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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
Redline IDF Carburetors Low Speed Circuit Tuning
KYMCO Webber IDF Adjustment
Weber IDF Parts Breakdown
Item # Quant Desc Part # Item # Quant Desc Part # 1 1 Air Horn 52848.003 32 1 Pump Cover 32486.085 2 1 Pump Control Rod 61285.042 33 1 Pump Diaphragm 47407.207 3 1 Carburetor Top Cover 93.0078.05 34 1 Fixing Pin 58445.001 4 2 Stud 64955.108 35 1 Pump Diaphragm Spring 47600.250 5 1 Pump Rod Spring 47600.252 36 2 Vacuum Tube-Off Cover 61015.005 6 1 Carburetor Main Gasket 41705.085 37 2 Air By-Pass Screw 64750.029 7 1 Fuel Filter 37022.002 38 2 Air By-Pass Screw Nut 34705.001 8 1 Fuel Filter Gasket 41530.024 39 2 Throttle Plate 64005.126 9 1 Fuel Filter Cover 61002.010 40 4 Throttle Plate Fixing Screw 64520.027 10 2 Pump Demand Valve 64290.016 41 1 Air Horn Gasket 41655.020 11 4 Pump Jet Gasket 41530.012 42 1 Throttle Return Spring 47610.035 12 2 Pump Jet 76210.050 43 1 Throttle Lever 45034.109 13 1 Pump Spill 79701.055 44 2 Choke Tube Fixing Screw 64830.002 14 2 Auxiliary Venturi 69002.450 45 2 Choke Tube Fixing Screw Nut 34715.010 15 2 Venturi (Choke Tube) 71506.280 46 2 Fixing Screw 64700.013 16 2 Stud 64955.114 47 2 Main Jet 73401.115 17 1 Throttle Lever 45034.110 48 2 Emulsion Tube 61450.031 18 2 Throttle Spindle Nut 34710.003 49 2 Air Corrector Jet 77401.200 19 2 Lock Washer 55520.004 50 2 Emulsion Tube Holder 52580.006 20 2 Spring Ring 55530.005 51 1 Float 41030.008 21 2 Race Bearing 32650.003 52 1 Float Pin 52000.001 22 2 Idle Jet 74405.050 53 1 Needle Valve 79511.175 23 4 Idle Jet 'O' Ring 41565.002 54 1 Needle Valve Gasket 83102.100 24 2 Idle Jet Holder 52570.006 55 5 Top Cover Fixing Screw 64700.005 25 1 Throttle Stop Screw Spring 47600.007 56 1 Fuel Inlet Pipe 10525.023 26 1 Throttle Stop Screw 64625.015 57 1 Pump Adjustment Rod 12750.126 27 1 Pump Rod Spring 47600.251 58 1 Spring Clip 32610.003 28 2 Mixture Screw 'O' Ring Cap 58000.006 59 1 Pump Rod Nut 34725.008 29 2 Mixture Screw Spring 47600.062 60 2 Fuel Pipe Gasket 41535.003 30 2 Mixture Screw 64750.003 61 1 Threaded Cap 61002.022 31 4 Pump Cover Screw 64700.004 62 1 Throttle Shaft 10005.243 Original Specs 40 IDF 70 Main Jets 115 Emulsion Tubes F11 Air Corrector Jets 200 Venturis 28mm Idle Jets 050 Aux Venturis 4.50mm Pump Jet 050 Needle Valve 1.75mm Pump Exhaust Jet 055
CB Performance - Weber Carbs - Dual IDF Kits
CB Performance - Upgrading Dual IDF Carburetor Linkage
Triadwest - 914 Carburetor Linkage - Weltmeister design
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.
WEBER 40 IDF Factory
SizesSingle
CarbDual
CarbsAIR CORRECTION 2.00 1.60 2.00 EMULSION TUBE F-11 MAIN JET 1.15 1.45 1.30 PUMP JET .50 .80 .50 IDLE JET .50 .65 .55 VENTURI 28mm NOTE: Drill 2mm Hole in each Throttle Valve of Single Carb. WEBER 44 IDF Factory
SizesSingle
CarbDual
CarbsAIR CORRECTION 1.75 1.65 2.00 EMULSION TUBE F-11 MAIN JET 1.35 1.60 1.55 PUMP JET .50 .80 .50 IDLE JET .50 .65 .60 VENTURI 36mm NOTE: Drill 2mm Hole in each Throttle Valve of Single Carb. WEBER 48 IDF Factory
SizesSingle
CarbDual
CarbsAIR CORRECTION 1.80 1.80 2.00 EMULSION TUBE F-2 MAIN JET 1.50 1.70 1.75 PUMP JET .50 .80 .50 IDLE JET .55 .65 .60 VENTURI 40mm NOTE: Drill 2mm Hole in each Throttle Valve of Single Carb.
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