Power Valves Affecting Idle Mixture?

[Infomaniac]

OK I'm never too old to learn something so someone please explain how a power valve opening up at idle in a Holley carb changes the idle mixture.
When the power valve opens it adds fuel to the main fuel well after the main jets. This would give more fuel to the discharge nozzles in the main metering system only. The idle system pulls fuel from the main well through the idle restrictions (jets). It has to be pulled above the fuel level in the float bowl and then out the idle discharge nozzles.
The only way to increase idle fuel mixture would be to:
Raise the fuel level in the main fuel well (this determined by float level only)
Open the idle fuel screws
Increase the size of the idle jets.
Increase engine vacuum
Decrease idle air bleed size
I can't see a power valve opening doing any of these.

[Havasu Hangin']

My Holleys are very sesitive to butterfly position at idle. I would assume that any additional fuel in the main metering system would find it's way into the idle circuit through the butterflies being slightly open?
On a side note, when we were trying to tune my carbs, float bowl level did make a difference in idle characteristics.
Just my worthless 2 cents.

[Infomaniac]

Well HH only if the main well fuel level was increased. And it will not increase above the level in the float chamber. The power valve will only let more fuel into the main system. Will not change the level. The idle system only flows a fraction of what the main system can supply. So additional fuel available should not matter.
I am trying to find a holley carb schematic. Anyone have one to post?

[steelcomp]

The idle system gets it's fuel from the main well through the main jet. Fuel is then bled off from the main well into an idle well where it's mixed with air from the idle air bleed hole. The idle well leads directly to the idle discharge port and the idle xfer syst. It seems to me that introducing more fuel down stream of the main jet into the main well, which is what a power valve does, (higher in the main well above the idle tube) is what will richen the idle mixture. Just a thought.

[INEEDAV]

OK I'm never too old to learn something so someone please explain how a power valve opening up at idle in a Holley carb changes the idle mixture.
When the power valve opens it adds fuel to the main fuel well after the main jets. This would give more fuel to the discharge nozzles in the main metering system only. The idle system pulls fuel from the main well through the idle restrictions (jets). It has to be pulled above the fuel level in the float bowl and then out the idle discharge nozzles.
The only way to increase idle fuel mixture would be to:
Raise the fuel level in the main fuel well (this determined by float level only)
Open the idle fuel screws
Increase the size of the idle jets.
Increase engine vacuum
Decrease idle air bleed size
I can't see a power valve opening doing any of these.
Ron
Mine seems to drip from the booster venturi's when the power valves are above 5.5 and are staying open at low speed idling.
If you want to come to Chetopa, KS this weekend(90 min north of Tulsa) I can throw the 8.5 powervalves back in the carbs and you can try to make it idle. Bring your own tools though, enough of mine were thrown into the woods near my house in Colorado while trying to make the boat idle at 10,000 ft with no air for either it or me to breathe. :(

[Moneypitt]

If the butterflys are open too far the idle circuit won't work at all. There must be a vaccume pull on the idle bleed slots in the wall of the throttle bores. If the idle speed screw is holding the butterflys open, the vacuume isn't there for the idle circuit. Have you actually got the idle to change by adjusting the idle mixture screws? A sure sign of throttle opening is when the mixture screws make no difference, like all the way in and the motor doesn't change. MP

[Infomaniac]

The idle system gets it's fuel from the main well through the main jet. Fuel is then bled off from the main well into an idle well where it's mixed with air from the idle air bleed hole. The idle well leads directly to the idle discharge port and the idle xfer syst. It seems to me that introducing more fuel down stream of the main jet into the main well, which is what a power valve does, (higher in the main well above the idle tube) is what will richen the idle mixture. Just a thought.
Interesting discussion guys. I would agree that introducing more fuel would richen it if the idle system were using enough fuel. The idle system has a tiny little jet in the metering block as compared to the main jet. If you introduce more fuel when the system is only using a fraction of what is available should not matter. Only if that additional fuel raised the level of the main and idle wells should it change the mixture.

[Infomaniac]

A little carb FYI for those that do not really know how they work. If you do know then disregard. :D
What makes a carb flow fuel is pressure differential. The higher the differential the more "force" there is to flow fuel. A mechanical restriction (jet) is what works along with the pressure differential to help meter the fuel.
The float chamber has atmospheric pressure and it requires a lower pressure at the other end of the metering circuit somewhere to pull fuel above the level in the float chamber and out the discharge nozzle. The higher the float level the easier it is to flow fuel. The lower the float level the harder it is to pull fuel.
The idle discharge nozzle is below the fuel level so it has to be raised above the level of the main system in order to flow. This is not a problem because there is more pressure differential at idle than any other time. Engine vacuum at idle is always more than a venturi pressure drop. A venturi pressure drop is what flows fuel above idle. The main jets meter the fuel.
The idle system is in series with the main system. Idle fuel has to flow through the main system before reaching the idle system. The smallest orifice in series is what will meter the fuel. The idle jet or adjusting screws will do this. The main jets are too big to meter the fuel at idle and opening the power valve only allows more fuel to be available. The engine has to be able to pull the additional fuel. And the idle jets and mixture screws are after the power valve.

