This 870 carb I scored on Ebay has 78 front w/PV, and a 82 rear w/PV.

Any benefits to running a rear PV? There must be if Holley put one there I guess. I plugged the rear PV and left the 82's in there, dropped the primary to 74's. I lean now up top (AFR 15-16).

If you keep dropping the primary jetting, should the rear be upped to compensate?

First, figure out what the stock jetting was supposed to be.
Go here and look up your carb by it's list number:

http://www.holley.com/data/TechService/Technical/Carb%20Numerical%20Listing.pdf

Starting with the stock jets, when you remove the secondary power valve, you need to INCREASE the secondary jets 8-10 jet sizes.

If you measure the size of the Power Valve Channel Restrictions, and the Jet size, compute the areas, and add them together, you can then pick a new jet that equals this area and pretty much nail the jetting. Otherwise, follow the 8-10 jet sizes and tune up or down from there.

Ron

Ron, the consesus here is that 82 - 84's are good for the rear. Factory for this carb is 82, so I figured if I block the powervalve I'd be good.

You have to compensate for the PV and jet up 6-10 sizes minimum. The HP series carbs come with PV's and I still use mine, got 4.5's in it, front and back. Some say you can uncover the powervalve if your car pulls very hard down track, but I pickup between 24-25 mph on the backhalf so I'm not uncovering anything.

Should I put like a 78 in the rear, and run the PV? I just think 82 and a PV is alot. That would be equiv. to 88-80 if I understand this correctly.

The carburetor is calibrated for the 82's and a powervalve, so all should be fine if thats the stock jetting. I've jetted mine up from stock 78's square to 82 square when I put it on the 505. I was running 78/80's on the 454. I think I'm still a tad fat, but rich is better than lean. I may drop back to 81's square and see what happens this weekend.

If you go w/o a rear PV, 8-10 sizes up in the rear is correct (as a starting point anyway). Personally, I don't think there is any real benefit to running a rear PV - I think jetting is a more precise way to meter the fuel. If you're not in the throttle fairly hard, the secondaries aren't open anyway.

Looking at 'factory' settings is fine but you'll want to tune your carb to specific conditions and what your motor is capable of. On my BBC Camaro, for the car to run right there is a big difference in jetting between August 98 degrees and humid and November 50 degrees and dry.

Start with the stock freakin jetting. Holley got em dang close, if not right on.
If you have to change jets more than 4 sizes from stock, something else is amiss. I gave you the link, go look it up, and start from there.

If you remove the secondary PV, jet up 8-10, or 6-10 sizes in the rear only, and tune from there.

When the PV opens, it ADDS fuel. So when you plug it, you have to add the fuel with the jets.

Bob, uncovering the PV is only an issue at the launch, not on the second half of the track.

I don't think blocking the PV is necessary for most of our rides, even when we race. I did all that in my HP950, was running right at 11 flat at the time, and it made zero difference.

It's hard to say why Holley used rear PV's in some carbs, and not in others....seems to be hit or miss. I'm sure they had their reasons.

Ron

Bob, uncovering the PV is only an issue at the launch, not on the second half of the track.

I beg to differ, it can also be uncovered down track in a hard pulling car as the fuel level drops in the bowl from increased power demand. The PV will uncover long before the jets will be uncovered, and how many use jet extensions?

Easy Ron...

I did try the stock jetting, its just a little rich thats all.

I'd like to lean the primaries till I get a light surge, then I'll bump it up a notch. I'm almost there with the 72's - my afr is around 14 with them. Being that I dropped the primaries so much I'll probably need to keep the rear overly rich to compensate for wot.

I beg to differ, it can also be uncovered down track in a hard pulling car as the fuel level drops in the bowl from increased power demand. The PV will uncover long before the jets will be uncovered, and how many use jet extensions?jet extensions are for the launch too though, right? i would think if you're uncovering the jets or power valve down track then you have a fuel delivery problem. obviously though, your's isn't having that problem if it's picking up almost 25mph on the back half of the track.
i agree with the statement Ron454 made. start with the factory jetting and if you are going past 4 jet sizes front and rear, then there's something else wrong. usually factory jetting is pretty darn close on an out of the box holley carb. now with these trick racing carbs, i'm not sure. i've never been fortunate enough to own yet.

Sorry, didn't mean to sound cranky......
I have always used jet extensions, they just make sense.
I thought that removing the secondary PV would make a difference in a high ten second car, but it didn't.
Ron