My white '74L came with a model 3310S Holley 750 vacuum secondary
carb. Being a 4160, it doesn't allow for changing jets on the
secondary side so I purchased a conversion kit and pulled the carb
off. The previous owner had it jetted very rich but it still bogged
like it had a bad accelerator pump. I verified the pump is squirting.
After setting the floats and adjusting the idle mixture, I measured
the idle vacuum at 18". I'll be hooking up a wide band O2 sensor to
tune the carb but before I bolt it back on, does anyone have a
recommendation on where to start on the jets, accelerator pump cam
and power valve?
Except for the Holley carb, Edelbrock Performer 4V intake (with 1" 4 hole
spacer) and Hall big bore headers and mufflers, the engine is a stock
long block 351C-4V with open chamber heads and 4 degrees retarded CJ
cam. The car has less than 17K miles and still has the OEM dual point
distributor. The vertical spark retard port on distibutor has been
capped off and the advance port connected to carb vacuum (not manifold
vacuum). I've not yet verified initial advance. I'll install a new set
of plugs once the O2 sensor is wired up.
Out of the box, the O-3310S comes with 70 main jets, #21 secondary
plate (equivalent to 75 jets), 0.031" pump discharge nozzle, a 6.5"
power valve and a black secondary spring. From weakest to stiffest,
the secondary springs are:
White - Weakest
Yellow (Short Spring)
Yellow
Purple
Plain (Steel grey)
Brown
Black - Stiffest
I have a reference with data for a 350 SBC and vacuum secondary that shows
the RPM at which the secondaries begin to open and the RPM at which they
are fully open as:
Spring Color RPM to open RPM at full open
Yellow (short) 1620 5680
Yellow (long) 1635 5750
Purple 1915 6950
Plain 2240 8160
Brown 2710 8750
Black 2720 NFOAMAF
Note: NFOAMAF = not fully open at maximum air flow
IIRC, the open chamber 4V's with CJ cam peaked between 5400 and 5600 RPM,
so I'd expect to shift under 6000 RPM. I was planning on starting with a
plain spring but if the above information is accurate, it would never open.
Dan Jones
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