[Infomaniac]

Lets say you have two chambers filled up half way with a liquid and they are connected beneath the liquid level with a sizeable connection.
Start draining liquid from one container from a hole only a fraction the size of the connection between the containers. Then add another connection between the containers below the liquid level. It will not drain out any faster.
This is basically what is happening when you open a power valve during idle.

[Jim B.]

Ron
Mine seems to drip from the booster venturi's when the power valves are above 5.5 and are staying open at low speed idling.
That may be true, but fuel dripping from the booster area is a classic sign of the needle/seat assembly sticking open due to dirt, debris, etc. I'd check pull a needle/seat assembly and make sure they are free of crap...

[Taylorman]

That may be true, but fuel dripping from the booster area is a classic sign of the needle/seat assembly sticking open due to dirt, debris, etc. I'd check pull a needle/seat assembly and make sure they are free of crap...
Wouldn't float levels to high and to much fuel pressure cause fuel to drip from the boosters also? Just a question. I really know nothing about Holley carbs. Ive always had edelbrocks. I just bought a Mighty Demon and installed it a couple days ago. I cranked it yesterday for the first time and messed around with the idle mixture screws.
Heres my question. The manual really stressed that the butterflies should be set to a certain spot or an off idle stuble could occur. It says to set the butterflies so about .020 of the little slots were showing from below. So how do you adjust idle speed.

[Taylorman]

Also, how does a powervalve work? Does low rpm vaccum hold it closed and when vaccum does down its allowed to open? How do you select the correct size power valve?

[Jim B.]

A little carb FYI for those that do not really know how they work. If you do know then disregard. :D
What makes a carb flow fuel is pressure differential. The higher the differential the more "force" there is to flow fuel. A mechanical restriction (jet) is what works along with the pressure differential to help meter the fuel.
The float chamber has atmospheric pressure and it requires a lower pressure at the other end of the metering circuit somewhere to pull fuel above the level in the float chamber and out the discharge nozzle. The higher the float level the easier it is to flow fuel. The lower the float level the harder it is to pull fuel.
The idle discharge nozzle is below the fuel level so it has to be raised above the level of the main system in order to flow. This is not a problem because there is more pressure differential at idle than any other time. Engine vacuum at idle is always more than a venturi pressure drop. A venturi pressure drop is what flows fuel above idle. The main jets meter the fuel.
The idle system is in series with the main system. Idle fuel has to flow through the main system before reaching the idle system. The smallest orifice in series is what will meter the fuel. The idle jet or adjusting screws will do this. The main jets are too big to meter the fuel at idle and opening the power valve only allows more fuel to be available. The engine has to be able to pull the additional fuel. And the idle jets and mixture screws are after the power valve.
Hey Ron, your giving me flashbacks of when I was in A & P school learning about aircraft carbs for recips!! Ha! Ha! I learned more about carburetion in that school than anywhere else. Way to break it down man!
I do have something interesting to add to the subject. I'm sure some of you have had this problem and probably thought it was something else, and didn't even know it. A buddy of mine recently replaced a power valve one one of the carbs. After that fired the boat up and noticed that it was running fat at idle. He screwed with everything (timing, idle/mixture screws,new plugs, etc..) and then called me when he finally gave up and couldn't figure it out. I came over and pulled the bowl/metering block to look at the power valve he had just replaced. I didn't even notice at first, but realized that the power valve gasket was slightly off center and there was a small gap exposed. and was slightly wet. Then it hit me was a ton bricks. Fuel was flowing from the bowl through the threads between the power valve/metering block and right down through the manifold vacuum passages in the main/throttle body and into the motor. The additional fuel entering the motor was enough to effect the idle mixture........At least I think is was.. Sound logical?
JiM

[Jim B.]

Wouldn't float levels to high and to much fuel pressure cause fuel to drip from the boosters also?
That is correct. I was making the assumption that his floats and fuel pressure were already properly set..... :cool:
JiM

[Infomaniac]

Hey Ron, your giving me flashbacks of when I was in A & P school learning about aircraft carbs for recips!! Ha! Ha! I learned more about carburetion in that school than anywhere else. Way to break it down man!
JiM
Dude I taught recip engines and systems in A&P school for 8 years. LOL
Spartan School of Aeronautics and Tulsa Technology Center.
Also if the powervalve is somehow affecting some other circuit other than the idle metering circuit. That will explain how the power valve affects idle mixture. But as mentioned above. The other systems would be malfunctioning.

[INEEDAV]

That is correct. I was making the assumption that his floats and fuel pressure were already properly set..... :cool:
JiM
Fuel pressure is a rock solid 7psi. I use an Enderle return check valve and have it shimmed about as close to 7psi as I care to chase. It allows full pump flow if needed while still always keeping the pressure very near 7psi. Float settings are good, or seem to be, never ran out of fuel and never saw fuel come out of my sight windows while fiddling with the carbs.
It just seems that the power valves coming in and out while using the higher numbered power valves caused more fuel to enter the main well and work its way into the carb. Just personal experience, not gospel.

