on my engine all the vacum i can get at idle is 9"hg @ 1000rpm due to to the huge cam i'm running.i know your supposed to go down 2 steps on power valve sizes but what would you think would be better a 6.5 hg or 7.5 hg power valve.currently i'm running a 8.5 and getting a little surging/rich idle and low rpm driveability issues

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1972 chevy camaro ss 350 sbc 425 hp/356 @ the wheels,350 trans,373 gears,daily driver with restification in progress for that old school day 2 look ------- future plans- turning it into a baldwin motion Z30 clone
"IF IT'S TO LOUD YOUR TO OLD"
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yeah stick the 6.5 in if you already have one try it
What size cam,motor,compression,carb?

[This message has been edited by Mwilson (edited December 27, 2004).]

6.5

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70 SS 454 CI Dynoed 684 HP, 702 TQ All Aluminium Fuel Injected Small Block , plus 200 - 500 HP NX nitrous system.

thanks http://www.nastyz28.com/ubb/smile.gif http://www.nastyz28.com/ubb/smile.gif

<font face="Arial,Verdana" size="2">Originally posted by Mwilson:
yeah stick the 6.5 in if you already have one try it
What size cam,motor,compression,carb?

[This message has been edited by Mwilson (edited December 27, 2004).]</font>

11.5:1 comp. 360 sbc with the 1rst gen off road mechanical cam 257/260 @ .050 and 493/512 lift running a holley 750 double pumper with choke horn milled off

Muscule Car, I hope one of us or someone else can clear this up. My understanding is close to yours, but with one major difference. You are suppose to get that reading at idle in drive. Then you are suppose to divide that number by 2. So you should have a 4.5 in there. I hope someone can correct me if I am wrong but I got that from a holley book and the local speedshop.

<font face="Arial,Verdana" size="2">Originally posted by z28smokin:
Muscule Car, I hope one of us or someone else can clear this up. My understanding is close to yours, but with one major difference. You are suppose to get that reading at idle in drive. Then you are suppose to divide that number by 2. So you should have a 4.5 in there. I hope someone can correct me if I am wrong but I got that from a holley book and the local speedshop.</font>

i was told by a holley tech record your vacum reading then drop 2 sizes down and that's your power valve size.
i checked vacum in neutral even though i have an auto.because of the size cam i shift into neutral @ stoplights.

thats right 2 sizes down perfect world but when your like me with a big carb and small cubes you have to push it a little to prevent stumble by installing a p-valve thats ready to open "FAST" any time you change motor configurations you just threw the book out the window,
I have less than ideal conditions for my carb 850 dp, automatic, 350ci, 3.42 gear, 3520 lbs!

When you customize the motor you can count on customizing the carb if you want optimal performance at that point the book becomes a guidline
but i took the time to adjust everything to work(alot of time)it payed off big time at the track over a 650 or 750 for me, but is it worth it to someone else?

[This message has been edited by Mwilson (edited December 29, 2004).]

Try drilling your throttle blades-start with a 1/8" hole in each blade and see if that helps your bad idling situation. Also, if you are running ANY vacuum acessories, you are in a stumble from the get-go with a reading of only 9 inches at idle....a cam with a little less duration would help, as would a tighter ring gap, or hell-go with a cannister.

<font face="Arial,Verdana" size="2">Originally posted by gmachinz:
Try drilling your throttle blades-start with a 1/8" hole in each blade and see if that helps your bad idling situation. Also, if you are running ANY vacuum acessories, you are in a stumble from the get-go with a reading of only 9 inches at idle....a cam with a little less duration would help, as would a tighter ring gap, or hell-go with a cannister.</font>

the carb worked perfectly no issues at all until 2 weeks ago when i had a bad needle/seat in the secondaries.now i'm going nuts trying to get back to a baseline.going from a 8.5 to a 6.5 created a lean tip in on light throttle and surge at low rpm cruise but gave me a cleaner/crisper sounding motor