JaBoT

Does anyone make or have a trick to get a power valve rating above 10.5?

duckmanquacker

yup its called a quadra-jet

kd5icr

iTrader: (2)

Holley does make them. Here is a link to the one you want.

http://www.holley.com/125-1005.asp

JaBoT

That's a 10.5. I'm looking for one above 10.5.

chas0218

Can I ask why you need a power valve that big? Does your motor not drop below 10.5 inches of vacuum on acceleration?

JaBoT

It makes almost 18" at cruise. so yes when you give it a little gas to accelerate it does not drop below 10.5 and goes lean on the wideband.
This wasn't a problem out of the box because the main jets were causing it to cruise at 13.0 afr so it was already rich enough. Once i jetted the primarys to 14.7 that's when the problem started.

chas0218

What carb?
Vacuum Secondary or Double Pumper?
Whats your timing curve look like?

Here is a 12.5:
http://www.summitracing.com/search/Part-Type/Carburetor-Power-Valves/Primary-Stage-Opening-Vacuum/12-5-in-Hg/?Ns=Rank|Asc

JaBoT

I was looking into those but you can't use those with a pvcr bigger than .060” so there out. Although a great idea if they would flow enough fuel.
650 double pumper

chas0218

What squiter size are you running? Is the afr lean only when you romp on it then it richens when you stay in the throttle?

JaBoT

its not the squirters. There fine. it's the power enrichment circuit not coming in soon enough. The jets and squiters are perfect. if u nail it it goes right to 12.8 and pretty much stays there minus a little fluctuation you get from any carb car.
It only happens under light acceleration and it stays lean till it gets to a cruise because the power valve isn't opening under high vacuum light acceleration.

pancherj

I remember a guy on speedtalk not long ago talking about testing and modifying power valves. As I recall, he was having a similar issue. You may want to get on that website and do a search (or maybe google).

chas0218

Ok what does the car make for vacuum when in gear and sitting there? Check out this site might help some.

http://www.chevelles.com/forums/showthread.php?t=179125

Holley says to take your vacuum at idle with the car in gear then divide by 2 and that is the size of the power valve you need. There is a sticky that explains the process as well at the top of the section. If you pull 18" of vacuum then you need a 9.5 power valve.

five7kid

iTrader: (14)

JaBoT, how lean is it going under these conditions? Is it affecting the way it runs? I've read through your posts a couple of times, and I only see you saying that the wideband reads lean, not that it isn't running right under those conditions.

Are your throttle blades set to give you proper transition slot exposure?

I assume you have a stock main body and stock metering blocks. You might be getting into the area of needing to modify air bleeds and PVCR's.

JaBoT

chas0218:
That link was about a bog off the line at wot. That's not my problem the squirters are perfect.
The whole divide by 2 thing for smaller camed cars that are properly jetted for 14.7 at cruise is wrong and holley has finaly admitted it.
Read this. It's holley's updated info
http://www.holley.com/data/TechServi...ech%20Info.pdf
And a quick little thread about it.
http://www.holley.com/forums/showthr...alve-selection
If you make 18" of vac then u actually need a 15.5 pv which they don't make.

five7kid:
It hits about 16.0 unless i give it a little more gas to get the vacuum low enough to open the power valve. It doesn't actually stumble but i would just like to get the pv to open when it should so it doesn't go lean..
Yes my blades are set fine. The problem is not so much off the line. It's when cruising and i give it a little more pedal to lightly accelerate the vacuum doesn't drop low enough to open the pv.
The airbleeds will just make everything richer or leaner. This is a problem with the enrichment circut not opening soon enough. And drilling the pvrc's will just make it richer once the pv opens.

five7kid

iTrader: (14)

If you hold the throttle at the point where it is lean, does it stay lean, or eventually go richer?

Wondering if you have a restriction in the vacuum signal.

wrenchbender

When you are at your cruising speed, do you think your throttle blades have already cleared the transfer slots fully? If not, you might could open up the idle feed restrictor one step at a time to richen it up. Your idle mixture screws would still control the idle discharge holes while the transfer slots feed directly from the idle channel. If the blades are cleared of the slots,here's a couple of crutches: raise fuel level a couple of flats as this will bring in mains quicker and a smaller main air bleed will also bring in mains quicker. Just a couple of thoughts......

JaBoT

five7kid:
It will stay lean unless i accell harder or go back to a cruise. I have a 10.5 power valve, once i go lower than 10.5 vac it goes right to 12.8 (what i tunned it for).
wrenchbender:
It's not the transfer slots because it will happen anywhere in the throttle if i go from a cruise to a light accell. If i use a smaller main air bleed will it also make it richer if i'm cruising off the mains?

pancherj

Tuner told me to make a tester out of an old spark plug boot and a length of vacuum hose. I put a gauge with a bleeder valve in the line and it works really well. The big end of the boot fits perfectly over the PV.

JaBoT

I'm gonna give that a shot. Now i just need to find springs that will fit the pv. I was also thinking of doubling up on the springs. I'll make the tester and see how it goes. Should work though.

pancherj

I was thinking about adding some spacer washers to the existing springs. That might be the easiest way.

Damon

If I could share some experience I have on this subject.....

The old Holley advice about choosing a PV that's half of your idle vacuum still works and always did as a good starting point for your choice. The purpose of the PV is to give you the additional fuel needed to extract the MAXIMUM POWER from each combustion event. It was NEVER intended to provide precise tuning or prevent lean stumbles from slight loading or tip-in bogs. You can try to use it as such but then you end up asking questions like "where can I get a power valve over a 10.5?" When it opens it's like instantly going up 8-10 jet sizes. Choose one that opens every time you breathe on the throttle a little heavy (too high a PV number) and your fuel economy goes right in the toilet. You don't want it opening every time you pull away from a stop sign.

Shooting for a 14.7:1 A/F ratio is dicey, considering a universal replacement Holley is not a precision fuel metering instrument (it can't be calibrated out of the box to work prefectly on every engine combination, as you can imagine). 14.7 is a pretty lean mix and it doesn't take much to tip over into a mix that's basically too lean to burn effectively.

So if you want to make it work well you have basically 2 courses of action:

1. CHEAP AND EASY. Don't shoot for such a lean A/F ratio. Shoot for something in the high 13s/14.0 range instead. That provides enough safety margin to keep you from stumbling when the carb happens to get to a slightly leaner part of the calibration. This is not the "cop out" option you might think. Running a little fatter might sound like a fuel waster, but compared to an engine running so lean it sags and stumbles, you'll actually use less fuel overall in most cases, believe it or not.

2. PREPARE FOR BATTLE. You will need to tune every part of the carb for proper mixture control. Jets and air bleed tuning is just for openers. It can get a lot more involved than that. This is far beyond what the average guy is going to do, but the resuts can be amazing if you're willing to spend the time. For instance, when you're crusing steady at, say 50 MPH, are you aware that probably about half the A/F mix supplied by the carb is still coming from the idle circuits (which are often calibrated a little fat straight out of the box)? Think about what THAT does to your jet/PV tuning efforts when half the mix being supplied isn't affected by the changes you are making!

3. PUT ON A QJET. Just kidding! Couldn't help myself.

duckmanquacker

3. PUT ON A QJET. Just kidding! Couldn't help myself.


Ouch, thats twice said. I think thier headed in the right direction though.