[Jim B.]

Also if the powervalve is somehow affecting some other circuit other than the idle metering circuit. That will explain how the power valve affects idle mixture. But as mentioned above. The other systems would be malfunctioning.
I totally agree. I was thinking if fuel was leaking into the vacuum passage for the power valve like I stated before, then the power valve itself wouldn't be recieving the correct vacuum signal due to fuel plugging the passage.....

[SmokinLowriderSS]

Holley power valves are designed to open upon recieving a low manifold vac. condition such as lugging a motor at low revs with a large throttle opening. This condition reduces fuel pull fron the venturis so the power valve helps in a "flat-spot". If the power valve rating is too high for a given engine setup, it will open at idle and flow fuel through it's own circuit, thus enriching the idle mixture. Large-lift cams with lots of overlap tend to have very low manifold vac. conditions, lots of pulsing too. This is made worse the larger the carb gets. If you have an 85 power valve, it opens at 8.5" mercury vaccume. if you have a manifold signal that gets to 9 or so at idle, then the power valve can open some of the time. The rating should be aprox 1/2 of what your idle vac is in the engine. Hence an idle vac of 10" requires a 50 power valve to avoid problems. You can then tune the opening up or down (55 to 60 or 45 to 40) to perfect the fit of it's opening so as to remove any flat-spots or sogginess in acceleration pull. The power valves shouldn't even do much on a jet as the hi-load/lo-rev condition just doesn't exist with a jet pump drive. I've seen this question a lot of times and researched a lot of info on it. A lot of the above is from Holley website.

[Infomaniac]

Can you send my that link Smokin? The powervalve's own circuit is a passage from the float chamber through the power valve restrictions (jets) to the main well and it ends there.
And I am looking for a technical (theory) explination on how the power valve actually affects the idle Circuit.
My main discussion point is the idle jet and mixture screws are the final metering device. A change in fuel level or engine vacuum are the only other ways I can see to enrich the idle mixture. Everything else functioning properly

[INEEDAV]

Can you send my that link Smokin? The powervalve's own circuit is a passage from the float chamber through the power valve restrictions (jets) to the main well and it ends there.
And I am looking for a technical (theory) explination on how the power valve actually affects the idle Circuit.
My main discussion point is the idle jet and mixture screws are the final metering device. A change in fuel level or engine vacuum are the only other ways I can see to enrich the idle mixture. Everything else functioning properly
Ron
I am looking for a good article on this, my Holley Car book has a lot of information, but it is spread out. Here is a drawing I think will clarify some things for people in this discussion. Good topic by the way.
http://www2.***boat.com/image_center/data/506/169powervalve.jpg
This is out of Super Tuning and Modifying Holly Carbueretors, written by Dave Emanuel. Printed by CarTech INC.
http://www2.***boat.com/image_center/data/506/169powervalveverbiage.jpg

[058]

If a Holley is working correctly the power valve will not have any effect on idle. The power valve channel restriction [pvcr] dumps directly into the main jet well and has nothing to do with the idle circuit just the same as the main jets have no effect on the idle. The idle circuit draws fuel from the same main jet well as the main jet and the pvcr is located but since the idle feed restriction [IFR] is in the range of .026" to .050" it is the metering device that controls idle fuel not the power valve, PVCR or the main jet. If there is a problem with the carb where there is fuel dripping from the booster venturi then the float level is not correct [that can be due to several unrelated reasons] throttle blades open too much at idle or too much fuel pressure. Another problem that can contribute to fuel dripping into the engine is cross channeling [fuel leaking from one channel to another] due to a warped main body or leaking metering block gasket. This fuel cannot be seen unless you look closely to the idle transfer slot and the idle discharge port. Since the idle circuit draws from the main jet well float level will have a minor effect on idle adjustability and quality. The 2 most common causes of fuel level control is crap in the needle/seat assembly or the small O ring sealing the needle/seat goes flat allowing leakage. Ruptured power valve diaghpram will dump fuel into the engine much same way as cross channeling but will leak thru the vacume port that is at the base of the carb.

[Moneypitt]

If the venturi are dripping, there are only a couple of reasons. Either the floats are too high, OR the butterflys are cracked open and drawing vacuume............the pump is exceeding the needle and seat pressure, dirt.....MP

[Blown 472]

Can you send my that link Smokin? The powervalve's own circuit is a passage from the float chamber through the power valve restrictions (jets) to the main well and it ends there.
And I am looking for a technical (theory) explination on how the power valve actually affects the idle Circuit.
My main discussion point is the idle jet and mixture screws are the final metering device. A change in fuel level or engine vacuum are the only other ways I can see to enrich the idle mixture. Everything else functioning properly
The only way to change the air fuel ratio of the idle is to mess with the bleeds or the restriction the screws only meter the amount of emlusified air fuel into the discharge.
The pv can affect idle if the diaphram is torn other then that no way.

