CARBURETTERS Section 4-C CARBURETTER TUNINGCOMPLETE The following instructions apply only to new carburet- ters or carburetters which have been serviced as described in CARBURETTER SERVICING. The tuning must be carried out with the engine emission control equipment connected and operating. SINGLE CARBURETTERS 3 U1 r; a min , /* XQ A (309A. The type HS carburetter 1. Jet adjusting nut. 4. Fast-idle adjusting screw. 2. Jet locking nut. 5. Throttle adjusting screw. 3. Piston suction chamber. 6. Piston lifting pin. 7. Jet adjustment restrictor. Initial setting A640: (1) Disconnect the mixture control (choke) wire if tted. (2) Unscrew the fast-idle screw (4) until it is well clear of the cam. (3) Unscrew the throttle adjusting screw (5) until it is just clear of its stop and the throttle is closed. 4'10 426 (4) Set the throttle adjusting screw one full turn open. (5) The jet adjusting nut must not be altered at this stage as it will be initially set to a datum setting at the factory or during the carburetter servicing procedure. Tuning conditions To ensure that the engine temperature and mixture requirements are stabilized, tuning must be carried out in accordance with the following setting cycle. (1) Connect a tachometer and an approved exhaust gas analyser in accordance with the instrument- makers instructions. (2) Warm the engine at a fast idle to normal operating temperature preferably with the car standing in an ambient temperature of between 16 and 27 C. (60 to 80 F.). Run the engine for at least ve minutes after the thermostat has opened; the thermostat opening point can be detected by the sudden rise in temperature of the radiator header tank. (3) Set the engine speed at 2,500 rpm, at no load, and run for one minute. (4) Tuning operations may now be commenced and must be carried out in the shortest possible time. If the time for settings exceeds a three-minute period, open the throttle and run the engine at 2,500 rpm. for one minute then resume tuning. Repeat this clearing operation if further periods of three minutes are exceeded. Tuning procedure (l) TOp up the piston damper with the recommended enginefoi'l until the level is v} in. above the top of the hollow piston rod. Engine Emission. Issue 4. 18959