Holley/Demon Carb stumble

Wow 10.5??????????????
Well i sure don't remember you or anyone else recommending a power valve that that stays open from Idle to top rpm.
Real confused there for sure.


Ok, I'll type the short version.

Power valve opening has ZERO to do with idle. Nothing. Zip. Nada. And it doesn't matter who says it does, because it's just wrong.

There may be (might) a case when an open power valve can affect idle. Mattax can explain it better, but the short of it is if the main jet is small enough and the idle bleed is big enough than maybe PV opening may affect idle. I could have that wrong. If I do, Mattax can correct it for me.

That being said, we do not chose power valve opening by idle vacuum. Ever. If it's a street car, you use cruise vacuum to pick PV opening. If it's a race car, put it on the chip, or flash the converter and set the PV opening just under whatever the vacuum says. And yes, I'm a fan of using a PV on a race car.

In my case, I cruise at 15 inches. I'd love to have a 12.5 PV but I can't find one. So I have to use a 10.5 until I do something. I'm going to buy a PV tester and then I'm going to play with springs and spring installed height to make the PV open where I want it.

So think about how the PV works. It's a system that allows you to use a smaller main jet for cruise and then the PV opens under load (low vacuum) and adds fuel.

The later that PV opens, the harder it is for the booster/accelerator pump to cover up the delay in fuel delivery. Even if you are on the booster, without the extra fuel of the PV, you get a big lean spot.

I think it's worth mentioning here that when you do as you are doing in your video, every time you smack the throttle like that, any fuel in the intake system that was vaporized turns back to solid fuel. Almost at the touch of the throttle. Solid fuel doesn't burn. It also almost never gets back to a vapor. That means it becomes wall flow and it just runs right down the manifold walls as a liquid and essentially runs right through the chamber where it stratifies and ends up out the exhaust pipe.

So anything you can do to help getting that wall flow down to a minimum helps with power.

That's the short version. Mattax and PRH can add somethings I'm forgetting or don't know.

One of the biggest tune up sins with a Holley is incorrect PV opening. It's been wrong forever. And Holley is STILL teaching people the incorrect way to select power valve opening.