[Infomaniac]

Well I was pretty sure I knew how carbs work but am willing to be corrected if needed and keep an open mind to learn something new.
Not picking on anyone but I read all the time how the power valve is messing with someone's idle, causing blower surge at idle etc.

[Blown 472]

Well I was pretty sure I knew how carbs work but am willing to be corrected if needed and keep an open mind to learn something new.
Not picking on anyone but I read all the time how the power valve is messing with someone's idle, causing blower surge at idle etc.
I had that happen due to having the blades open too far and was into the transfer slot and the starting to pull fuel from the venturis. I ended up drilling holes in all four blades and putting them back down to get out of the transfer slot and it cleaned it up a ton.
I drilled the air bleeds out a bit to lean it down and it helped the idle and the off idle is kick ass now.

[INEEDAV]

Personal experience tells me that the power valve did effect the amount of fuel in the main well and effected the way my motor idles, maybe I am wrong. Wouldn't be the first time, just is what I saw. It acted like it was loading up, and would idle like crap.
Here is another article on the subject in Hot Rod.
Hot Rod Article (http://www.hotrod.com/techarticles/588/)

[steelcomp]

OK...I have a question...if it dosen't matter if the power valve is open during idle or not, then why have it?? Why bother with it? I still say that if you're introducing extra fuel in to the main well and not using it at the rate it was intended, you're going to screw with the dynamics of the system. Fuel has weight, which can be referred to as head pressure or static pressure in a system, and if you increase system pressure, you increase flow through a given restriction.

[Infomaniac]

OK now this is just interesting discussion.
Well the power valve is there for a reason but not for idle reasons.
The power valve is the power enrichment system. It is there to deliver "extra" fuel during high power conditions. When the throttles get wide open or close to wide open the manifold vacuum drops to a point to where it can no longer hold the power valve CLOSED. The valve will open and deliver extra fuel to make sure the engine does not go lean during high power conditions. that is why the valve delivers fuel after the main jets. Because it suppliments the main system. Some older carbs had mechanical power enrichment systems that lifted a needle off the seat at about 70% throttle opening.
Sure you can give the main system extra fuel during idle but the fuel in the main system still has to go through the idle system before it goes into the engine. The idle jet, mixture screw or even the idle discharge nozzle will still meter the fuel. And the extra fuel introduced into the main well is introduced below the fuel level in the carb. The extra fuel is available from the power valve but not being used.
Just like the two container analogy I posted before. open and close a passage between two containers below the level in each and it does nothing to the level in either.
Some folks boost reference their blower carbs. The carb on top of a blower is not seeing engine vacuum, it is seeing blower vacuum. The blower can draw a vacuum during high power that is enough to suck a power valve shut. Just the opposite of how an engine produces vacuum.

[058]

OK...I have a question...if it dosen't matter if the power valve is open during idle or not, then why have it?? Why bother with it? I still say that if you're introducing extra fuel in to the main well and not using it at the rate it was intended, you're going to screw with the dynamics of the system. Fuel has weight, which can be referred to as head pressure or static pressure in a system, and if you increase system pressure, you increase flow through a given restriction. The power valve is used to control fuel only when there is flow thru the booster venturi, when the carb is into the main circuit. Some people have referred to the power valve as an economizer valve as that allows the carb to run a smaller main jet at part throttle but will allow enough fuel thru to the engine at WOT or when the manifold vacume drops to the set value of the power valve.

[Infomaniac]

Could not get my Photoshop skills going tonite.
Here is the drawing posted earlier.
The line drawn is approx where the fuel level is in the carb. everything below that line is full of fuel (dots) lol. The float chamber is one container and the main well is the other container. open and close that power valve all you want to and it will not affect the fuel level in the main well. It is way below the fuel level in the carb.
http://moritzmachine.com/level.jpg

[058]

OK now this is just interesting discussion.
Well the power valve is there for a reason but not for idle reasons.
The power valve is the power enrichment system. It is there to deliver "extra" fuel during high power conditions. When the throttles get wide open or close to wide open the manifold vacuum drops to a point to where it can no longer hold the power valve CLOSED. The valve will open and deliver extra fuel to make sure the engine does not go lean during high power conditions. that is why the valve delivers fuel after the main jets. Because it suppliments the main system. Some older carbs had mechanical power enrichment systems that lifted a needle off the seat at about 70% throttle opening.
Sure you can give the main system extra fuel during idle but the fuel in the main system still has to go through the idle system before it goes into the engine. The idle jet, mixture screw or even the idle discharge nozzle will still meter the fuel. And the extra fuel introduced into the main well is introduced below the fuel level in the carb. The extra fuel is available from the power valve but not being used.
Just like the two container analogy I posted before. open and close a passage between two containers below the level in each and it does nothing to the level in either.
Some folks boost reference their blower carbs. The carb on top of a blower is not seeing engine vacuum, it is seeing blower vacuum. The blower can draw a vacuum during high power that is enough to suck a power valve shut. Just the opposite of how an engine produces vacuum.
Info, This is an interesting discussion and this issue need to be addressed. There is alot of misunderstanding surrounding the Holley due to Holley publishing information on p/v selection and use the "idle vacume" as a base line for people to choose their power valve. I see it as the same misinterpatation as the accellerator pump setting of having .015" clearance on the pump diaghpram at WOT, many people see this as adjusting a .015" gap on the pump lever at closed throttle.

