Introduction
The type HIF (Horizontal Integral Float Chamber) carburetter has been designed primarily to meet the requirements of exhaust emission control carburation systems. Functionally similar to previous S.U carburetters, the HIF uses the variable choke/constant depression principle to achieve the precise induction of mixture required to control the toxic elements of exhaust emission to within statutory requirements.
Float Chamber
The float chamber is incorporated in the main body casting (see fig.2) ; access to the chamber is obtained by removing the bottom cover-plate (9). The moulded float (2) is shaped so that it surrounds the jet tube and is pivoted along a line parallel to the inlet flange. The float is retained by a spindle (6) which screws into the body casting.
Entry of fuel into the float chamber is via a brass tube (7) in the side of the carburetter body to a needle valve assembly is of a spring-loaded design to prevent engine vibration affecting seating of the valve.
The jet is pressed into the top of an aluminium tube which is in turn pressed into a plastic moulding (3). This hollow moulding known as the jet head is open at its lower and allowing fuel to enter the jet tube.
Mixture Adjustment
The jet tube of the HUF type carburetter is moved in the vertical plane to provide mixture adjustment only. Unlike previous types of S.U. carburetter the jet tube is not lowered down the jet needle to provide cold start enrichment.
The jet adjustment assembly is comprised of a right-angled adjusting lever of unequal length arms riveted to a bi-metal blade (1. fig. 2).
The blade is cut out to accept the jet head (3) and the shape of the jet head is formed so that any movement of the bi-metal blade is transmitted to the jet head, moving it in the vertical plane.
The right-angled adjusting lever and bi-metal blade (1) is attached to the body casting by a spring-loaded retaining screw (5) positioned in the short arm of the lever. This attachment lever to be pivoted at the outer edge of its short arm and is loaded by the spring towards the jet adjusting screw. (4)
The jet adjusting screw (4) is located at the outer end of the long arm of the adjusting lever; screwing the adjusting screw inward will lower the jet, enriching the mixture and unscrewing the adjusting screw will allow the spring to return the lever together with the jet, weakening the mixture.
After the mixture has been set the jet adjustment can be sealed by fitting a plug into the jet adjusting screw recess of the carburetter body.
Fuel Temperature Compensation (Viscosity Compensator)
This device alters the jet position in relation to the metering needle to compensate for changes in fuel viscosity which takes place with changes in fuel temperature.
When the fuel temperature rises, the viscosity is lowered, and in an uncompensated assembly, this would allow more fuel to flow for a given jet/needle relationship.
In the HIF jet assembly the jet head is attached to a bi-metal blade (1). This bi-metal is immersed in the fuel in the float chamber and will move in the vertical plane in response to changes in fuel temperature. The jet will be raised to a weaker position on the jet needle when the fuel temperature rises and will be lowered to a richer position when the temperature falls.
From this it will be seen that once the jet position has been selected by adjusting the mixture screw, alterations of fuel temperature will bring about slight alterations in jet position to compensate for the change in fuel viscosity.
The effect of this device is that drivability is improved over wide ranges of temperature, and the exhaust emissions can be kept within closer limits during cold starting and the warm-up period. Temperature compensation also allows carburetters to have the mixture setting pre-set and sealed before a vehicle is delivered.
