is most important to verify all linkage and levers are installed without
binding and the linkage opens to full throttle and is allowed to close to
the Idle Speed Screw. This is the number one and two reasons for tuning
errors, improper linkage installations and over tightened linkage nut,
causing binding in the linkage assembly.
Individual Runner carbs, IDF, have individual Idle jets and mixture screws
for each barrel. They also have an additional air bleed screws and lock
nuts. This is not used for idle adjustment or idle quality. The
settings for these screws should be closed.
Standard IDF Settings:
screw ¼ to ½ turn in after contact with lever maximum.
Screw 1 turn out from lightly seated.
Best Idle” Procedures
confirming the linkage allows the throttle lever to seat against the Idle
Speed Screw. Back off the Idle Speed Screw, then turn the screw in until
it contacts the throttle lever and turn it in ½ turn. Turn in the Mixture
Screw in until it “LIGHTLY” seats, then back it out 1 full turn.
Loosen the 8mm wrench size nuts on the “air bleed” screws, turn in the
air screws until it seats then tighten the nut.
engine, it will run slow and like a tractor. As long as it will stay
running, the idle speed is not important at this point.
in the mixture screw until the engine runs worse, then back out the screw
¼ turn at a time. The engine should start to smooth out. Continue to back
the screw out ¼ turn at a time until the screw does nothing or runs
worse. Then turn it back in to the point where it ran best. You want to
tune the engine by sound. Adjust each mixture screw to the best, fastest
and smoothest running point. Do this procedure with each mixture screw.
Now you may
adjust the Idle Speed Screw. It should be sensitive and only require ¼
turn in or out to achieve the idle speed you like.
are commonly used in pairs, this makes the synchronization important, be
sure to bring the high flowing carb down to the low flow carb. Then bring
them both up to “proper” Idle speed. The Idle Speed Screws are not
opened more than ½ turn in.
synchronizing multiple carbs, reconfirm steps b. c. & d.
“Simple Rules for Calibration”
your mixture screw is out more than one turn like 11/2 turns then your
idle jet is too lean, go up one half size on the Idle jet.
you mixture screw is not out one full turn, something like only 1/2 turn
out from seated then your Idle jet is too rich, go down one half size on
the idle jet.
is all based on the important fact
that your speed screws are not open
more than ½ turn if they are then that is also an indication that you
have a lean Idle circuit. You
are cheating by opening the throttle plates and exposing additional
progression holes in the transition.
The pump by-pass valve is designed to allow fuel into the pump circuit and
when depressed, by-pass a percentage (hole on the side of the valve) of
the fuel delivered to the accelerator pump nozzle/jets. We have included
the zero by-pass valve in our jet kit. This will deliver all of the
available fuel to the engine and not by-pass fuel back to the fuel bowl.
This will increase the duration and volume of the pump shot with the
original pump jets. To decrease the duration and increase the fuel volume
we have included in our jet kit the larger 55 pump nozzle/jets. 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 carburetor.