I'm pulling about 14-15" of vacuum at idle. I've always heard that it's best to use a power valve that open about 2" less than idle vacuum. I'm running a 6.5 now (stock holley PV). Should I go to a 10? It's the largest number I've seen. Will it help with the little off-idle stumble I have?

Make sure you are checking your vacuum with the car in gear, i.e. "drive" if an automatic, take it from there.

------------------
I gotta get away.

yep make sure you're checking engine vacum @ idle in gear like 600-800rpm.now you are correct what ever your vacum reading is subtract 2 and that's your power valve.another trick is to get the power valve to open as quickly as possible. that is done by hooking up a vacum gauge and driving around recording your readings.on my sbc my vacum @ idle is a mere 9"hg. at 900rpm and tried a 6.5 and was getting a lean tip in and cruise surge. i then went to a high flow 8.5"hg. power valve and got awesome throttle response and better driveability.
now you can play around with you squirters and increase there sizes to get rid of your lean tip in "stumble". what you do is increase in size until you get a puff of black smoke from your exhaust upon acceleration.also you'll need to play around with the pump cams also to tailor you pump shot.
on my holley 850 DP my set up is 35 squirter /orange pump cam primary side and 37 squirter /red pump cam secondary side both in the # 1 position on the cams.i hope this helps ya and good luck in getting your carb dialed in

------------------
1972 chevy camaro ss 350 sbc 425 hp/356 @ the wheels,350 trans,373 gears,daily driver with restification in progress for the old school / day 2 look
future plans- turning it into a baldwin motion Z30 clone

"IF IT'S TO LOUD YOUR TO OLD"
-------------------------

I stepped up to 8.5 on mine too but i dont know what my vacumme is? Runs better though!

------------------
http://www.supermotors.org/getfile/163599/fullsize/Idlin%20smooth.WMV

I have a question about this topic too. What are the symptoms of a power valve being too big or too small.

If the valve opens late youll have a hesitation too soon or worse at idle youll be loading up and flooding.

The PV circuit does not activate at idle.

<font face="Arial,Verdana" size="2">Originally posted by Rick WI:
The PV circuit does not activate at idle.</font>

unless you have the wrong vale!!!

It still won't open at idle, if it's set up correctly. Most people w/hairy cams will crank down on the idle screw, causing the blades to uncover the main circuit slots. That when a blown power valve will cause flooding at idle.

------------------
79Z28 Bought it new, honeymoon car, same wife, same car.

No the vac hole for the P valve is on the bottom doesent matter where the throttle blades are they have no effect on P-Valve

When I was waiting for a 8.5 for my 780cfm I tried a 4 and the car had no power when I stepped on it.

------------------
Mark D
1970 Z28
http://http://ca.geocities.com/mlvd@rogers.com/ (http://ca.geocities.com/mlvd@rogers.com/)
WTB: 70 SS396 Chevelle 4-spd, flapper hood car, no rag tops, 1-2 owner car.

It takes 4.5" of vac to hold a 4.5 valve closed and 8.5" for an 8.5