[steelcomp]

Ron...I've been doing Holleys for 30 yrs...my questions aren'r from lack of understanding on how the system works...more trying to provoke thought. If it dosen't matter if the valve is open at idle, then I ask again, why bother with it? Seems to me there are lots of Holleys running no power valve. I understand the theory behind a PV, but if the PV didn't effect idle mixture or quality, then why would they regulate where it opens or closes? Why not richen the mixture across the board and be done with it? Answer...because there isn't a requiremenet for extra fuel during idle. The PV is a way to enrichen the mixture off-idle as vac drops. If the system is open during idle, it is providing too much fuel to the system. Why or how is obviously unclear here, but the fact that it dose effect idle quality is clear. It may not be there for idle reasons, but it does effect it somehow. Let's keep[ looking into this to find out why. OK now this is just interesting discussion.
Well the power valve is there for a reason but not for idle reasons.
The power valve is the power enrichment system. It is there to deliver "extra" fuel during high power conditions. When the throttles get wide open or close to wide open the manifold vacuum drops to a point to where it can no longer hold the power valve CLOSED. The valve will open and deliver extra fuel to make sure the engine does not go lean during high power conditions. that is why the valve delivers fuel after the main jets. Because it suppliments the main system. Some older carbs had mechanical power enrichment systems that lifted a needle off the seat at about 70% throttle opening.
Sure you can give the main system extra fuel during idle but the fuel in the main system still has to go through the idle system before it goes into the engine. The idle jet, mixture screw or even the idle discharge nozzle will still meter the fuel. And the extra fuel introduced into the main well is introduced below the fuel level in the carb. The extra fuel is available from the power valve but not being used.
Just like the two container analogy I posted before. open and close a passage between two containers below the level in each and it does nothing to the level in either.
Some folks boost reference their blower carbs. The carb on top of a blower is not seeing engine vacuum, it is seeing blower vacuum. The blower can draw a vacuum during high power that is enough to suck a power valve shut. Just the opposite of how an engine produces vacuum.

[steelcomp]

Some folks boost reference their blower carbs. The carb on top of a blower is not seeing engine vacuum, it is seeing blower vacuum. The blower can draw a vacuum during high power that is enough to suck a power valve shut. Just the opposite of how an engine produces vacuum.
Let's not forget one thing here...engines don't suck. Either do blowers. They only provide a void for atmospheric pressure to fill.

[Infomaniac]

Yea Steel I have heard about the PV affecting idle so many times I am hoping someone can explain exactly how. Even though it's function is far from idle.
I am always willing to keep an open mind and learn something new.

[steelcomp]

Info, This is an interesting discussion and this issue need to be addressed. There is alot of misunderstanding surrounding the Holley due to Holley publishing information on p/v selection and use the "idle vacume" as a base line for people to choose their power valve. I see it as the same misinterpatation as the accellerator pump setting of having .015" clearance on the pump diaghpram at WOT, many people see this as adjusting a .015" gap on the pump lever at closed throttle.
Idle vacuum is the best way to baseline your choice for a PV, but that's only a base line. I don't see how that's similar to plain ol' mis-reading clear instructions.

[steelcomp]

Could not get my Photoshop skills going tonite.
Here is the drawing posted earlier.
The line drawn is approx where the fuel level is in the carb. everything below that line is full of fuel (dots) lol. The float chamber is one container and the main well is the other container. open and close that power valve all you want to and it will not affect the fuel level in the main well. It is way below the fuel level in the carb.
http://moritzmachine.com/level.jpg
Under a static condition, I agree...liquid always seeks it's own level, but I'm thinking that somehow the dynamics of pressure differentials here have something to do with it. I certainly don't have the answers. It's all speculation on my end. Educated ignorance. :rolleyes:

[Wicked Performance Boats]

Idle vacuum is the best way to baseline your choice for a PV, but that's only a base line. I don't see how that's similar to plain ol' mis-reading clear instructions.
Setting the acc pump wrong is the 2nd most common problem on holleys. And yes I have seen many power valves that are bad, dump fuel in the carb and cause an overrich problem. I believe once the motor is running, it pulls enough vaccum that if the power valve is bad, fuel starts dripping and affects idle quality. But personally I still prefer power valves in a normally asperated motor. Budlight

[Floored]

only time I have had a prob at idle is the PV blown and fuel drawn through the PV diaphragm into the manifold. Wrong PV will give a hesitation in my jet on part throttle transition that could be masked with bigger acc pump shot, but getting the PV timed correctly makes a smoother transition from idle to main jets for pulling skiers etc. I find that I dont just hammer my boat I drive it around so smoothness is my goal. I backed my boat into the water(strapped to trailer) and let it run to warm up and checked idle vacuum(6.5 pump loaded) as I slowly brought the R's up my vacuum went up slightly to 8.5 then dropped down again, so I put in a 7.5 PV and my part throttle hesitation went away and has run great for years now. With my originel 5.5 PV I had a hesitation that drove me nuts. My idle vacuum is actually less than the PV opening point but no effect on idle quality.

