I just got a new Holley 750 cfm. It came from the factory with a 4.5 power valve. My engine draws about 14 inches or so vacuum. Is it necessary for me to upgrade to a 6.5? Or will the 4.5 still work alright?

You need to run a vacuum gauge and rig it so you can check the vacuum reading while you are cruising, not at idle. Then select a power valve that is 1.5 - 2 less than your cruise vacuum reading.

Auto or stick?

If it's a stick, do as above; but check to make sure the PV isn't above the idle vacuum. If it's an auto, run a PV that's 1.5" below the vacuum idling in gear.

A factory 4.5 PV?

What is the list number of the carb you got?

It's an auto.

Carb is a 4150 series model 82750. Instructions said it has a 4.5 pv in it.

factory 4.5? well, while it's still new, you could probably easily remove the primary fuel bowl and check it out really quick. i thought they all came with 6.5" pvs

That's a new model.
I can't find any info on it other than it's a 750,

and it has vacuum secondarys :(

You must want to build a cruser or to save gas.

Not necessarly true....double pumpers are for radicly cammed engines with little vacuum.

This engine is not radicly cammed and does have some vacuum. Plus it's going to be used both street and strip, not just street.

The instructions said it has a 4.5 pv but I will pull the bowl tomorrow just to double check. BTW I've never pulled a bowl on a Holley before...anything I need to be aware of before I do this?

double pumpers are for radicly cammed engines with little vacuum Not necessarily true.... they work fine on a street car, better than vacuum secondaries, IF the driver is re-calibrated. The other is basically an "old wives' tale". But that's all beside the point; you have what you have, now it's time to make it work as good as possible.

4.5 (or 6.5 for that matter) is too low. Get yourself a vacuum gauge, see what your vacuum idling in gear is, and get one an inch or 2 below that. I'm guessing it's going to end up being a 8.5 or 9.5 or maybe even a 10.5. Then, lean the jets 3 sizes at a time until it just barely starts to surge while cruising, and then go back up 2 sizes. You'll have to re-adjust the idle each time, as idle affects the cruise mixture. Adjust the idle mixture for max vacuum idling in gear, with the 2 screws turned an equal amount out. You should be able to get it to where it has instant throttle response, doesn't smell rich, and burns clean on the plugs. Then after you get the primaries set right, put in a secondary PV about 2 numbers lower than the primary, and tune the primary jets for best MPH in the quarter; then tune the vac diaphragm spring for no bog when it opens.

Do those things, and you'll get far better gas mileage with alot crisper throttle response.

Ok idling in gear my vacuum reading is 13 hg. And I've been told you take your vacuum reading and divide it by 2....in this case 13 divided by two is 6.5....so now are you telling me I need a 4.5-5 hg powervalve?

Also what would happen if I left the current pv in and ran it instead?

Here is some good info that you can read:
Carb Tuning (http://www.bob2000.com/carb.htm)

Here's the answer straight from the horses mouth!! Note there are different ways to determin what PV to use depending on if hi-performance or near stock....

http://www.holley.com/HiOctn/TechServ/TechInfo/TI-222A.html

Thanks. Well, now I need to know how to change the valve? Do I just pull off the fuel bowl and the block behind it or ???

Disregard that last post I figured it out....yep it's a factory installed 4.5 pv....swapped it for a 6.5 pv, lets hope this thing performs better than it would've.

Originally posted by Junkyard Dawg:
....double pumpers are for radicly cammed engines with little vacuum.

I have used Double Pumpers on very mild street combos (milder than yours) with great success.

A vacuum secondary carb may provide a "slight" MPG advantage because the idle circuit is calibrated a bit leaner.

A Double Pumper will provide the best ET and throttle responce as long as you don't over do it on the ventiri size which kills booster signal.

I have read exactly what you have said in a couple of books.
The FACT is that you will never know for sure unless you try it for yourself, which I have done.
I stand by what I have said because of the old, been there done that smile.gif

If your car has 13" idling in gear, I'd go with a 9.5 or a 10.5 PV; and most likely, it would end up with far leaner jets (and better gas mileage) than it came with out of the box.

The goal with the PV is that as you give the car more throttle, it doesn't take much to open the PV; that is, the vacumm doesn't have to drop by very much before it opens. If you have 13" of vacuum at idle, and a 4.5" PV, that means that the PV will remain closed (non-power-enrichment mode) until you mash the gas far enough to drop the vacuum to 4.5".... a sure recipe for a bog, sag, stumble, maybe even a backfire. The only way around taht is to calibrate the main system (jets) so rich that it covers up the lean-out. That's how those carbs come set up from Holley. They assume that the builder is putting too big a carb on the car (best for max power) and the cam is too big (best for max power) unlike what most of us are actually doing these days.

