The SU Type HD Design, construction. parts and identification.
The Type HD (Diaphragm-jet) carburettor was introduced after the original Type H version. It was manufactured now in larger sizes only — 1 1/2 in the HD4, 1 3/4 in the HD6 and 2 in HD8, and was therefore generally fitted to larger and heavier cars.The characteristic features of the Type HD are the methods of metering fuel for idling, and of sealing the jet base.
The Type HD Construction has the piston and suction chamber assembly is of the same general design as that of the original Type H. The dashpot bore may be either dustproofed or non-dustproofed, and may therefore have un-drilled or drilled piston damper caps respectively.
The float chamber assembly is of similar design to Type H, in that it houses a brass float sliding on a central spindle, and the inlet valve is closed by a hinged lever mounted in the detachable float chamber top. An ignition timing vacuum tapping is provided in the roof of the bore, the fine hole emerging at the edge of the throttle plate when in the closed position. The hole is bored through from a flat projection behind the suction chamber. Connection is made by an adapter plate with a stub pipe, secured to the body with two screws. A gasket is fitted between the body and the plate.
The jet slides within a bearing, secured to the underside of the body, by the same method as in the Type H, and is surrounded by an open-ended cylindrical housing, cast integrally with the body. The housing is closed by a jet housing, incorporating the jet actuating lever assembly hinged in integrally-cast lugs, and an extension of the float-chamber, the parts being secured to the body by four screws. A flexible diaphragm (fixed to the base Of the jet assembly) is clamped between the float Chamber extension and the underside of the jet
housing, and divides the housing cavity into two chambers. The upper chamber contains the jet actuating (lowering) mechanism and the lower chamber is linked to the float chamber via a drilling in the extension, and is therefore flooded with fuel. The fuel flows up through the hollow jet to the orifice at the top. A helical compression spring, fitted between a cup at the base of the jet assembly and a locating spigot in the float chamber extension, loads the jet upwards.
All HD4 and HD6 units are fitted with throttle spindle sealing glands, which minimise inwards air leakage when the spindle bearings become worn. Some HD8 units are also so equipped, while others have no sealing glands and are fitted with replaceable PTFE bushes. The glands have tapered faces, and are fitted into conical-ended counterbores in the unit body. A helical compression spring, retained by a steel sleeve pressed into the end of the counterbore, compresses the gland so that the bore closes Onto the throttle spindle and forms a seal. The glands do not require servicing, and no provision is made for their easy removal.
PTFE bushes are inserted into counterbores in the unit body from inside the bore of the unit. They are retained in position by circular spring clips fitted to the throttle spindle.
The underside of the body has cast ribs, and there is a cast projection at each side of the main bore. The ribs are internally-drilled to form a passage which bypasses the throttle plate when it is closed, One of the side projections houses a screwed needle valve, the end Of which intercepts the drilled passage, and functions as a slow running volume Screw. The Other side projection has a vertical bore in which slides a cam rod, forming part of the cold start enrichment/throttle opening interconnection mechanism. The top end of the cam rod is fitted with a plate, carrying an adjusting screw which bears on the throttle spindle arm. The bottom end is rivetted to an arm, at the end of which is a roller. The roller bears on a cam at the end of the jet actuating lever spindle.
