DHLA Adjustment

CARBURETOR SYNCHRONIZATION AND IDLE ADJUSTMENT

Proper carburettor synchronization and idle adjustment requires an exhaust gas CO meter, a rev-counter and a four-column mercury manometer.

Proceed as follows:

• Disconnect the throttle control rod and lever (1).
• Unscrew the idle speed screw (2) out of contact with lever extension (3).
• Unscrew the screw (4) of the balance lever (5) until the throttles of both front and rear carburetors are fully closed. Check they are with light upward pressure on lever (1) While maintaining pressure on lever (1), tighten screw (4) until it contacts the tongue of lever (1), thus fully closing the throttles of both front and rear carburetors.
• Tighten the screw (2) one more turn after it first contacts the extension (3) of lever (1).
• Remove the anti-tamper seals from the mixture screw housing (6) and unscrew each screw five turns from the fully-closed position.
• Check that the idle air bypass screws, to which anti-tamper paint has been applied, are completely closed- Remove the depression blanking plugs (8) and connect each barrel to the four-column mercury monometer using the screw-in adaptors.
• Reconnect the Throttle operating rod to lever (1).
• Start the engine and leave it running to attain normal operating temperature.
• First ensure that any airlocks are removed from the mercury columns which will otherwise result in inaccurate readings.
• Align the lower mercury column of the front carburettor with the lower one of the rear carburettor using balance screw (4).
• Then, if necessary, adjust the levels of the other two cylinders to match those of the lowest pair using the bypass screws on these barrels With the No 1 cylinder blow-by pipe temporarily clamped shut, all readings should be brought to the same level. Unclamped for normal running, No 1 cylinder’s mercury level will always be lower than the other three cylinder levels.
• Insert the CO tester probe and connect the rev counter.
• Preferably only by using the special Dellorto screwdriver, adjust the running of each cylinder by turning the mixture screw (6) to get the most even running with the engine idle speed at the correct level and holding the exhaust CO level well below 4,5%. Remember that by unscrewing these screws you increase the CO level and vice versa. The mixture screw of barrel (1) will need to be opened more than the others due to the effect of the blow by hole on that cylinder- Remove the vacuum adaptor plugs and refit the blanking plugs in their place.
• Fit new anti-tamper seals on the mixture screw housings (6).

If all the above-mentioned equipment is not available, it is still possible to obtain the correct idle setting by simply resetting the idle speed screw (2) at the balance adjusting screw (4).

DHLA 40-45-48

Applications:
ALFA ROMEO
1300- 1600 Giulia Super
1600 GT Coupe’ - 1750 Saloon
2000 Saloon - GT Coupe’ - Spider
LOTUS CARS
Europa Twin-cam - Elan -
Elan Plus - 2S

1. FEATURES

• Twin-choke side draft body.
• Barrel sizes, 40 - 45 - 48 mm.
• Interchangeable chokes available in sizes ranging from 25mm. to 43 mm. inclusive.
• Butterfly throttle valves.
• Centrally-placed constant-level float chamber with double float.
• Main circuit using centrally-placed main jets and emulsion tubes.
• Idling circuit with centrally-placed idle jets drawing fuel directly from the float chamber.
• Independent starting circuit with single starter jet and valve and separate channels to each barrel.
• Diaphragm accelerator pump, mechanically-operated, with single inlet valve, two delivery channels and individual valves and pump jets.

2. OPERATION

a - Starting
Fuel at the union (2) passes through the filter (1) and reaches the seat (14) where the needle (15), attached to the float (17), controls the fuel flow into the float chamber thereby maintaining a constant level. The float chamber is vented to the atmosphere through the vent (4) in the chamber. On opening the choke valve (7), fuel metered through the starter jet (12) passes into the emulsion tube (11) where it is mixed with air from the channel (16) and then enters the passage (6) further mixing with air from the vent (5) and reaches the valve chamber. From here, it is distributed via the two ducts (9) which lead into the main barrels (10) downstream of the throttles. On closing the choke valve, communication between the main barrels and the starting circuit is broken as well as communication between the two barrels due to the sealing action of the split bushing.

b - Idling
Fuel from the float chamber is metered through the idle jets (20) and mixes with air from the well (3) through the channels (19). Mixture through channels (21) reaches the idle mixture screws (22) and, when regulated by them, reaches the main barrels (10) downstream of the throttles (13).

c - Progression
On first opening the throttles (13), that is, when passing from idling to full throttle, mixture also reaches the two barrels (10) through the progression holes (23).

d - Acceleration
On opening the throttles (13), the lever (28) attached to the throttles spindle (27) by means of the rod (29) and spring (34) pushes the pump lever (32) which acts directly on the pump diaphragm (31) which is normally held out by the spring (30). The pump diaphragm then pumps fuel into the two main barrels (10) via two separate channels through the delivery valves (24) and the pump jets (26). On closing the throttle, the diaphragm returns to its Full position pushed by the spring (30), drawing fresh fuel from the float chamber through the inlet valve (25). The nuts (33) adjust the pump injection quantity.

e - Full throttle
At full throttle, fuel from the float chamber is metered through the main jets (37) entering the wells (36) and mixing with air metered through the air corrector jets (9). This mixture then passes through the channels (39) to reach the auxiliary Venturi (38) where it is further mixed with air drawn into the main intake and finally flows into the Venturi (40) to reach the barrels (10).

3. ADJUSTMENT

a - Idling
Always adjust the idle with the engine hot, screwing in the idle speed screw (a) to obtain a slightly higher idle speed than normal. Then adjust the mixture adjusting screws (b) until you find the most even running; remember that unscrewing them results in a richer mixture and vice versa. Then steadily unscrew the idle speed screw again until the normal idle speed is reached.

b - Accelerator pump
Adjust the accelerator pump injection quantity by fitting the carburettor to the special support with the proper gasket and connect the carburettor to a reservoir so that it is continuously supplied with fuel.
Put the two graduated measuring tubes, each having a capacity of 10 cc, under the drain pipes on the support in order to collect all the fuel pumped out. Open and close the throttle completely 20 times, with a few seconds’ break in between each time, and check that the amount of fuel collected in the tubes corresponds with the correct specification and is the same for both barrels. It not, adjust the pump delivery by resetting the nut and locknut fitted on the pump operating rod; remember that screwing the nuts up increases fuel delivery and vice versa. It there is any difference in volume between the two barrels, remove the pump jets and blow through them vigorously to correct this. Recheck until the correct setting is obtained and then ensure that the nut and locknut are retightened.

c - Float level
Check that the float has the actual weight which is marked on it, is undamaged and also tree to rotate on its pivot pin. Hold the carburettor cover vertically so that the float arm is in light contact with the needle and with the spring in the needle not compressed. In this position, check that both half-floats are at the correct distance from the float chamber cover measured to the top cover gasket fitted to it.

float	A
7298.1 7298.2	14,5 ÷ 15 16,5 ÷ 17

4. MAINTENANCE

To keep the carburettor in good condition, especially after operating faults have occurred, proceed as follows. Dismantle the carburettor, washing the components in fuel and blowing dry. Special care is needed with the jets, emulsion tubes, needle valve seat, fuel tilter and all the drillings in the carburettor body. Check the condition of all the components before reassembling and replace them wherever necessary only with new parts. When reassembling the carburettor, renew all the gaskets and O -rings.