Power Valve Selection: The Definitive Answer

[MOPAROFFICIAL]

I agree, I believed the 2" below idle vacuum method and used a 6.5 PV. I have an A/F gauge and noticed a pretty severe lean condition at part throttle. I run about 17-18 inches of vacuum at cruise, so I will probably switch to something like a 10.5 PV so I will get some enrichment to help the part throttle stumble. Once I sort that out i may be able to lower the primary jetting.

Automatic transmission?

2" below idle is a stick car starting point. Fwiw.

 

[RustyRatRod]

One of the issues with PV timing (well two actually) is that some carbs are so rich that changing PV timing doesn’t affect anything. It is so rich that bringing it in later won’t make a difference.

The other is the opposite. You are lean enough that opening the PV sooner still doesn’t add enough fuel to make a difference.

The way I do it (and there are other ways but I’m pretty old and lazy) is I block the power valve off. Then I drive the car at a cruise so I’m on the main jet. Then I start taking main jet out of it until I get a slight lean surge at cruise. The I go up two sizes on the primary main jet and leave it alone.

Now you can put the power valve back in. Then I disconnect the secondary throttle blades and I wire them shut so they can’t open.

The I go out and mash on it real hard and I start looking at the plugs. If it’s lean, I open up the power valve channel restricters.003-.004 and go test it again. I keep opening up the PVCR’s until it’s correct at WOT.

Of course, of you are fat at WOT, then you would close up the PVCR’s a few thou until it’s cleaned up.

Then you can hook up the secondaries and test for WOT. Only change the secondary main jet to correct any issues you have.

Once you get the cruise right, you’ll find out how important it is to get the power valve open on time. The later you open it, the bigger the hole in the fuel curve you have to fill with accelerator pump shot.

It takes time to do it, but IMO it’s worth doing because the carb will reward you with incredible performance and drivability.

I put the 9.5PV and jetted it up from 55s to 59s today. The part throttle bucking is gone. I really didn't drive it far enough to tell if I could see what difference the PV made. Probably drive it to town tomorrow.

 

[MOPAROFFICIAL]

I put the 9.5PV and jetted it up from 55s to 59s today. The part throttle bucking is gone. I really didn't drive it far enough to tell if I could see what difference the PV made. Probably drive it to town tomorrow.

You said it lightly surged at cruise iirr.
That's the jet. 4 jet sizes at a time? Be nice to see one change at a time, like 2 jets @57 Awesome to hear it's gone, just keep an eye on the plugs though... that's a really big step in enrichment...and IF correct, means the carb is a hair too small. That would be suprising to me.

 

[RustyRatRod]

You said it lightly surged at cruise iirr.
That's the jet. 4 jet sizes at a time? Be nice to see one change at a time, like 2 jets @57 Awesome to hear it's gone, just keep an eye on the plugs though... that's a really big step in enrichment...and IF correct, means the carb is a hair too small. That would be suprising to me.

Well, like I said in the PM, that was the quickest I could get a bigger jet. They couldn't even tell me when or IF they could get something smaller. THose were in the DC in Atlanta so I got them. It still BARELY and I mean BARELY does it, so I think it's about right.

Also, think if this. I KNOW it clearing that up was not the PV, because my cruise vacuum was 12hg and the PV is a 9.5 so it's not even opening then, so it had to be the jets that cleared it up. It seems to be running better, but like I said, I have the 3.5PV coming as well and I plan on putting that in just to see what the difference will be. I've never really played around with one this much, so I am learning as I go. I think it might can stand a smaller pump shot too. It has a 25 in it and I have a 21 coming just to "see" what effect that has. I can always back up if I see something negative.

 

[RustyRatRod]

You said it lightly surged at cruise iirr.
That's the jet. 4 jet sizes at a time? Be nice to see one change at a time, like 2 jets @57 Awesome to hear it's gone, just keep an eye on the plugs though... that's a really big step in enrichment...and IF correct, means the carb is a hair too small. That would be suprising to me.

I've thought of that as well.....and it would surprise me too. I HAVE an old style Holley 600DP I may sling on it just for kicks. LOL