power valve circuit myths

[ClassAct]

I was stating that my 6.5 pv’s do not pass fuel at my idle vacuum. I understand now that pv circuit is with the boosters and not the t slot. Cruise vac is 14” and I want to try 8.5’s to cover a slight hiccup at full throttle jab in high gear. I figure it needs more fuel quicker. T ram set up with twin 650’s on a 511. Thanks

If you have 14 inches at a cruise, I’d use a 10.5 and see what happens. If you think about it, where you have it now with the 6.5 in there you have all that TIME for the vacuum to drop before the PV even STARTS to open. Time being a relatively short span but in PV operating time, it’s a LONG time from 14 to 6.5 inches. Even the 8.5 is a bit late for 14 inches of cruise vacuum.

The longer you delay the PV, the bigger the hole in the fuel curve you have to fill with pump shot.

Just my .02

[treyrags]

A blown or ruptured power valve is not the same as an open power valve.

[rustbucket79]

I have you tried stiffening up the secondary springs? Check balls for the diaphragms still installed?

My boss went through a bunch of 1850 holleys to find 2 that were half way decent and reasonably matched, you might have one that is more of an issue than the other.

[shoedoos]

I agree with Classact above....and OP, if you look at your youtube video again where the meter reads 22+ for a fraction of a second, that could well be the difference in PV opening points.....

[mopardave]

That has been my theory as well. I'll get some 10.5's to try. I was just gonna square jet it without pv's, but i think it would cruise way too fat. PV is the way to go for street i think. Thanks guys

[oscaracme]

it will run pig rich with a blown pv

at idle it won't run pig, cow or chicken rich with a blown power valve.

It certainly will run super rich with a blown power valve because fuel goes right past the diaphragm in it, directly through the vacuum supply hole for it.

i stand corrected......i should have known better as it's far from my first rodeo with carb operation. The "pig rich" phrase always rubs my ruhbarb the wrong way.

[77cruiser]

Maybe filthy rich.

[mopardave]

rustbucket79, I'm using DP 650's.

[63qcar]

On my 4150 Dbl Pumper ... 482 with Ebrock dual plane , 10.5 CR , dyno at 650 hp , I had a similar flat spot around 2700 to 3100 rpm coming out of turns on the track . I was just balancing the throttle until I got the car straightened and then rolled into WOT . I played with TS restrictors , IAB`s , accelerator pump cams and various squirters with little effect . Then it hit me ..... that was where I was just bringing the main ckt in and because I had fairly hi vacuum , the 6.5 PV wasn`t opening and I was still on the TS and IFR`s . Ran a few tests with a vacuum gauge to verify and changed to a 10.5 PV and the problem went away and I was able to get the IAB and TS back where they should have been .
Same situation that ClassAct described earlier .

[mopardave]

Thank you. Thanks for all the info guys. I have some 8.5's and 10.5's ordered. Should fix it.

[levisnteeshirt]

If the power valve opens at 8.5 and he's idling at 6, the valve will sit there open but not flowing until there's enough depression to draw from the boosters, correct?

With the power valve busted or open , it can make the idle circuit richer ,, those that dont believe this must not have ever had it happen to them ,, fuel is not coming out of the boosters but the exhaust will burn your eyes up . Ive been called several bad names on here for claiming this ,, but its real

[RevTheory]

If the diaphragm isn't ruptured and fuel isn't being drawn from the boosters, where does the excess fuel come from then?

[fatblock]

Power valve sealing gasket out of position will do the same thing...dumping raw fuel into the PV cavity.

[bill jones]

---several times I have found that the power valve body sticks out far enough from the meter blocks it will actually bind up in against the bolt boss for the main body that is right there inside the power valve cavity.

---older main bodys have rounded outer edges on these bolt bosses that are only have about .015" to .025"clearance to the power valve bodies

---some of these older main bodies have now been filed and/or machined to get rid of distortion and/or to get rid of bad erosion from corrosive fuels

---so machined main bodys with the rounded bolt bosses need to have a flat machined onto those main body bolt bosses

---this binding of the power valve to those main body bolt bosses holds the meter block gasket away so it'll seep fuel into those power valve cavities and straight down the vacuum hole into the engine

---the ones I have seen leave a rounded imprint on the bolt bosses and a rubbed spot showing on the power valve

---it's pretty easy to use a set of calipers to measure the protrusion of the power valve as installed in the meter block with a gasket on the meter block---and then touch that caliper to the main body and see how close the caliper end is to touching the bolt boss

[levisnteeshirt]

Power valve sealing gasket out of position will do the same thing...dumping raw fuel into the PV cavity.

It richens the idle circuit ,, it applies more head pressure to the mainwell that feeds the IFR , makes it easier for the siphon effect to draw fuel