CARBURETTOR Most Mini models are fitted with an SU type HS4 carburettor, the exceptions being early 850/1000 models with manual transmission up to 1974. These latter cars are equipped with an HS2 carburettor. Although both types are of similar design they differ mainly in respect of the position of the float chamber and the throttle lever assembly which are on opposite sides for the two types.
Idle Adjustment The procedure for adjusting the carburettor slow running is fully described in the TUNE-UP section previously, and reference should be made for details.
Removal and Installation Unscrew the wing nut or wing nuts on the air cleaner lid and lift the air cleaner assembly off the carburettor air intake duct. On some models it may be necessary to first disconnect the engine breather hose and/or the connector pipe from the air temperature control valve flange.
Disconnect the fuel feed hose from the float chamber, the engine breather hose from the carburettor adaptor (where applicable) and the distributor vacuum advance pipe .
Disconnect the throttle return spring from the throttle lever and its abutment bracket, then disconnect the throttle cable.
On models with automatic transmission it will also be necessary to disconnect the fork end of the kick-down control rod from the throttle lever. Disconnect the choke cable.
Remove the two nuts and spring washers securing the carburettor to the inlet manifold studs and lift off the carburettor assembly complete with the cable abutment plate.
If required, the air intake duct can now be detached from the carburettor intake.
Installation is a simple reversal of the removal procedure.
Use new gaskets between the manifold face, abutment plate and carburettor flange if they have been damaged during removal.
When reconnecting the choke cable, allow approximately 1/16 in (2 mm) free movement before the cable starts to pull on the fast idle cam lever. The throttle pedal should also have approximately 1/8 in (3 mm) of free movement before the throttle starts to open.
When installation is complete, check the idle and mixture settings as described in the TUNE-UP section previously.
On automatic models, check the adjustment of the kick-down control linkage as described in the AUTOMATIC TRANSMISSION section.
Cleaning the Float Chamber On some models it may first be necessary to remove the air cleaner to gain access to the float chamber.
Disconnect the fuel feed pipe from the float chamber lid.
Mark the relative positions of the float chamber lid and body to ensure correct alignment on reassembly. Remove the three screws securing the lid to the float chamber body and detach the hd assembly, together with its gasket . Retain the part number tag fitted to one of the retaining screws.
Clean any sediment from the float chamber - this can best be done by soaking up the fuel in the chamber with a suitable absorbent lint-free cloth, then blowing any remaining sediment out of the chamber with an airline.
If required, the fuel float can be removed and the fuel needle withdrawn from its housing to check its condition. Hold the float hinge pin at its serrated end when withdrawing it.
Examine the fuel needle for wear i.e. small ridges or grooves on the seat end of the needle. Also check that the springloaded plunger on the opposite end operates freely, (later type needles only). Renew the needle valve and seating if worn. The seating is a screw fit in the float chamber lid.
Reassemble the components in the reverse order of removing.
The fuel needle is inserted, coned-end first, into its seating.
On early models which have a brass or nylon float with a metal float lever, the float level setting should be checked as follows:

With the float chamber lid held upside-down, insert a drill or gauge rod of suitable diameter between the hinged lever and the machined lip of the float chamber lid . The end of the lever should just rest on the rod when the needle is on its seating. Use a 5/16 in (8 mm) rod for the brass float, and 1/8 in (3 mm) rod with the nylon float.
If adjustment is necessary, this should be carried out at the point where the end of the lever meets the shank. Do NOT bend the shank, which must be perfectly flat and at right-angles to the needle when on its seating.
Centring the Jet
If the piston does not fall freely on the carburettor bridge with a distinct metal click when the jet adjusting nut is screwed to its uppermost position, the carburettor jet must be centralised as follows:
It should be noted that this procedure applied only to early type carburettors, as later carburettors have a springloaded needle which is self-centring .
Remove the air cleaner assembly to allow access. Support the plastic moulded base of the jet, then remove the screw retaining the jet pick-up link and link bracket to the jet head . Unscrew the sleeve nut from the base of the float chamber and disconnect the jet flexible feed tube.
