Tuning & Adjustment
Most Critical! Be sure for initial carburetor set up all air by-pass
screws should be in closed position. These are not commonly used in
standard carburetor adjustment.
Set the idle speed screw at ¼ to ½ turn in after
contact with throttle lever. When doing multiple carburetors all linkage
should be disconnected. All carbs should be bench adjusted to same setting.
Set the idle mixture screw to one turn out form
lightly seated. When checking the seated position to make only light
contact with seat, aggressive seating will damage needle and seat of
Start engine as long as engine starts and runs do
not set up idle speed first. First adjustment if possible should be to find
smoothest idle with each mixture screw on all carburetors. Some prefer to
do one barrel of each carburetor then come back and do the second barrel
After preliminary lean best setting of idle. Check
carburetors for synchronization. Commonly done by checking lead or front
barrel of each carburetor.
You will always want to bring high flow carburetor
down to match the low flow carburetor. If this cannot be done you will need
to recheck bench assembly for binding throttle in high flow carb. Once you
have matched both carburetors you will need to set the idle to the desired
idle speed setting. This will be done by adjusting both carbs up or down
the same amount and re checking for synchronization.
Make one last check of lean best (smoothest running
position) idle on all mixture screws one last time.
Best idle should end up with the mixture screws at
or near one turn off seat. Check rule of thumb for idle jet selection in
Redline Weber basic idle adjustment instruction.
Pump By-pass Valve:
The pump by-pass
valve is designed to by-pass a percentage of the fuel delivered to the
accelerator pump nozzle/jets. The “zero” pump by-pass valve delivers all of
the available fuel to the engine and does not by-pass back to the fuel
bowl. This will increase the pump shot duration and the volume of fuel with
the original pump jets. To decrease the duration and increase the fuel
volume, you may increase the pump nozzle/jet size. This is for maximum fuel
delivery from the accelerator pump circuit. The accelerator pump by-pass
valve is located in the bottom of the fuel bowl; one is required per