In carburetors, the top of the fuel jet with reference to the level in the float chamber is kept:

Explanation:

Principle of Carburetion:

Both air and gasoline are drawn through the carburettor and into the engine cylinders by the suction created by the downward movement of the piston.

The Simple Carburettor:

• The simple carburettor mainly consists of a float chamber, fuel discharge nozzle and a metering orifice, a venturi, a throttle valve and a choke
• The float and a needle valve system maintain a constant level of gasoline in the float chamber.
• If the amount of fuel in the float chamber falls below the designed level, the float goes down, thereby opening the fuel supply valve and admitting fuel
• When the designed level has been reached, the float closes the fuel supply valve thus stopping additional fuel flow from the supply system
• The float chamber is vented either to the atmosphere or to the” upstream side of the venturi.
• The jet coming out from nozzle is kept a bit higher than the float chamber as can be seen from the image given above.