[058]

I don't see how that's similar to plain ol' mis-reading clear instructions.
I dunno, You will have to ask the people that had a problem understanding clear instructions but as W.P.B. said its the 2nd most common problem on Holleys.

[HP350SC]

I don't have a PV laying around here to verify, but my understanding was fuel leaks from a torn diaphragm, obviously affecting mixture. I tend to agree with Ron, looks like no affect if the valve was stuck open (but not leaking from diaphragm).

[Fiat48]

And all this is why injectors are the way to go.
You knew I was gonna say that if Gofastracer did not beat me to it. :D
Anyway...my understanding of a "power valve" is a fuel enrichment circuit that would not be open at idle...therefore not affecting idle UNLESS...the engine did not make enough vacuum due to camshaft or vacuum leak or bad power valve diaphram which would allow the power valve to open AT IDLE...which it is not designed to do...and would enrichen the idle circuit, flooding, etc.
The power valve number stamped on it (ex 6.5) is the inches of vaccum or lower which the power valve circuit opens.

[steelcomp]

The PV is a way to enrichen the mixture off-idle as vac drops. If the system is open during idle, it is providing too much fuel to the system.
Exactly what I've been trying to say.

[tbanzer]

I would gues that its similiar to why float level differences have a small affect on overall jetting. With the power valve open it supplies more fuel to the main well possibly causing more pressure to feed the idle circuit. The idle circuit uses such a small amount of fuel that even small variances have a noticable affect.

[GofastRacer]

Wow another thread that's blown out of proportion, I love it!.. :D :D

[GofastRacer]

And all this is why injectors are the way to go.
You knew I was gonna say that if Gofastracer did not beat me to it. :D
That's only because I been too busy over the weekend!.. :D

[GofastRacer]

Yea Steel I have heard about the PV affecting idle so many times I am hoping someone can explain exactly how. Even though it's function is far from idle.
I am always willing to keep an open mind and learn something new.
Ron, the big confusion is because we know how a carb works. The PV has absolutely nothing to do with the idle "SYSTEM" as we all know BUT if you have a ruptured PV you will have a leak and have extra fuel dumping at idle and screws up the idle, if you can turn the idles mixture screws all the way in and it keeps on running then you have a ruptured PV. That's how it affects the idle, not the system!..

[Fiat48]

That's only because I been too busy over the weekend!.. :D
I knew I could flush you out! :D

[GofastRacer]

I knew I could flush you out! :D
LMAO, seriously though I have been too busy to hang out on the boards!.. :D

[INEEDAV]

Wow another thread that's blown out of proportion, I love it!.. :D :D
Art
How could you say that this is blown out of proportion? It is nice to see a thread where someone with knowledge like Ron is sharing his ideas and knowledge with people like me who could use more understanding and theory.
I have learned a ton off these boards, if it was full of people who understood and knew everythign how boring would this be?
Billy

[SmokinLowriderSS]

The power valve has a suction porting to provide a vaccume signal to it so to regulate it's opening and closing. This port is below the butterflies and is seperate from either the idle bleeds and the venturis (which are several inches higher and have zero vac signal at idle (or darn near zero). If the manifold vac is lower than the valve's opening point, it opens by a spring pressure. If the vac is higher than the set point, it pulls the PV closed. If the PV is damaged, not gasketed correctly, or if it is exposed to a low vac signal (such as a wide throttle opening at low RPM (per design) or due to a long-overlap cam with large intake runners (a very hi-rpm optomized intake system)) then the PV will open, including at idle.
The extra fuel is supplied through it's own enrichment circuit seperate from the venturis and released into the carb throats well below the venturis. Due to design, venturis need a strong signal to suck fuel uphil. PV system works in absence of a good vac to allow fuel to run "downhill" to enrich in a condition that would otherwise cause leanout. The reason for the PV system is to "fill in" when there is insuficient vac or airflow at large throttle opening with low RPM to pull from the venturis, thus causing a leanout. I'm looking for a picture to show this better.
http://www.holley.com/data/TechService/Technical/power_valves.pdf
Per Holley, your power valves need to have a set point AT LEAST 2"hg below the lowest idle vac reading on a fully warmed up engine.
This page even references the problems that occur and need special attention if running PV's with blowers. In the picture you can see the small bleed port that will allow any fuel leaving the PV to enter the airstream at any time, regardless of idle mixture screws or venturis through the port in the bottom of the baseplate where the vac signal goes to the PV.
http://www.holley.com/data/TechService/Technical/Power%20Valve%20Tech%20Info.pdf
Another picture on Pg 5:
http://www.holley.com/data/TechService/Technical/Carburetor%20Tech%20Info.pdf

[cruser]

Niether a guru or expert. Just observed from the comments here a point htat I think was missed. There is a vacuum signal that regulates the operation of the power valve. I twas stated that when the diaphram on the Pv is torn, fuel drips into the intake, causing the motor to run rich at idle.
I propose that when the diphram is ripped, the extra fuel goes through the PV control circuit directly from the fuel bowl, by passing the Idle restriction orifaces altogether.
This does not address the rich condition that some see at idle due to incorrect PV choice.

