>I'm running a TR8, Offy/JWR Dual Port manifold
Hey, I used to have an Offy/JWR Dual Port. Haven't seen it in
a while though :-)
>This made me wonder whether I'm not getting a lean mixture
>from the secondaries since they would be opening up and adding
>significantly to A/F ratio under these conditions. I've heard that you
>"MUST" convert the 390 to the adjustable metering plates (4150?),
>is this why?
Yes, you're going lean. The 8007 390 is a 4160 Holley which means
it has a plate with fixed size orifices for metering. These plates are
replaceable with similar plates of differing orifices but they cost
much more than jets. You also have to disassemble the carb
each time you want to change the mixture. If you have to try more
than a couple of plates (and you probably will), you'll want to convert
to the 4150 secondary metering block which has removable jets.
However, you'll still have to pull the fuel bowls off to change the jets,
unless you install the Holley quick change fuel bowls. They have a
threaded plug that allows access to the jets without disassembly of
the carb. Highly recommended.
One drawback is the quick change bowls are only compatible with
center hung floats and the 8007 has side hung floats. If you plan
on hard cornering, you'll want the center hung floats since they are
much less susceptible to cornering loads and fuel slosh. Side hung
floats have a pivot at one side of the bowl (the pivot pin runs
front-to-back in the carb, hence "side pivot") and a long float that
extends to the other side. With that arrangement, mechanical
leverage is significant and a slight slosh at the far tip can cause the
entire float to move. With the center pivot floats, the pivot pin runs
side-to-side and the mechanical leverage is much lower, so slosh in
the corners doesn't lift or drop the floats as easily. You can change
from side hung to center hung floats by simply by changing bowl and
float assemblies. Holley will be glad to sell you the parts or you can
pick up a core carb at a swap meet.
The 8007 also comes with a 6.5 power valve which you'll need to
replace to match your engine's vacuum characteristics. Check the
vacuum at idle (warmed up and in gear if the vehicle is equipped with
an automatic transmission) and at various *steady* cruise speeds. The
power valve should be picked to open at 1 1/2" to 2" Hg below the
minimum steady vacuum reading. For instance, if 11" Hg is the
minimum observed, pick a #90 power valve (trips at 9.0" Hg).
Note that the power valve only determines when the additional fuel is
added. The amount of fuel added is determined by the diameter of the
power valve channel restrictors (PVCR's). To properly tune the power
valve circuit to provide best power and best fuel economy, the PVCR
diameters will probably need to be altered. Unfortunately, this means
making permanent modifications to the metering block, either drilling
the passages larger or epoxing them shut and re-drilling smaller. IIRC,
you already had Ted's carb guru work your carb over and one of the
mods was to enlarge the PVCR diameters. If you want to set a carb
up to cruise at a stochiometric fuel-air ratio (high vacuum, power valve
closed, best fuel mileage), and have the ideal mixture for maximum
power at wide open throttle (power valve open), the PVCR's should be
approximately one half the diameter of your main jet orifices when you
are all done jetting. That ratio of orifice size to jet size will give you
around a 25% increase in fuel flow, the difference required to go from
stochiometric to best power fuel air mixture (for pump gas). Actually,
you can go a little leaner than stochiometric for best fuel economy but
you have to be careful to retain driveability.
You may also need to change the vacuum diaphragm spring which
controls the opening rate of a secondaries. Holley makes a spring
assortment kit (P/N 20-13) for tuning purposes. The color-coded
springs run from light tension to heavy:
White - Weakest
Yellow (Short Spring)
Yellow
Purple
Plain (Steel gray)
Brown
Black - Stiffest
A lighter spring will allow the secondary throttle plates to open more
quickly. Likewise, a stiffer spring will delay the secondary opening.
To make spring changes easy, Holley makes an inexpensive quick
change kit (P/N 20-59). It consists of a two-piece secondary
diaphragm housing cover which allows spring replacement without
housing disassembly.
I did the quick change fuel bowls, the quick change secondary kit
and added power valve protection to the Holley 735 I run in my
Pantera and it makes the Holley much more user friendly. Still
not as nice as a Carter AFB but tolerable.
Speaking of the Carter AFB, are you interested in trying my Carter
400 when I get it back? It's currently installed on a co-worker's
'66 Mustang for testing purposes but I'll be getting it back sometime
soon. It might be interesting to see how it stacks up against the
Holley 390.
Dan Jones
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