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 Carburetors, DCOE, 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 DCOE 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.
Start the 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.
First, turn 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
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.
These Carburetors are commonly used in pairs, this makes the
synchronization important, be sure to bring the high flowing Carburetor
down to the low flow Carburetor. Then bring them both up to “proper”
Idle speed. The Idle Speed Screws are not opened more than ½ turn in.
After synchronizing multiple Carburetors, reconfirm steps b. c.
“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.
Tuning & Adjustment
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
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 Carburetors should be bench adjusted to same setting.
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 carburetor,
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
preliminary lean best setting of idle. Check carburetors for
synchronization. Commonly done by checking lead or front barrel of each
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 Carburetor. 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 Carburetors up or down the
same amount and re checking for synchronization.
last check of lean best (smoothest running position) idle on all mixture
screws one last time.