Carburetors MK1 model.

The series 1 Sd1 6 cylinder cars were supplied with twin sidedraugth SU type HS6 unicon carburetors.

Basic construction.

The majority of car engines rely on carburetors to provide them with the finely atomized fuel/air mixture necessary for satisfactory performance. A carburetor provides a good air/fuel mixture under all operating conditions. The method used to do this in all carburetors is to speed up the velocity of the air by means of a venturi or choke and to use the consequent reduction of pressure in the venturi to draw fuel from the float chamber thought a suitable jet orifice into the air jet stream.

The perfect carburetor is an instrument, which supplies its engine with optimum mixture for maximum performance throughout the full throttle range and for minimum consumption under all conditions. This only can be achieved when correctly tuned.

The SU carburetor, a closer look.

The diagram shows a SU HS carburetor, the float chamber moved out of position.
The depression is communicated to the suction chamber (18) by means of the depression transfer drillings (16) in the bottom of the close-fitting piston (13).

Cold start enrichment is provided by clockwise rotation of the fast idle cam (24). This is usually affected by a Bowden-type flexible cable, which is operated from a position somewhere in the car, in reach for the driver. Initial rotation of the cam takes up lost motion built into the jet drop link and brings the cam into contact with the fast idle screws (25). This causes the throttle to start opening. Further rotation of the cam increases the throttle opening still further and also pushes the link (23), which lowers the jet, and with that providing more enrichment.

The diagram also shows the fuel jet (14), suction disc (15) throttle disc (17) atmospheric vent passage (19), jet needle (20), mixture adjusting nut (21), the hydraulic damper (22) and the float chamber vent (26).

The basic means of adjusting the carburetor are.

A) Jet adjustment for mixture strength
B) Slow run screw adjustment for engine idling speed
C) Fast-idle speed adjustment by means of fast-idle screw (25)

HS6 Dismantling

1) Clean the outside of the carburetors.
2) Standard suction cambers, remove the piston damper, (1) and washer if fitted.
3) Unscrew the suction chamber retaining screws (3).
4) Lift the chamber assembly of the body vertically (4) without tilting it.

The SU HS6 has two types of ball bearing suction chambers.
The early type, hold the piston firmly and pull the suction chamber, be carefull not to bend the damper rod. When the bearing retainer is freed from the piston rod (5a) remove the damper.

Later types.
Remove the piston damper, lift the piston and remove the bearing retaining circlip.

1) Separate the suction chamber, the piston and the piston assembly and empty the oil from the piston rod. (6)
2) Unscrew the needle guide locking screw, then withdraw the needle, guide and spring (7).
3) Remove the piston lifting pin circlip and spring and withdraw the pin from the body (8).
4) Release the pick-up lever return spring from its retaining lug (9)

Tune and adjustments

For a proper adjustment of the engine special equipment is needed, these tools are: exhaust gas analyzer, accurate tachometer, timing device and a air flowing balancing meter. For a exhaust gas analyzer a so called Colour tune plug can be used, afterwards the engine must be checked at a garage if engine does not produce to many CO levels.
For the best performance the engine must be timed correctly first.

1) Remove the airfilter unit from the carburetors.
2) Check the throttle for correct operation, and ensure it does not stick.
3) Remove the tamperproof seals from:
a) the mixture controle nut
b) the idle adjusting screw.
4) Turn the fast idle adjusting screw anticlockwise until well clear of the cams, and disconnect the mixture controle cable at the trunnion.
5) Turn the idle adjusting screw “B” until just clear of the throttle levers with the throttles closed, then turn the screws 1 1/2 turns clockwise.

6) Raise the piston of each carburetor in turn and check that the bridge falls freely onto the carburetor brigde. If either pistons shows any tendency to stick, remove and clean the piston and suction chamber.

7) Lift the piston and support it, or remove the damper complete so that the jet is visible.

8) Turn the jet adjusting screw “A” to bring the jet flush with the carburetors brigde.

9) Check that the needle shank is flush with the underside of the piston and that the needle is centered.

10) Turn the jet adjusting nut down two full turns.

11) Fill the damper with SAE 20 or engine oil. To do this, pull out the damper to its fullest extent, fill the retainer recess with oil and push the damper down until the cap touches the suction chamber.

12) Repeat this procedure until oil is just visible at the bottom of the retainer recess with the piston fully down.

13) Screw the damper cap back on top of the damper, don’t use any tools!

14) Start the engine and run at a fast idle speed unit it has the normal running temperature, and continue for a further 5 minutes.

15) Increase the engine speed to 2500 rev/min for 30 seconds.

16) Using the gas exhaust analyzer, insert the probe into the exhaust pipe.

17) Slacken both the clamping nuts and bolts on the throttle spindle interconnection.

18) Slacken both nuts and bolts on the jet control interconnection.

19) Using an airflow balancing meter and check the carburetors for balance and adjust by turning the throttle adjusting screws whilst maintaining the correct idling speed.

20) Turn the jet adjusting nut on both carburetors, down to enrich or up to weaken the mixture by the same amount until the fastest speed is indicated on the tachometer.

21) Turn each adjusting nut up one flat at a time until the engine speed just commences to fall.

22) Turn the adjusting nuts down by the minimum amount until the fastest speed is regained.

23) Check the idle speed, and adjust if necessary by turning the throttle adjusting screws by the same amount. Check that the CO reading on the gas analyser is within the accepted limits at idling. Max. up to 4.5%

24) If the CO reading falls outside the limits, reset both jet adjusting nuts by the minimum amount necessary.

If any adjustment exceeding three flats or half a turn is required to achieve this, the carburetors should be renewed.