Adjusting Twin Weber DCNF Carburators

 

Especially interesting subject when fitting twin carburettors on a Murena (not standard). But before one starts adjusting the carburettors there are some pre-checks to be done. The decriped adjustment is based on measurment of vacuum levels on each cylinder, adjusting by sound is possible in a similar way.

Pre-checks

  • Valve clearance
  • Spark plugs
  • Air filter
  • Fuel-pump
  • Ignition timing



A Weber carburettor system will not be right, unless it's synchronized to ensure that each carburettor is doing exactly the same as the next - the name of the game is perfect cylinder tuning. The synchronization procedure can either be a breeze or a nightmare, depending on whether you have a well-designed linkage system or not. The secret to a good linkage set-up is that it must allow independent adjustment of each carburettor without affecting all the rest as you go through the procedure. Here again, if someone tells you they're absolutely impossible to synchronize, you might study his linkage. Chances are, it's incorrect and he's fighting himself.

Synchronization-linkage

  • Turn the four adjusting screws (A) till their max. position without tightening (locking with) the nut.
  • Turn the four adjusting mixture-screws (B) carefully till their max. position, then turn them two revolutions back.
  • Release idle-stop-screw (C), allowing throttles of left carburettor (flywheel-side 1&2) be placed in fully closed position.
  • Turn synchronisation-screw (D) outward.
  • Push lever (E) pushing spring of catch (F) together and closing all throttles.
  • Keep lever (E) at this position and tighten synchronisation-screw (D) until it just touches lever (E); throttles of right carburettor must remain in fully closed position.
  • Turn the idle-stop-screw (C) until it touches, now turn one revolutions further.
  • Mount the choke cable and make sure that it functions correct with low friction.
  • Mount throttle cable and make sure that it functions correct with low friction. Make sure you can not depress the gas pedal too much as causing a bent synchronization-linkage on your carburettors. A simple stopper underneath your throttle-pedal can prevent this and could be needed.

Synchronization running engine

First of all make sure your carburettors are well cleaned and check jetting if unclear. Idle speed has to be set at 950 rpm, but first it is more important to get the engine started and running. Make sure to first get the engine on a  good temperature (oil 85°C should be enough), if you start adjusting with a cold engine it will not run as nice in normal hot conditions.

  • If your carburettors are equipped with vacuum-connections then you have the best way of getting them synchronized by connecting 4 separate vacuum meters for measuring the levels. If not another vacuum meter has to sought which can be placed on top of each barrel (cylinder).
  • Start the engine, use idle-stop-screw (C) to get idle speed at 950 rpm. Now forget all about idle speed and continue synchronization.
  • Check if the vacuum levels of cylinder 2 & 3 are the same; correct if needed by adjusting synchronization-screw (D).
  • Now check if the vacuum levels in cylinders of one carburettor (1&2 - 3&4) are the same; correct by turning screw (A) of the barrel with the highest vacuum level. When all levels are the same secure the positions by turning tightening the nut. If needed use again synchronization-screw (D) to get vacuum levels the same for both carburettors.

Adjustment of idle speed

  • Make sure the engine is hot and adjust idle speed again at 950 rpm with idle-stop-screw (C)..
  • Increase idle speed as high as possible by turning mixture-screw (B) of each barrel; use idle-stop-screw (C) to stay as close as possible to 950 rpm.
  • Now decrease idle speed again in steps of 15-20 rpm by turning mixture-screw (B) of each barrel.
  • Use idle-stop-screw (C) to set idle speed at 950 rpm.
  • CO-percentage should be between 1.0 and 2.5 (%).

Once the unit is synchronized and the idle mixtures are set to give you the smoothest possible idle, you can hang up your screwdriver till next spring, because now it's set! And when it's set, it's set!!
 

Valve clearance

Adjustment Cold (ambient) Hot (oil 90°C)
Intake valves 0.25 mm 0.30 mm
Exhaust valves 0.30 mm 0.35 mm

Spark-plugs

Type: Marchal, GT34.5 HA
Electr. distance 0.60 mm

Air filter

Clean the air filter and PVC-system. Fit new filter-element or complete filter if needed.                  

Fuel-pump

Clean filter or fit a new one to make sure the fuel-delivery is without problems. Check the fuel delivery pressure. Remove fuel line from inlet and clamp it (for safety). Clamp return line and attach pressure gauge to outlet. Engage starter engine and measure pressure. Pressure must be between 20 and 30 kPa.

Ignition timing

Ignition sequence of the Talbot 1.6 engine is 1-3-4-2. Typical for a French brand the cylinders are numbered starting at the flywheel side with nr. 1 ending at the distribution side at cyl. nr. 4. Use a stroboskopisch-light to check / correct the timing at 11°  before TDC at idle speed 950 rpm, without vacuum advance. Also check the vacuum advance on good function: Min. - Max. (°/mmHg) 1/140 - 6/281. Last the centrifugal mechanical advance: Min. - Max. (°/rpm) 2/1200 - 12/5000. Note that the values given for vacuum/centrifugal advance are not absolute values but show their additional influence

Torque-list

Spark plugs 30 Nm
Bracket (clamp) distributor 10 Nm
Bracket distributor - cyl.block 20 Nm
Carburettor-manifold 20 Nm
Airfilter-carburettor  2.5 Nm