carb tuning question

[CUDA JAS]

My engine pulls about 10 - 10.5 in of vacume at idle in drive.

My carb (750 Holley) has the factory power valve (6.5). 

My understanding is that the power valve should be 1.5 - 2 numbers less then the vacume reading at idle in drive. Which means mine shoudl be an 8.5 or 8.

Will haveing the power valve lower than the recomendation cause any issues.  Will having the correct power valve increase performance?

Jason

[Supercuda]

If you experience no lean condition under heavy throttle (should come on about the time the engine starts to really sing, and feel like a flat spot), you shouldn't worry. In theory, if you have everything set up correctly, having the power valve come on that late will cause a flat spot. Here in the real world, if the car runs fine, the plugs read fine, and you're happy, it's time to leave well enough alone. If you can't tell if there's a problem, and you have the time and parts to commit to it, change it out and see how it does. No difference? Change it back to the lower opening point. Noticeable improvement? Keep the earlier-opening one in there.

[MEK-Dangerfield]

I would leave it as is.   

[Aracer]

At 10 in. of vacuum, I like the 3.5 valve or else it pops open too soon, you need some vacuum play, or a range of vacuum to drive normal. 10 in. drops quickly to 6.5, so you pop it open too soon. Power valve channel restriction, or PVCR can then be opened to .070 from stock @ .045. This will allow you to use a smaller jet to cruse. Hot cams are pulsing worse than stock and if the distributor springs are stock it will drop vacuum more when they reduce the advance on idle. So, you see that it gets tricky.

[ireland383]

I always ran a power valve 1/2 the vacuum in gear. I'm running a 4.5 with 11 in gear. Plugs read perfect!

[Chryco Psycho]

 
Approx 1/2 the vacuum reading

[CUDA JAS]

At 10 in. of vacuum, I like the 3.5 valve or else it pops open too soon, you need some vacuum play, or a range of vacuum to drive normal. 10 in. drops quickly to 6.5, so you pop it open too soon. Power valve channel restriction, or PVCR can then be opened to .070 from stock @ .045. This will allow you to use a smaller jet to cruse. Hot cams are pulsing worse than stock and if the distributor springs are stock it will drop vacuum more when they reduce the advance on idle. So, you see that it gets tricky.

Thanks guys, lots of good info here. 

I have been dealing with a part crusie issue (car bucks and jumps bad a part throtle) but I seem to have fixed by playing with the vacume advance.

I think I will leave the power valve for now...

Jason

[Aracer]

     One more thing about power valves is that they wear out. First, they get brittle and act stiffer than rated and then leak from cracks in the rubber diaphragm. That's why I use the 3.5 instead of the 4.5, it works better for me along with the bigger PVCR's.   
     Good for about two years, then replace along with the acc. pump diaphragm.

[moper]

If you were chasing a problem and had to tighten the vacuum advance to address it, I'd go up to the 8.5 power valve. A very telling tuning exercise is to extend the vacuum gage hose and tape the gage to the windshield. Then go drive the car. Write down the readings at cruise, light throttle, etc. The PV has to enrichen when needed the accelerator pump is either already done, or when there's not a lot of throttle movement (i.e. light throttle cruise). Witht hat much vacuum you won't hurt anything by running it. The truth is, any valve tight enough to be closed at idle in gear might be the right one. Could be 1/2 tha vacuum reading, could be 1 size below. I find as you get to lower vacuum levels the "1/2 the reading" idea is more the ideal case, while the higher the reading the "2 sizes under the reading" idea works better. Just my experience.

[Aracer]

  Yup. tape the gauge to the window, and the rest moper, is right on.