850 carb. what size power valve to use and why?

[Drew Pojedinec]

I agree with Ross, and will toss it out there that I view jets the same way.  As small as possible, take up the slack with the PVCR.

I wish folks would stop talking about Power valve "size"  it's misleading and a horrible description for something that only functions as a vacuum switch.

Might be that your large pump shot is simply covering the hole between circuits.

[andyf]

Best way to figure out the power valve spring pressure is to find a stretch of freeway with a long up hill section. Drive up the hill and slowly accelerate while watching the vacuum gauge. At some point past 1/2 throttle you'll feel (or hear) the need for the power valve. One thing to remember is that the power valve is fairly slow to open. If you test it with a vacuum pump you can see that they only start to open at the rated number but they might not be fully open as long as there is any vacuum. The low rated ones are especially susceptible to this. A 2.5 power valve might only be half open at WOT while a 6.5 is most likely fully open at WOT.

[2ndgear]

Thanks Ross and I went with the 30 cc pump in the back. I also went with the pink cams front and rear. May try the blue cam up front. Had good luck before with the blue. I also jetted the front with # 80 and the rear with # 88 and plugged the rear pv. The front pv is # 6.5 which seems to work. Only put 60 miles on so far buy seems good. Need to drive a little more.

[My427stang]

Thanks Ross and I went with the 30 cc pump in the back. I also went with the pink cams front and rear. May try the blue cam up front. Had good luck before with the blue. I also jetted the front with # 80 and the rear with # 88 and plugged the rear pv. The front pv is # 6.5 which seems to work. Only put 60 miles on so far buy seems good. Need to drive a little more.

Good to hear on both the PV and the ignition. 

If you do not need the blue cam, it is better to avoid it as it increases volume and if it doesn't bog, no need to introduce more fuel, but if it indeed wants it, the blue cam works well because it increases both rate and volume

[Tobbemek]

Good work Next ting to do is go down on PRI MJ to 78 and find out if the engine starts to surge if not go to 76 and so on until it starts to surge and den go back up to the ones it not surging with and find the leanest  PRI MJ size that the car runs good with.
 And take nots of att what HG int vac it starts to surge.  And all this with out WOT. Just playing with the pedal down to 6.5 HG int vac
and then down to say 4,5-5.0 HG to find out the right PW for your combo. Its this " investigation" driving that tells you what needed PW
you want in combination with leanest possible PRI MJ, you then have to calculate PVCR size "area" to get PRI MJ size "area" to gether with PVCR in the 88 PRI MJ size.  This is the fun part of carburetor tuning right.