Tuning - Single

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HD Type Carburetter: Tuning (single Carbs)

1

(a) Run the engine up to normal running temperature.
(b) Switch off the engine.
(c) Unscrew the fast-idle adjusting screw (2) to clear the throttle stop with the throttle closed.
(d) Screw down the slow-running valve (1) onto its seating, then unscrew it 3.5 turns.

2

(a) Remove the piston/suction chamber unit.
(b) Turn the jet adjusting screw (3) until the jet (5) is flush with the bridge of the carburetter.

3

(a) Replace the piston/suction chamber unit.
(b) Check that the piston falls freely onto the bridge when the lifting pin (4) is released. If not, Routine Servicing.
(c) Lower the jet by turning the jet adjusting screw (3) down 2.5 turns.

4

(a) Restart the engine and adjust the slow running valve (1) to give the desired idling speed.
(b) Turn the jet adjusting screw (3), up to weaken or down to enrich, until the fastest idling speed consistent with even running is obtained.
(c) Re-adjust the slow-running valve (1), if necessary, to give correct idling.

5

The effect of mixture strength on exhaust smoke

A. TOO WEAK: Irregular note, splashy misfire, and colourless.
B. CORRECT: Regular and even note.
C. TOO RICH: Regular or rhythmical misfire, blackish.

6

(a) Check for correct mixture by gently pushing the lifting pin (4) up about 0.8 mm (1/32 in) after free movement has been taken up.
(b) The graph illustrates the effect on engine r.p.m. and indicated mixture strength when the piston is raised.
RICH MIXTURE: r.p.m. increase considerably.
CORRECT MIXTURE: r.p.m. increase very slightly.
WEAK MIXTURE: r.p.m. immediately decrease.
(c) Re-adjust the mixture strength if necessary.

7

(a) Reconnect the mixture control wire with about1.6 mm (1/16 in) free movement before it starts to pull on the jet lever.
(b) Pull the mixture control knob until the linkage is about to move the carburetter jet operating arm and adjust the fast-idle screw (2) to give an engine speed of about 1,000 r.p.m. when hot.
(c) Return the control knob and check that there is some clearance between the fast idle screw (2) and the throttle stop.

8

Finally top up the piston damper with thin oil of grade S.A.E. 20 until the level is 12.7 mm (1/2 in) below the top of the hollow piston rod. Mutli-carbs: Remove the air cleaners and carry out items 1, 2 and 3 (single carburetters) on each carburetter.

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