8. EFFECTS ON FUEL DELIVERY
PERFORMANCE WHEN ADJUSTING CALIBRATION SCREWS
If the fuel delivery of a metering unit does
not conform to specification, the calibration screws must be
adjusted. Each calibration screw has a particular effect on fuel delivery, and
it is advisable to study these so that the minimum of adjustments can be
made when calibrating.
Setting the datum track angle (see
section 5, page 22) is the first part of the calibration sequence and it
may be necessary to re-adjust the setting when bench testing. It may also be
necessary to adjust other calibration screws even though they were not
disturbed during dismantling and re-assembly.
It will be seen from the fuel delivery specifications, that testing starts
with maximum fuel delivery, and is
then checked step by step down to minimum fuel requirements, against a rising
vacuum.
The fuel delivery curves shown in Figures 24 and
25 are typical only. They
illustrate how the setting of the various calibration screws affects fuel
delivery.
(a) Outer Screw A1
This screw acts as a
stop for both the large and small springs in the depression chamber, and
determines the pre-load on them during maximum fuel conditions (wide open
throttle). Varying its position will alter the position of point ‘B’ and
curve line ‘B’ to ‘C’, see Figs. 24 and 25. If the screw
is turned too far anti-clockwise the fuel delivery at full throttle will be
reduced due to the small depression in the manifold, caused by the air
cleaner, etc.
(b) Intermediate Screw A2
The position of this screw determines the
travel of the spring thrust plate. When the thrust plate touches the end of
this screw, the diaphragm deflection is controlled by the larger spring. This
is position ‘C’ on the fuel curve. Screwing anti- clockwise will reduce
fuel delivery between points ‘C’ and ‘D’, see Figures 24 and 25.
(c) Centre (Minimum Fuel Stop) Screw A3
This screw (Fig. 25) limits the travel of the linkage and rollers towards the
minimum fuel position. Its main purpose is to prevent the cam follower forcing
the control stop fully home and so reducing the fuel to zero. This stop only
comes into operation under
“high” vacuum conditions on overrun of the engine. Point ‘D’ on the
fuel curve Fig. 24.
(d) Maximum Fuel (Full Throttle)
The datum track is held by spring pressure against
an adjustable stop. The position of this stop can be
altered by slackening the locknut and screwing the stop in or out. Clockwise
rotation will increase the full load (maximum fuel supply). It will also
affect the curve, as shown in Fig.
(e) Datum Track Adjustment Screws
Altering the angle of the datum track, by slackening
the two fixing screws and moving the datum track on its carrier (about the
centre pivot) will mainly affect the engine performance at idling speed and in
light load conditions. See fuel output curve, Fig. 24.
9. CALIBRATION
ADJUSTMENTS DURING FUEL DELIVERY TESTS
As stated earlier, the performance of the
metering unit is checked against the specification figures for the particular
vehicle. Several checks are made at specified points on a rising vacuum to
ascertain whether the fuel delivery is within specified limits. If fuel
delivery rate is outside the stated limits, re-calibration must be carried out
before proceeding to the next stage. This is because the calibration screws
(sleeves) are interlocked. When the position of a calibration screw is altered
(by screwing in or out) the position of the others is also altered, and will
require individual adjustment. The large outer
screw and the maximum fuel screw should be adjusted to correct the line
‘A’ to ‘B’. The intermediate screw controls the line crossover point
‘C’. If the initial datum track angle procedure has been carried out
correctly the line ‘C’ to ‘D’ should normally be correct. If not,
re-adjustment starting at the datum track will be necessary.
Always start re-testing from the
beginning after making any adjustment. Also, ensure that the sight glasses are
empty before starting to re-test.
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