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I ran into a tuning issue last year that was much simpler then I thought, but became a nightmare. I did some tuning with my HEI and everything was great for about a week, then my engine seemed to fall flat on it's face at high rpm (6000 up). I returned everything back with no change, only to learn my valve springs were the cause. I'm ready to install new springs and return the HEI back to the tune I had, I'm also going go through my Holley Carb.

I have an 850 Vacuum secondary, that works well with a 3.5 power valve on the primary side as per Holley tuning and my cam, but I can't find any info on how to measure or tune for the secondary side power valve. All I've found is, if I remove the rear PV I should increase jetting X amount of jet sizes. How do I go about tuning the to find the optimum PV for the rear or should it be the same size as the primary one?
 

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Same size for primary and secondary. But it is preferred to remove the secondary one and jet 8-10 sizes richer than what you have now (or what is in the primary).

Choose your PV based on vacuum when driving, not in gear at idle. You will be running one which does not open soon enough. At cruise rpm while driving (2500-3000 rpm) see what the vacuum is and go 1-1.5" below that for your PV opening.

Contrary to what some will say, an open power valve at idle cannot richen the idle mixture, unless you have fuel coming from the boosters at idle.

The only way a power valve can richen the idle is if you have a blown PV, leaking gasket.
 

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Taz3, i wouldn't run a power valve in the secondary PERIOD! The reason being, under heavy acceleration the fuel moves away from the metering block uncovering the power valve and effectively shorting you 10 jet sizes. You should be running jet extensions in the rear and this will show the obvious concern of uncovering the power valve.

I would remove the power valve in the secondary, add 10 jet sizes (or jet back to the original jet with no power valve) and if you don't have them already, add jet extensions.

Happy Motoring!
 

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Same size for primary and secondary. But it is preferred to remove the secondary one and jet 8-10 sizes richer than what you have now (or what is in the primary).

Choose your PV based on vacuum when driving, not in gear at idle. You will be running one which does not open soon enough. At cruise rpm while driving (2500-3000 rpm) see what the vacuum is and go 1-1.5" below that for your PV opening.

Contrary to what some will say, an open power valve at idle cannot richen the idle mixture, unless you have fuel coming from the boosters at idle.

The only way a power valve can richen the idle is if you have a blown PV, leaking gasket.
This was how I found the correct PV size for the primary side. I'll remove the secondary PV and add some jet size accordingly.

Thank you

Andrew, thank you for the info, that makes a lot of sense and I'll order Jet extensions for the secondary side.:yes:
 

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This was how I found the correct PV size for the primary side. I'll remove the secondary PV and add some jet size accordingly.

Thank you

Andrew, thank you for the info, that makes a lot of sense and I'll order Jet extensions for the secondary side.:yes:
AED makes a kit with the notched float and tube style jet extensions, which don't change the afr ratio. They are better than the style which extend the jet out into the bowl. These do not give as consistent an afr.

At cruise rpm of 2500-3000, what vacuum do you see? That is where to base your PV selection on.
 

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AED makes a kit with the notched float and tube style jet extensions, which don't change the afr ratio. They are better than the style which extend the jet out into the bowl. These do not give as consistent an afr.

At cruise rpm of 2500-3000, what vacuum do you see? That is where to base your PV selection on.
This is what i'm using; i think they refer to them as "slosh" tubes.
Pockets; nice explanation of pv selection..... I thought I was the only one that didn't believe that pv's had no effect on idle .
 
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