Bob,
It would seem to me that there are 2 things at work here. Power Valves and
the Vacuum Secondary Diaphragm Spring.
The power valve (unless I am very rusty) controls the flow of fuel to the
main fuel circuit only as an on or off switch. While an engine that dips
below the value of the power valve at Idle will run "fat" and usually floods
out and dies, my understanding was that that was the extent of the
relationship of power valves in tuning. The correct valve is determined by
letting the car idle at its slowest idle setting 650-850 (SB 4.5-8.5in as
memory serves but I did a lot of high stall automatic cars) depending on
cam. Then drive the car starting in gear and watch to ensure that you never
go below that lowest value as a reading. Once you determine the vacuum at
idle choose a power valve 1 full range higher. Keep in mind that the term
"power" is a misnomer as it simply directs fuel to the main fuel circuit and
it acts like a switch either on or off with it preferably off at idle.
If you wish the car to "come on" earlier you need to install a softer
secondary diaphragm spring in the vacuum secondaries. This will result in
the secondaries coming in a little earlier. Another option is to play with
the accelerator pump cams to tweak the squirters to better suit the demands
of the engine under transition.
I hope that helps.
Best regards,
Tim
-----Original Message-----
From: Bob Palmer [mailto:rpalmer@ames.ucsd.edu]
Sent: Thursday, August 10, 2000 9:20 AM
To: Tiger List
Subject: Power Valve
Listers,
I have mentioned several times in the past that my engine
runs best with
way more spark advance than is normal. This, and other
problems were all
completely resolved when I replaced my old, faulty spark
coil. I'm now
running happily with 11 degrees initial and 36 degrees total
spark advance.
In trying to fix the symptoms, I made some changes to the
carb, including
changing the power valve. Now I want to adjust the carb
optimally. I hooked
up a vacuum gauge with a long tube so I could watch it while
driving and
determined that I probably wanted the power valve to kick in
at around 10"
of mercury. The 600 cfm, vacuum secondary Holley carb came
originally with
a 7.5 power valve and 64 jets. This was a bit lean for my
motor, so I
finally settled on 67 jets, but kept the 7.5 power valve. A
few days ago I
tried a different setup; I went to 63 jets and a 9.5 power
valve. So far I
like it a lot better than the previous setup. I may try a
8.5 power valve
with the same jets and see if this still works as smoothly.
I'm trying to
improve gas mileage AND keep the motor responsive too. I
realize the
precise power valve and jets depends on the motor
parameters, but what
combination have any of you found that works well.
Bob
|