Power Valve Plug??

[maydayneal]

Thought I would move this issue over to the carb section of the forum....

Trying to get to the bottom of some performance bugs on the combo....one of which is a poor idle. Doing the standard stuff in testing out the entire ignition system....checking for vacuum leaks around the intake...etc.

Finally moved on to the carb as some possible issues.....one thought was a blown power valve....but upon inspection I found that there was no valve only the Holley plug for the valve.....so obviously not a blown PV issue.

But what the heck is the reasoning behind plugging the power valve??....I suspect for racing application....but I want to understand how adjustments are made to increase fuel flow when needed apart from the job the PV would of been doing.

Thought??

And thanks!

[Tod74]

You don't need a power valve when running wide open all the time.
When you remove it you must increase jet size 8-10 numbers. The power valve allows for less jet to make the thing more drive-able at part throttle...When vacuum drops below the spec on the PV it opens and allows the extra fuel to make up for the 8-10 jet sizes that are not there, but need to be to prevent a lean condition at WOT.

That's my understanding of it Carb guys feel free to corrcet me if needed.

[maydayneal]

You don't need a power valve when running wide open all the time.
When you remove it you must increase jet size 8-10 numbers. The power valve allows for less jet to make the thing more drive-able at part throttle...When vacuum drops below the spec on the PV it opens and allows the extra fuel to make up for the 8-10 jet sizes that are not there, but need to be to prevent a lean condition at WOT.

That's my understanding of it Carb guys feel free to corrcet me if needed.

Hmmmm....that makes since. The jets are 81....does that sound close to being right?

Anyway....with the PV out of the equation...maybe I'm not chasing a rich condition for the poor idle....we really only suspected this because it seemed to get better as we moved the butterflies open (more air). But now I'm not so sure....maybe we were opening the butterflies up enough on the primary idle adjustment to get into the "transfer slots" which pulls in more fuel and bypasses the idle system.

I did however check the intake vacuum at idle while in drive....barely making 5 inches. That is really low....I dont know the cam profile, but this makes me think its fairly aggresive. Maybe I can improve that with idle screw adjustments.

Any thoughts on that would be appreciated too!

If nothing else....I'm sure learning a bunch.

Thanks Tod.

[Tod74]

If you are uncovering too much of the transferv slots, you will know because you will not be able to get the thing to responde to the mixture screws. What size carb is it?

[maydayneal]

If you are uncovering too much of the transferv slots, you will know because you will not be able to get the thing to responde to the mixture screws. What size carb is it?
750 the 4150 series.

And the thing that really bugs me about this carb is the idle adjustment on the secondaries.....which is impossible to get to unless you purchase the part to make adjustment easier....which I dont understand why they didnt put that on anyway. I actually put a piece of cardboard under the secondary just to open the butterflies just a bit so that I could lower the primaries to make sure I wasnt uncovering those transfer slots. I think I have that done now.....so tomorrow I plan to play with the mixture screws with the vacuum gage attached and see if I can improve the pressure and improve the idle.

[sm44]

any tell tell signs of rich? plugs black or black smoke?

[maydayneal]

any tell tell signs of rich? plugs black or black smoke?

No smoke.....but I just changed the plugs and they were pretty black.

[JimmySmith]

If you have reason to suspect PV check the gasket under the PV plug. Later J.Smith 4621 racing

[fatjodycat]

power valves can help ideling. If you do replace the plug with a PV, take a vacuum reading and divide it in half for your PV #. Good luck

[Tod74]

just an FYI


You asked about the 81's being too big....

I looked on Holley's website and most 750 cfm 4150 's are jetted around 70 on the primaries..depending on which model it is..may have been 69 or 71 that is with a power valve so yours is 11 or 12 bigger without the PV. So you are on the fat side of the recomended increase in jet size when the PV is removed.

[Scooterz]

just an FYI


You asked about the 81's being too big....

I looked on Holley's website and most 750 cfm 4150 's are jetted around 70 on the primaries..depending on which model it is..may have been 69 or 71 that is with a power valve so yours is 11 or 12 bigger without the PV. So you are on the fat side of the recomended increase in jet size when the PV is removed.

I have found this too be accurate too. I just replaced one that had 72's in the primaries. I have had the same issues as you describe. Although the others here are more knowledgable than I, I have found that if you are on the street at all, you will benifit from a primary PV... like someone else said, at approx 2 steps lower than inches of idle. Another thing is allowing the the idle circuts to do thier job may require more air too (by drilling small holes in the primary butterflies). Also, make sure your floats are set correctly & needle & seat is not damaged. BTW: I have found the carb I have now works fine on the street w/a secondary PV. Scooter

[maydayneal]

Great info guys.....I've really applied lots of this knowledge and have got the engine ideling much better.

Best adjustment seemed to be to open the secondary idle adjustment just a bit....really pain in the butt to get to that small screw from the underside of the baseplate but I was able to get a couple of rotations out of it to move those butterflies just a tick, adding some additional air. This allowed me to back off the primary idle screws so that I could make sure I was operating on the primary idle circuits.

Now I'm moving on to the accellerator pumps. When I hit the throttle I get a little hesitation before picking up. The holley tech site says that its almost a sure bet that the squirters need to be increased. Let me run down the numbers and tell me if from your experience this is in the ball park??

Primary squirter = 25 <-- this seems low to me
Secondary = 31

Just to recap....this is an strip only bracket car running a chevy 350 +.040, approx 11.5:1 using Brodix Track 1X heads......other carb specs are below.

Model 80528-1 4150 HP - 750cfm mech double pumper
Primary jets = 81 (PV is plugged)

Really enjoying all of the learning....so if you have any thougts about those squirter sizes....love to hear them.

thanks again!

[fatjodycat]

Are you running a 50cc pump back and front? If not on the front I would try that first. It,ll give you a longer shot with the smaller squirter (on front) and may kill that flat spot.

[fatjodycat]

Jet extentions could solve that hesitation also.

[cepx111]

"Primary squirter = 25 <-- this seems low to me "


Ok that 25 (prim) is way to small for a healthy big breathing motor like yours.
I'd start out with 32's squared (prim & sec) and go from there.
Also if you can find them, get the squirters with the long brass nozzels sticking out, they seem to work better than the stubbys.


Ditto> on the 50cc accell pump (prim) and jet extensions (sec) and don't forget to install a notched float with the extensions.
Reason being, if you don't the float has the potential to not drop down far enough because of the extensions which wont let the needle drop down far enough which in turn can hinder adequet fuel supply during hard accelleration.
Sorry for the book, I just like to let people know why rather than say just do it, it helps with the learning curve.


Goodluck, Cp[/quote]

[Tod74]

I think 50 cc pumps would be overkill and if anything just be a bandaid fix...I had a 950HP that had 30 cc pumps front and rear and worked beautifully on a big block.

I completely agree with the squirters being way too small. Go to Holley's website and find out what came stock in that carb and start there...sounds like someone has frak'd with that thing way too much.