Triumph Spitfire MK3 Handbook page 65

RUNNING ADJUSTMENTS
When this is satisfactory, adjust the mixture by screwing both
jet adjusting nuts (3) up or down by the same amount to give the
fastest idling speed consistent with even firing. Press the jets (4)
upwards during adjustment to ensure that they are in contact with
the adjusting nuts.
Should the engine speed increase as the jets are adjusted,
unscrew the throttle adjusting screws a little, each by the same
amount, to reduce the speed.
Lift the piston of the front carburettor approximately 1/32"
(0.75 mm.):
64
(a) If the engine speed increases, the mixture strength of the
front carburettor is too rich
(b) If the engine speed immediately decreases, the mixture
strength of the front carburettor is too weak
(c) ]f the engine speed momentarily increases very slightly, the
mixture strength of the front carburettor is correct.
Repeat the operation at the rear carburettor and, after
adjustment, re-check the front carburettor, since the two
carburettors are interdependent.