[Squirtin Thunder]

Niether a guru or expert. Just observed from the comments here a point htat I think was missed. There is a vacuum signal that regulates the operation of the power valve. I twas stated that when the diaphram on the Pv is torn, fuel drips into the intake, causing the motor to run rich at idle.
I propose that when the diphram is ripped, the extra fuel goes through the PV control circuit directly from the fuel bowl, by passing the Idle restriction orifaces altogether.
This does not address the rich condition that some see at idle due to incorrect PV choice.
Yep !!!
It seems that most peps run too low of a PV and never see the benifits from the PV. After reading a artical on PVs from Holley I noticed that I have been doing things all wrong for many years.
ABTW- They sure know what they are talking about !!! What a difference !!!
Also if it is leaking they will not work right, I mean the gasket !!!

[SmokinLowriderSS]

That is what I have been saying that everyone seems to be missing. There are 3 seperatew feed circuits in a holley carb, idle, main, and power valve, all with seperate feed mechanisms and passages. if the PV either leaks from failure, poor sealing, or by incorrect "size", it feeds fuel through it's own passages bypassing the idle circuit. It is designed to bypas the main circuit venturis in correct operation. Any supply by it at an incorrect time causes richness, whether it enriches too soon in the proper manner or seeps/drools at idle.

[GofastRacer]

Art
How could you say that this is blown out of proportion? It is nice to see a thread where someone with knowledge like Ron is sharing his ideas and knowledge with people like me who could use more understanding and theory.
I have learned a ton off these boards, if it was full of people who understood and knew everythign how boring would this be?
Billy
Relax man, just funnin!... ;) :D :D

[steelcomp]

That is what I have been saying that everyone seems to be missing. There are 3 seperatew feed circuits in a holley carb, idle, main, and power valve, all with seperate feed mechanisms and passages. if the PV either leaks from failure, poor sealing, or by incorrect "size", it feeds fuel through it's own passages bypassing the idle circuit. It is designed to bypas the main circuit venturis in correct operation. Any supply by it at an incorrect time causes richness, whether it enriches too soon in the proper manner or seeps/drools at idle.
SLRSS...I don't know where you got the info that says the PV has it's own mechanism, but AFICS, there are only two ports leading away from the PV passage and they lead directly into the main well. There's only two places the fuel can go from there...past the gasket, or through the dia. There's no "secret" passages, or seperate passages that I can see.
I have to admit, after looking at this more closely that I ever have, I understand Ron's point that logically, it dosen't seem as if the PV can effect idle, but again, I ask the question...if id dosen't effect idle, then why have a system that differentiates between idle and not? Or is that just a by-product of it's design, needing something like a vacuum signal to activate it? What about carbs with mechanical enrichment systems like Carters and Snodchesters? Are there idle issues when the needles are mis adjusted?
And here's a wrench in the monkeyworks. As a kid, when I was tuning the 500cfm Holley 2bbl on my sister's 289, I swear that by changing the main jets it made the engine idle leaner. It used to start ice sold and idle with no choke. My old man said it was too rich, so we changed jets, and it solved the problem. (He knew nothing about Holleys at that time, and neither did I) Now we may have inadvertantly done something else to the carb, but not intentionally. So all this time I had thought that by changing mains you would also effect your idle mixture, and have to adjust acordingly. I've always noticed a difference in idle after jetting...or maybe I just thought I did. :idea:

[Infomaniac]

logically
LOL I think this is where we get into trouble. Trying to bring logic into it. But it is fun debating logic and theory.
I'm thinkin that Holley has you select your PV from idle vacuum because that is a baseline to use that is easily measured. Most folks can hook up the vacuum gauge and stand under the hood to read it while the engine is idling. Naturally the best way would be to measure the manifold pressure wide open under a load and select the PV from that reading.
Or block it off and jet up. Or run injection and not worry about it. LOL

[Wicked Performance Boats]

LOL I think this is where we get into trouble. Trying to bring logic into it. But it is fun debating logic and theory.
I'm thinkin that Holley has you select your PV from idle vacuum because that is a baseline to use that is easily measured. Most folks can hook up the vacuum gauge and stand under the hood to read it while the engine is idling. Naturally the best way would be to measure the manifold pressure wide open under a load and select the PV from that reading.
Or block it off and jet up. Or run injection and not worry about it. LOL
If you have ever called the Holley tech line. By the sound of their accent, I think they made that pv test while they were leaning over the fender of a pickup truck in the parking lot drinking beer! Budlight

[steelcomp]

