Zipper's Performance G/D Carbs - Manual

ThunderTech Products - 6655-A Amberton Drive - Elkridge, MD - 21075 - PH: (410)579-2929 - Fax: (410)579-2835
READ INSTRUCTIONS FIRST!
OFF IDLE RESPONSE &
LOW TO MID-RANGE POWER
MID-RANGE TUNING
MID-RANGE AND TOP END
Jetting instructions continued on
the back of this page!
WE STRONGLY RECOMMEND THIS!
AIR FILTER
ACCELERATOR PUMP
INTERMEDIATE (LOW SPEED) JET
IDLE MIXTURE
THE AIR BLEED
MAIN JET
This is the first issue to
talk about because when making
changes here, it can change jetting
requirements. The foam filter restricts
airflow. We have found the best alternative
is a high flow K&N filter. It will give you
the performance you demand while doing
a better job of filtering incoming air than
a foam filter can. We stock these for all
S&S applications and also manufacture
alternative systems.
We recommend turning off
the accelerator pump when tuning the
intermediate circuit. It doesn’t supply fuel
at steady speeds and can mask problems
of a lean condition. This is evident with
plug readings that look good or even
rich and the next week you may have
two burnt pistons. To disable the pump
when tuning a high performance engine,
turn the adjusting screw clockwise until it
contacts the pump arm. CAUTION: DO
NOT OVERTIGHTEN. When tuning is
complete you can increase pump travel
to aid cold starts. For best results, keep
the volume low on this circuit. Clearance
should be .020” to .030” @ adjustment
screw
This is the next step after increasing
the intermediate jet. Disregard the factory
setting on this circuit. When you drastically
increase the intermediate jet, this mixture will
be rich. In some cases, we find the correct
setting may be only 1/2 turn out from the seat.
NOTE: Never richen this circuit to
compensate for lean conditions on the
intermediate circuit.
rpm before fuel is drawn from the Main
Jet, in order to correct a rich fuel overlap
from the Low Speed to Main Jet circuit
transition.
Now you can tune the Main
Jet in at a higher rpm level, when the
engine will want (and need) more fuel.
This circuit can also help you manipulate
problems you may be experiencing
with certain hydraulic lifter camshafts,
exhaust systems and other components.
These components might not allow the
engine to carburet very well in the middle
of the scale. The air bleed allows you
to remove some of the fuel volume when
the requirement is low. Other circuits are
used to compensate for any demand.
Keep a good selection of these on hand
to tune for best power
Now that you have a handle on
the air bleed and intermediate circuits, you
can use the main jet to increase the power
in the mid-range and top-end. When
you’re changing jets, keep in mind the
relationship of the air bleed to the main jet.
Chances are that if you had good
power at peak rpm before you installed
the ThunderJet, you will probably
drop two sizes on the main jet after
installation.
Check chart for starting point.
When engine efficiency has been increased
via improved airflow, compression, larger
displacement, etc., the volume of fuel in this
circuit must be raised. You increase fuel to
increase power but also to prevent engine
damage. With a wide variety of engine
sizes, components and exhaust systems,
we can only offer a broad recommendation
of two sizes larger than stock setting,
some engines will require substantially
more. Call us for special jets if you need
them. This is truly a hidden treasure.
TUNE IT CORRECTLY!
The Air Bleed is an air passage
connected to the Main Jet emulsion tube
that allows air to mix with fuel before it is
pulled into the carb throat. You may find
now that you’ve increased the Low Speed
jet size, the engine responds better off idle,
but now has a stutter or hesitation (flat spot)
as the rpm rises to a level that recognizes
the carb’s Main Jet. With the stock sized
(.042) Air Bleed, fuel starts being pulled
through the Main Jet at a relatively low rpm,
causing this rich condition. By increasing
the Air Bleed size, you are in effect raising
the rpm level needed to draw more fuel from
the Main Jet. Simply, it allows for increased
THE AIR BLEED CONT’D
After years of development on
S&S carburetors, we have noticed that
this circuit is the most misunderstood
in the overall picture of the fuel curve.
TOP END
THUNDERJET
This circuit is doing a lot of
demand work. Because of its built in
air bleed system, it becomes active at
approximately 4000-4200 rpms, while
the engine is under a controlled load.
Use ThunderJet to tune (with the main
jet) for maximum power increases at high
rpm. If you are tuning on a dyno, make
small jet changes to see the difference
it makes. You will be surprised at the
power increases.
S&S G/D I
nStallatIon anD
t
unInG
I
nStructIonS
These products are legal for sale in the State of California only for
racing vehicles which may never be used upon a highway.