The Weber DCNVA is a downdraft synchronous
double body carburetor. This means it operates as though it were two separate carburetors
that each supply two cylinders. In the Murena 1.6 it is build upon an inlet manifold which
is surrounded by coolant, thus preheating the mixture.
A mechanic fuel pump with return line is driven by an eccentric on the camshaft. A fuel
filter is located between the pump and the carburetor.
Start with the engine warmed up to operating temperature
and perform your standard ignition tune-up (timing, spark plug gap, valve clearance, etc.)
first. All of these things can affect the setting of the carburetor, which should be
adjusted last, if at all! After being properly set, the carburetor should rarely need
further adjustment. If you've got a timing light and a dwell meter, you can verify the
ignition components independent of the way the car is running. When it's warm, shut the
motor off and remove the air filter.
Of course, it helps if the carburetor is in good
mechanical condition as well. But you can consider a rebuild once you have gotten things
working first!
If you experience uneven idle, hunting, or an idle that
changes (rises or falls) as the engine's temperature climbs or drops, you probably have
vacuum leaks. The most serious fault on most old carbs is wear in the throttle shaft area.
To test for this, spray some carburetor cleaner on the outside of the throttle shaft;
carburetor cleaner is non-combustible, and if the engine speed drops, it means some of
this is getting into the air stream from outside the carburetor. You may also have leaks
from the manifold, from tubing such as the vacuum advance line to the distributor, or from
other places (concealing headlights!!); the carb cleaner trick works well for locating
those leaks as well.
You might want to read the article Reading
Your Spark Plugs before you do anything else.
Installing an inlet-manifold and two double carburetors
(Weber 36 DCNF 17/18) taken from a Bagheera S or X is relatively easy and looks very
attractive. However if an engine performs well, it is generally considered useless to
change the carburetor(s) if nothing is being done about the engines ability to 'breathe'.
So if you haven't changed the camshaft, or taken even more drastic measures like enhancing
displacement, I would strongly advise against installing Bagheera (or any other) carbs. It
will probably just cost you fuel and agony.
A donor-camshaft that would be suitable for the
Murena 1.6 engine, also comes from the Bagheera (or Simca 1100 TI or Simca 1308GT). The
lift of this shaft is slightly more (than the standard camshafts 9.07 mm for inlet and
outlet valves). The downside is that AFAIK the camshaft can only be replaced with the
engine removed...
Please also read Adjusting double carburetors by Andy Owler
| Idle speed (rpm) |
950 ( ± 50) |
| CO-percentage (%) |
1.0 - 2.5 |
| Fuel-pump pressure (kPa) |
20-30 |
| Fuel octane (RON) |
98 (unleaded) |
| Capacity fuel-tank (ltr.) |
56 |
| Fuel consumption (ltr/100km) |
at 90 kmh
6.0
at 120 kmh 7.3
city
10.5 |
Jetting
| Venturi (mm) |
29 |
| Auxiliary venturi |
4.5 short |
| Main jet |
150 ( ± 5) |
| Air nozzle |
175 ( ± 10) |
| Idle jet |
40 - 45 |
| Idle air nozzle |
165 ( ± 10) |
| Emulsion tube |
F 46 |
| Pump jet |
40 long |
| Float weight (gr) |
14.5 |
| Float level height (mm) |
42.5 ( ± 0.25) |
| Float valve needle |
175 |
| Mechanic choke opening |
8 - 9 |
| Pneumatic choke opening |
4.25 ( ± 0.25) |
| Carburetor to manifold |
20 |
| Fuel pump to engine |
20 |
Before:
Check valve clearance, ignition timing and sparkplug gap. Warm the engine up to operating
temperature. Remove hose of carter ventilation from header cover.
Procedure:
Use throttle screw 3 (figure 1) to adjust idle to 950 (± 50) rpm
Adjust mixture screws 1 and 2 to raise rpm as much as possible
After every raise re-adjust to 950 rpm using screw 3
Now turn screws 1 and 2 in one at a time by equal amounts, to lower revs by 25 rpm
CO reading should now be 1 - 2%, if not repeat last step
Adjust to 950 (± 50) rpm using screw 3
After:
Refit hose
Figure 1
Procedure:
Close choke valve completely
Adjust screw B (figure 2a) until the situation in figure 2c is reached
 |
 wrong |
 correct |
| Figure 2a |
Figure 2b |
Figure 2c |
Before:
Remove the top off the carburetor. The top is held on with six screws and a clip on the
choke linkage. Use the proper sizes of screwdrivers and take your time. You may be able to
buy all the little pieces for a Weber but you'll not like their cost!
Procedure:
Hold the top of the carburetor in a vertical position so the float barely touches the
float valve
The the distance A (figure 3) should be 42.5 (± 0.25) mm
The the distance B should be 52 (± 0.5) mm, adjust by bending 2
Figure 3
The manual choke is equipped with a vacuum box which
opens the choke valve a little when the engine runs.
Procedure:
Completely close the choke valve
Start the engine
A 4.25 mm round feeler gauge should fit beside the choke valve now; if not replace vacuum
box
The fuel pump is bolted to the engine. Underneath the
top cover (held by a single screw) is a filter (clean every 30,000km).
Testing pressure:
Remove fuel line from inlet and clamp it (for safety)
Clamp return line
Attach pressure gauge to outlet and engage starter engine
Pressure must be between 20 and 30 kPa
If any of the above information is incorrect:
please a mail to: matra@go.to
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