But, best thing to do, is leave it alone and put it on as it is, and see what it does; and go from there. You won't really know how close to or far from ideal it is until you try it. You might find that it's fine like it is. Or, you might find that it's horrible and needs lots of changes. You can't be sure until you've run it.

Originally posted by thrasher:
I have used Double Pumpers on very mild street combos (milder than yours) with great success.

A vacuum secondary carb may provide a "slight" MPG advantage because the idle circuit is calibrated a bit leaner.

A Double Pumper will provide the best ET and throttle responce as long as you don't over do it on the ventiri size which kills booster signal.

I have read exactly what you have said in a couple of books.
The FACT is that you will never know for sure unless you try it for yourself, which I have done.
I stand by what I have said because of the old, been there done that smile.gif Man I sure hope you're not telling me this very expensive air/fuel mixer I've just recently purchased is nothing more than a turd.

The only reason I mentioned dp's being for low vac. cammed engines is because everyone I've talked to has said on a mild and fairly hot street/strip engine run a vac. secondary, and on something that is more geared towards track only run a dp. And that doing vice versa with either carb will not give the best results. :confused:

No that's not it at all.
That is a good carb.

I'm just saying that since you have 4.10's, a good set of heads, and a cam that is not too big you could have bought a DP and ran a bit quicker.

It's all up to you.If it's more geared tward being a cruser then a VS carb was probably the correct choice.
If you inted to get the last ounce of performance from the combo the DP will get the nod.

I remember you saying that you were thinking that you might install 3.73 gears.If that is what you are going to do maybe you did make the correct choice.

You also indicated you wanted a lower rpm stall converter, in which case you did make the right choice.

If you intend to get a 10inch 3200+ converter with that XE274 cam and run 4.10's then IMHO a VS was the wrong choice.

Well actually when I get my XE274 cam I think I'm going to go with a 3000 rpm stall converter. Alot of people have told me a 3000 stall is borderline for street and strip.

One of the reasons why I bought that carb (750)is because the 600 VS on my V8 S-10 is starting to crap out on me, plus before it had the 600 it had a 650 VS which made it run a bit better, but it was already going south by about 2 months of ownership and I needed a replacement. I already have the very same 600 VS carb on the Chevelle and it runs alright but I figured if this carb doesn't work well on the S-10 then perhaps it will work better on the Chevelle, especially after the mods I'm about to do to the engine.

A 3000 stall will work fine on the street.
I wouldn't consider it borderline at all...
If you get a cheap one, that's another matter.

I think a 10inch 3000-3500 stall is fine on the street.I consider the 10inch converters rated at 3500+ borderline.With 4.10 gears it's not even a concern, those gears make a world of difference.
With 3.73's you might want to stick with 10inch converter rated at 3000-3500 stall.

Streetability will be lost by going with a cheaper converter that produces lots of slip.

Most who tell you that is borderline probably haven't had a quality converter.Most people just go with the old B&M TCI stuff because it costs a lot less.

There are also people who are going to tell you those two companies converters are fine.
I have been that road too, they can have'm.
Heck might as well throw GER in there too :D , that might be a stretch smile.gif

Originally posted by RB69SS396Conv:
[QB]4.5 (or 6.5 for that matter) is too low. Get yourself a vacuum gauge, see what your vacuum idling in gear is, and get one an inch or 2 below that. I'm guessing it's going to end up being a 8.5 or 9.5 or maybe even a 10.5. Then, lean the jets 3 sizes at a time until it just barely starts to surge while cruising, and then go back up 2 sizes. You'll have to re-adjust the idle each time, as idle affects the cruise mixture. Adjust the idle mixture for max vacuum idling in gear, with the 2 screws turned an equal amount out. You should be able to get it to where it has instant throttle response, doesn't smell rich, and burns clean on the plugs. Then after you get the primaries set right, put in a secondary PV about 2 numbers lower than the primary, and tune the primary jets for best MPH in the quarter; then tune the vac diaphragm spring for no bog when it opens.I think I'm going to replace the pv with an 8.5 and see how that performs and then change the jets if needed.

BTW I don't think the vac. secondary carbs have a 2nd pv do they? I thought they only had one? :confused:

Can't say with such a new model.
Just look.
Have new gaskets ready though.