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JETTING (MAIN JETS):
Due to varied applications that a universal performance carburetor will work with, a few tips on jetting are provided to help you
understand their purpose.
1. Out of the box jetting is extremely close for most applications.
2. Carburetors are calibrated at 70
at sea level. Decrease the jet size primary and secondary, one number for every 2000 ft.
increase in altitude.
3. Holley® jets are broached, flowed, and stamped according to flow rate.
NEVER
drill jets, as this seriously alters flow
characteristics. Stamped numbers are reference numbers and
DO NOT
indicate drill size.
4. In most cases it will be
unnecessary
to increase jet size more than four numbers greater than out of the box jetting.
Exceptions could arise when the carburetor is mounted on a very large volume, plenum-ram manifold.
5. Spark plugs provide the best indication of proper jetting. Consult an ignition manual for proper reading of spark plugs.
POWER VALVES:
The number stamped on a power valve, such as 65, indicates the manifold vacuum below which the power valve is operational.
In this case, all manifold vacuums below 6.5” Hg, the power valve is operating. Generally, a 65 power valve is sufficient for most
high performance applications that have a manifold vacuum of 12” Hg or higher. However, some problems can result with
radically cammed machines equipped with automatic transmissions. These vehicles often “idle” at 2000 rpm, approx. 6.0” Hg.
At this point the main nozzles are starting to feed and richen the mixture (supplied by the power valve) and the engine will
probably “load up”. To correct this problem, install a 45 or 35 power valve. If the engine has a manifold vacuum of 12” Hg or
less, a good way to determine power valve size is take the manifold vacuum at idle and divide that number by two. The answer
is the power valve size. This will provide idling and proper fuel flow under wide open throttle conditions when manifold vacuums
seldom rise above 1” Hg.
CARBURETOR SPECIFICATIONS:
CARBURETOR #
PRIMARY
METERING
JET
PUMP
DISCHARGE
NOZZLE
POWER
VALVE
0-7448
61
0.031
85
0-4412C
73
0.028
50
0-4412S
73
0.028
50
