Dellorto Carburetor DHLA Service Info
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 |
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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.
Dellorto DHLA carburetor and parts.
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