Pre-tuning

Adjust the Idle-Air Screw so that the butterfly valve and the side of the carburetor barrel wall on the side of the Idle-Air Screw is 0.004 inches.  An appropriate size feeler gage should slide out of this opening with just a bit of resistance.  This will most likely give you a high idle, but will help you get to a good starting point for your dual setup.

Gently turn the Idle Mixture Screws clockwise until they bottom out.  Do not force them.  Then unscrew them counterclockwise 3 turns each.

Gently turn the Air-Bypass Screws clockwise until they bottom out.  Do not force them.  Then unscrew them counterclockwise a half turn each.

Install

If you want the fuel lines on the same side (front or back), you will want to make sure that the Fuel Inlet Pipes are where you want them.

Depending on your setup, it may be easier to hook up the fuel lines on the carbs before you install these in your VW.

Now install the carb on the intake manifold and the manifold on the heads.  Note that the cylinder covers often need to be modified to allow these manifolds to fit.  Paper gaskets work better than the OEM stamped steel gaskets and should be used on the mating surfaces between the carb and manifold and the manifold and the heads.

Install the air filter bases and linkage.  Do not connect the linkage to the throttle lever just yet.

Get your fuel lines hooked up and get everything read to fire up your engine.

Synchronization

After a warm up, use an airflow meter to measure the airflow over the barrel nearest the idle air speed screw of each carburetor.  Now adjust the idle speed screw until both carburetors match.  This is called carburetor synchronization and is vital for optimal performance.

Adjust the your linkage so that it connects to the throttle levers without moving them.

Tuning

Now it's time for the real carb tuning.  Start with the barrel over cylinder #3.  Turn the Mixture Screw in clockwise until the engine misses (bogs, snapping, popping, etc).  When you get to this point that cylinder is too lean.  If you can't get to that point then you may have a vacuum leak somewhere, a clogged jet or a dead spark plug.  After this lean state, runt the Mixture Screw counterclockwise until things smooth out and runs at maximum RPMs.  Now repeat with cylinder 4, 2 and 1.

If things are still rough, you may need to tweak things.  Popping in the carb or exhaust usually means it is too lean, while a slow unresponsiveness or black smoke suggests a rich state.  If you are maxing out your setting, you may need to rejet your carbs.

Now set you idle speed back down to 750 to 850 RPMs.

If everything up to this point is fine, it's time to check air flow at each barrel.  If they are all equal - you are done.  If your air measurements vary, they you and adjust the Air By-Pass Screws until they are all the same.  Air flow is increased by opening the Air By-Pass Screws and conversely, air flow is decreased by closing the Air By-Pass Screws.  The Air By-Pass Screws should be opened up no more than 1.5 turns from it bottomed out or you will get a too lean condition.

Install your air filters (with oil). Check that everything is in order and go for a ride.

Harvey's Carburetor Clinic

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Weber IDF Parts Breakdown

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Gene Berg

Weber IDF Jetting Chart

This Jetting Chart provides a base line starting point provided by CB Performance. Every engine responds differently, and final jetting changes are most often determined by the engine builder or tuner. Engine size, cam, cylinder heads, ignition system, compression ratio, exhaust system, type of fuel, the vehicle and type of driving are all key elements in final calibration. The selection of main gas and air correction jets is determined by venturi size. The venturi sizes listed below are the sizes supplied by Weber in new carburetors. The use of larger venturi will increase airflow, and larger main gas jets than noted below will be required.