The one on the left is what was in my 4779 holley 750cfm double pumper. The one on the right came with a rebuilt kit. Is there a difference between them? Should i use the new one? My old one has numbers (6,5, E) stamped in the back. The new one has numbers (4,5, 1,I) stamped in the back.

 

I'm not sure I remember absolutely correctly, but the 6,5 pwr valve will open at 6.5 in. of vacuum, and the 4,5 will open at 4.5 in. Now here's the part I'm not sure of...

The 4.5 pv will open way fast!! Like barely above idle (4.5 in. of vacuum above idle, which should be 0)

The 6.5 pv is a better all around one, depending on the state of tune of the carb.

IIRC, the rule of thumb, is after finding the NORMAL vacuum draw on the engine at cruise, divide by 2 and that should give you the starting point for pv. Example, normal cruise vacuum, 70mph on highway gives 13 in. of vacuum, 13/2 = 6.5 in. That is the 6.5 pv.

The best advice would be to contact five7kid or Jester on this board. Frankly I'm surprised they haven't already responded...

Hope this helps.

 

He's right about what the numbers mean, but he has the result backwards. The 4.5 will open way late..not early. You rebuild kit came with that tho? you sure you don't have them backwords? All the kits I've seen come with 6.5's..what kind of kit was it?

 

OH!! Ok, so in other words, the vacuum has to come down from cruise vacuum to 4.5 in. instead of 6.5 in. which would make it come in much later?? Is that right?

So, if you're getting a bog when WOT, say from 55 mph on the highway, then power comes on hard, but late, then the pv is too low, right? You should step UP in numbers until you get the proper response? (this is provided your accelerator pump shot is acting properly?)

Enlighten me, oh wise one!! :hail: :hail: :hail:

I really need to learn this stuff...

 

well it's more likely that it's the pump being off, or you're just below you combos powerband at 55mph, but yeah thats the right idea.

The PV opens up the equivilent of about 6 jet sizes. It's significant, and can cause a flat spot, but it's not going to result in a noticably jerky acceleration.

 

The rebuild kit is by AED. You can see the picture of it here that i posted a few days ago:
https://www.thirdgen.org/techbb2/sho...hreadid=143940
And yes, i'm sure i didn't get them confused, the rebuild kit came with two of these same power valves. I have to go to the autostore anyway cause i broke the two rubbery umbrellas trying to get them in (the ones that go underneath the accelarator pump diaghram) so i'm gonna pick up a new 6.5 powervalve. Thanks for all the help.