per unit of suction increase, correcting mixture strength.

Why other options are incorrect

• Main-jet diversion: Not a simple diversion mechanism; it is a calibrated secondary circuit.
• Constant diameter/C_d: True facts, but they do not explain constancy with speed.
• Static head only: Would make flow rise strongly with venturi suction; it does not ensure constancy.

Final Answer
The effective jet area varies automatically with suction, keeping fuel flow nearly constant.