Power valve missing spring ***'y ... why?

[Mopar to ya]

I got my carb off and cleaned it up, changed the gaskets, new jet sizing and new power valves. When I got it apart, on the secondary side, the power valve was missing the spring assembly. It was just the diaphragm and the threaded stub. The missing pieces were not in the bowl, so someone at some point took them off. Why would someone do that? Am I missing something? I put in a new power valve and it runs beautifully.

 

[67Dart273]

You sure it didn't get sucked down "somebody's" engine?

 

[Mopar to ya]

How? The only place it could go is the float bowl. Whoever had this carb before either took it apart and didn't notice it, or did it on purpose. I am just wondering what that purpose might be. It was weird. Since I rebuilt it, the car runs smoother than it ever has and the throttle response is immediate. I don't know if I'm imagining it or not, but it seems to have less power. I went from 75 primary jets to 72. I may switch back and see if I feel the difference.

 

[fishy68]

That is an odd one. No idea why anyone would do that but most guys plug the secondary power valve because it's not really needed. The power valve is basically an economizer valve and if you stop and think of it by the time your into the gas hard enough to open the secondary's fuel mileage is out the window anyway. Holley stopped putting secondary power valves in most of their performance 4 barrels a long time ago.

If you do install a plug you need to increase the secondary jets several sizes. The size depends on the size of the power valve channel restrictors (PVCR). That's those little holes you see under the power valve.

The formula is

PVCR size divided by 2 then you square that and multiply that by Pi. That gives you the area of the PVCR.

Example

Lets say the PVCR size is .054" (a common size for a 750 PVCR)

.054" divided by 2= .027"

.027" squared =.00073"

.00073" x Pi= .0023" (area rounded to nearest decimal)

Lets say you've got 75 jets in the secondary's now. The area for a 75 jet is .0053"

Add .0053" and .0023 and you come up with .0076". That's the total jet area.

The area for a #84 jet is .0076" so in this example that's the starting point when you plug the PV. Actual conditions can vary that by a jet or 2 either direction.