Lucas MK1 and MK2 fuel / petrol injection



Lucas Service Training Centre
Petrol Injection Mk II




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 opera
tion 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.




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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