Withdraw the jet assembly from the bottom of the carburettor.
Unscrew the jet adjusting nut, remove the locking spring and refit the nut without the spring. Screw the nut up as far as possible. Now slacken the jet bearing locking nut until the jet bearing can be turned with the fingers. Refit the jet assembly in the jet bearing and hold it in the uppermost position with a finger.
Remove the damper piston from the top of the suction chamber and, using a pencil or similar instrument, apply gentle pressure to the top of the piston rod to push the piston down onto the bridge. Tighten the jet locking nut while holding the jet against the jet bearing. Ensure that the jet is in its correct angular position during this operation.
With the jet still in the fully raised position, lift the piston with the lifting pin then release it and check that it falls freely onto the carburettor bridge with a soft metallic click. Lower the jet with the adjusting nut and repeat the check. An identical sound should be heard with the jet raised or lowered. If a sharper click is heard with the jet in the lowered position, repeat the centring procedure.
If difficulty is encountered in correctly centring the jet, this can often be achieved by raising the piston and allowing it to fall onto the jet bridge. The slight impact should locate the jet in its central position in relation to the piston needle.
When the centring procedure is successfully completed,. remove the jet assembly and refit the adjusting nut with its locking spring. Screw the nut up as far as possible, then turn it down two complete turns (12 flats) to provide the initial setting.
Refit the jet in the bearing and reconnect the flexible feed tube to the float chamber. Ensure that the end of the tube projects a minimum of 3/16 in (5 mm) beyond the sealing gland before fitting the tube. Tighten the sleeve nut only until the gland is compressed as over-tightening can cause leakage.
Support the jet head and reconnect the pick-up link and link bracket with the securing screw..
Top up the piston damper with light oil, as necessary, to bring the oil level approximately 1/2 in (13 mm) above the top of the hollow piston rod. Refit the air cleaner assembly.
Finally, check the idle and mixture settings and adjust if necessary as described in the TUNE-UP section previously.
Cleaning the Piston and Suction Chamber A sticking piston will inhibit acceleration and smooth running and can be ascertained by removing the piston damper from the top of the suction chamber and raising the piston with the lifting pin, or a finger inserted into the carburettor intake.
The piston should move up quite freely when raised, and fall back smartly when released.
If sticking does occur, the whole assembly should be removed and cleaned as described below:
Remove the air cleaner assembly. Mark the position of the suction chamber in relation to the carburettor body to ensure correct alignment on reassembly. Thoroughly clean the outside of the suction chamber and the adjacent surface on the carburettor body.
Remove the damper from the top of the suction chamber. Remove the three securing screws and lift off the suction chamber.
Remove the piston spring. Carefully lift the piston assembly out of the carburettor body and empty the oil from the hollow in the end of the piston rod.
Carefully clean all fuel deposits, etc., from the inside of the suction chamber and the two diameters of the piston, using petrol, or preferably methylated spirit, then wipe the components dry. Do NOT use abrasives to clean these items.
The operation of the suction chamber and piston can be checked as follows if required: Plug the transfer holes in the bottom of the piston with rubber plugs or Plasticene.
Insert the piston fully into the suction chamber and refit the damper assembly. Secure a large flat washer to one of the suction chamber fixing holes with a screw and nut so that it overlaps the bore.
With the assembly held upside-down, hold the piston and check the time taken for the suction chamber to fall the full extent of its travel. This should take five to seven seconds. If this time is exceeded, check both the piston and the suction chamber for cleanliness and mechanical damage. If the suction chamber has been dropped at any time this may have damaged the bore and be the cause. Renew the piston and suction chamber assembly if the time taken cannot be brought within these specified limits.
Lubricate the piston rod lightly with a drop of light oil - one of the non-oil lubricants such as WD 40 may be used for this purpose. Reassemble the components in the reverse order of removing, not forgetting the piston spring. Ensure that the assembly marks made previously are correctly aligned, then tighten the securing screws evenly.
Finally, fill the piston damper with light oil until the level is approximately 1/2 in (13 mm) above the top of the hollow piston rod, then refit the piston damper.