HS type carburettor, tuning


It is essential, particularly where vehicles are equipped and tuned to comply with engine emission control regulations, that the carburetors are tuned in accordance with the vehicle manufacturer's tuning data. To achieve the best results when tuning, the use of a reliable tachometer, balancing meter and an exhaust gas analyzer (CO meter of the infra-red non-dispersive type or equivalent are required). These instruments are essential when tuning vehicles equipped to conform with exhaust emission regulations.

Before servicing or tuning a carburetor in an endeavor to rectify poor engine performance, make sure that the maladjustment or fault is not from another source by checking the following:

Valve clearance
Spark plug condition
Contact breaker (dwell angle)
Ignition timing and advance
Presence of air leaks into the induction system


Step 1: remove the air cleaner(s).
Step 2: Check the throttle for correct operation and signs of sticking.
Step 4: Unscrew the throttle adjusting screw (each screw on multi-carburetors) until it is just clear of the throttle lever with the throttle closed, then turn the screw clockwise 1.5 full turns (single), one turn on each (multi carburetors) (1).
Step 5: Raise the piston of each carburetor with the lifting pin (2) and check that it falls freely onto the bridge when the pin is released. If the piston shows any tendency to stick, the carburetor must be serviced.


Step 1:  Lift and support the piston clear of the bridge so that the jet is visible; if this is not possible due to the installed position of the carburetor, remove the suction chamber assembly.
Step 2: Turn the jet adjusting nut/screw up/anti-clockwise, until the jet is flush with the bridge or as high as possible without exceeding the bridge height (3). Ensure that the jets on multi-carburetors are in the same relative position to the bridge of their respective carburetors.
Step 3:  Check that the sintered needle guide is flush with the underside face of the piston (4).
Step 4: Turn the jet adjusting nut/screw (3) two turns down/clockwise (each nut/screw on multi carburetors).
Step 5:  Turn the fast-idle adjusting screw anti-clockwise (each screw multi-carburetors) until it is well clear of the cam (5).


Refit the suction chamber assembly if it has been removed and, using the lifting pin (2), check that the piston falls freely onto the bridge.

Note: If ball bearing suction chambers are fitted take care not to wind up the piston spring when refitting the suction chamber - see reassembly section.