It is important to verify all linkage and levers are
installed without binding and the linkage opens to full throttle and
closes to the Idle Speed Screw. The number one and two reasons for
tuning errors are improper linkage installations and over tightened
linkage nut, causing a binding in linkage assembly.
CALIBRATIONS MAY VARY DUE TO REGIONAL FUELS AND STATE OF
ENGINE TUNE AND PERFORMANCE. POOR RUNNING DOES NOT ALWAYS MEAN A BAD OR
POORLY ADJUSTED CARBURETOR. AN ADVANTAGE OF THE WEBER CARBURETOR IS ITS
EASE OF ADJUSTMENT AND TUNING.
Start set up by confirming Carburetor base line
settings. Do not depend on the existing settings.
All settings are done with choke disengaged or
warmed up so that the choke is fully opened and disengaged. This is
done on automatic choke carburetors by first opening the choke butterfly
by hand and inserting a wood block or wedge of some kind to hold open
while the linkage is cycled (linkage operated through its full movement )
to clear the choke cam. (You will hear a metallic click as the choke cam
is released. You can check the choke fast Idle screw under the choke
assembly to confirm that it is not in contact with the choke fast idle
Set the Idle Speed Screw by backing out the Idle
Speed Screw until it is not in contact with the throttle stop lever. Cycle
or Snap the linkage again to be sure that the linkage comes to close
without any assistance. (Checking for linkage bind) Now bring screw back into contact with the lever and
continue to open or screwing in, no more than 1/2 turn in, maximum.
Set the mixture screws by turning each screw in
until it lightly seats. DO NOT
FORCE OR BIND AS THIS WILL CAUSE DAMAGE TO THE SCREW AND IT’S SEAT IN
THE BODY OF CARBURETOR. Back
out the screw 1 full turn.
BE SURE TO FOLLOW THE NEXT INSTRUCTIONS IN THE
PROPER SEQUENCE, DEVIATION WILL CAUSE THE CARBURETOR TO NOT FUNCTION TO
ITS IDEAL SPECIFICATIONS AND MAY NOT PROVIDE THE PERFORMANCE AND FUEL
ECONOMY AS DESIGNED.
Start the engine, the engine will run very slowly
more like a tractor. As long as the engine stays running idle speed is not
important at this point.
The first thing, do not set “up” the idle
speed, set the Idle mixture screw to lean best idle setting. First,
turn in the mixture screw until the engine dies or runs worse, then back
out the screw (recommend turning ¼ at a time). The engine should pick up
speed and begin to smooth out. Back out 1/4 turn more, or until the screw
does nothing or runs worse then turn back to the point where it ran its
Use your ear, not a scope or tuning instruments at
this point. You want to tune the engine by sound. Adjust to best, fastest
and smoothest running point.
Now that the mixture screw is at its best running
location, you can adjust the Idle speed the screw. The idle speed screw
will be sensitive and should only take ¼ turn to achieve the idle speed
Check and set idle to your driving preference. Put
the car in gear and apply
slight load, (AC on) and set the Idle as you like it. Don’t set it too
high, as this will cause causes excessive clutch and brake
wear. The Idle only needs to be 650 to 800 RPM with light load or
Recheck timing and vacuum hook ups. Recheck mixture
screw to lean best idle again. If all is still best and smoothest idle
then confirm and note the final settings.
To confirm settings with the engine running. Start
by screwing in the mixture screw and count the number of turns it takes to
bottom out and note if the engine dies. If Idle Mixture screws are with in
½ turn of base line setting then all is well. Also check the speed screw
and note how many total turns from initial contact. You may have opened
(turned in) the speed screw. Your final setting should be under 3/4 turn
in. Reset the screws (back in) to the best final settings (Per your notes)
and go on a test drive. If the settings are other than described then you
may want to recalibrate the Idle circuit (low speed circuit) to your
engines needs. This is done by following the rule of thumb BELOW.
Rules for low speed calibration
If the mixture screw is more than 11/2
turns out turns then the Idle jet is too lean (too small). When the
mixture screw is ½ turn or less, then the Idle jet is too rich (too
large). These assumptions are based on the fact that the speed screw
setting is not opened more than 1/2 turn in. If the speed screw has to
be opened1/2 or more turns then this is also an indication of a lean
condition usually requiring greater change. At times it may appear to be
showing signs of richness or flooding it is really a lean condition. Please
understand the need to keep throttle plate as near to closed as possible
so as not to prematurely expose the transition holes. This is what
causes the visible rich condition, and confirms the need to increase the
jet size. JET KITS are available if needed.
the idle speed screw set at no more than 1/2 turn in after contact with
the stop lever; and the best idle occurring with the idle mixture screw
set at 1 1/2 turns from lightly seating, indicates the need for a larger
Idle jet. Achieving the best idle at less than 1/2 turn indicates the need
for a smaller idle jet.