LOL I think this is where we get into trouble. Trying to bring logic into it. But it is fun debating logic and theory.
I'm thinkin that Holley has you select your PV from idle vacuum because that is a baseline to use that is easily measured. Most folks can hook up the vacuum gauge and stand under the hood to read it while the engine is idling. Naturally the best way would be to measure the manifold pressure wide open under a load and select the PV from that reading.
Or block it off and jet up. Or run injection and not worry about it. LOL
:D :D

[SmokinLowriderSS]

I'll be pulling my 4150 open tomorow night anyhow putting my power valve back in. I have not ben pleased with the tests I have done involving re-jetting away from using a PV and blocking the PV off. I will report tomorow PM just how the fuel makes it from the float bowl to the manifold regarding the PV. I am not familiar enough with the passages and none of the pix posted are helpful to me. :cool:

[Bense468]

If the diaphram is blown it will affect idle. On blower motors I would not run a PV

[SmokinLowriderSS]

Well, I will correct some of what I have posted here recently on this subject but I cannot argue with Holley's claims of the phenomena. I am just at a bit of a loss as to explain how. As folows:
There are 2 small holes in the metering body casting that the PV passed fuel must enter they are the ONLY passages there are). These holes lead to two vertical slots in the metering body which run alongside, separated from BUT CONNECTED TO (because of holes drilled between them and) the main jet fuel passages. This is downstream (after) the main jets and BOTH these passages lead up the carb to the entrance to the main venturis.
This is my confusion. The jets are lower in the casting, the PV is higher, the fuel level is the same, under identical conditions, the engine is unable to draw fuel from the venturis from the jets but, if the PV is open it will draw fuel through the venturis? That is exactly where PV fed fuel goes. It just don't make sense but it is apparently a known "problem" if you use the wrong-setting power valve. Oh well. The spray-can of carb cleaner don't lie as to where the stuff sprays from.
Holley actually makes a set of carbs designed to be blower mounted with power valves. I have no idea as to how they feed vac data to the PV's (they DO have them) but they claim to.

[Oldsquirt]

but again, I ask the question...if id dosen't effect idle, then why have a system that differentiates between idle and not? Or is that just a by-product of it's design, needing something like a vacuum signal to activate it?
Steel, it is NOT differentiating between "idle" and "not idle". It is differentiating between LOAD and NO LOAD(or LIGHT LOAD). The vacuum that holds the PV closed is manifold vacuum. Under load, it drops below the preset value of the PV, opening the PV which then supplies the extra fuel to the main metering system. When the vacuum is high enough it stays shut.
So why is it there? As 058 said earlier it is sometimes called an "economizer" valve. It allows the main jets to be smaller allowing for better fuel economy at lighter loads(higher manifold vacuum). Picture driving down the road at 65. Flat road so you have light throttle opening, light load, but you are into the main fuel circuit. There is no reason to have the main jetted for max load demand under this condition. Plant your foot to pass somebody and manifold vacuum drops, opening the PV. Now you have fuel from the PV added to that of the main jets to provide your max load needs. If you were never going to have any throttle opening other than closed and WOT, you could easily eliminate the PV.
I'm gonna have to go stare at the Rochesters and Carter AFB out in the garage to answer about those, but they, too have a load- or demand-enrichment system.

[Infomaniac]

I'm gonna have to go stare at the Rochesters and Carter AFB out in the garage to answer about those, but they, too have a load- or demand-enrichment system.
Probably a taper or step in the needles man.

[Infomaniac]

I am just at a bit of a loss as to explain how.
Made ya look !! LOL
Dude I taught carbs in A&P school and I made every student bring me their carb in a thousand pieces. I picked components at random they had to identify and I made them explain how fuel traveled through each system passage by passage. It was a dreaded shop project but they learned a ton.

[Oldsquirt]

Probably a taper or step in the needles man.
Yep pretty sure that's how they deal with the fuel side. It's how they move the needle I wanted to check on. :)
In checking.....The AFB uses a pair of "power pistons" located one on each side of the carb. Pistons lift stepped needles through the main jet. Vacuum holds them down. Vacuum drops, pistons lift needles, up leaving a narrower part of the needle in the jet. Pretty sure most of the 1- and 2-barrel Rochester do it the same way.

[SmokinLowriderSS]

I.ve been in that carb a few times over the years but never tracked down the fuel passages before. The rebuilds of it have been done primarally because of not using Sta-bil. This particular discussion has been guite enlightening. :shift: The pix that were being posted (diagrams too) were being of no help and I am finding little explanation on the net. :confused:
I thought I was wrong once. Turned out, I was mistaken. ;)

[steelcomp]

There are 2 small holes in the metering body casting that the PV passed fuel must enter they are the ONLY passages there are). These holes lead to two vertical slots in the metering body which run alongside, separated from BUT CONNECTED TO (because of holes drilled between them and) the main jet fuel passages SLSS...the PVCR's (power valve channel restrictions) feed directly into the main fuel well, which is fed by the main jets. The extra fuel from the PV enters above the main jets, but before ANY of the idle mixture controls. The idle circuuit is fed from the main well. At idle, with the butterflys closed, there isn't enough air flow to "pull" the air/fuel from the main metering system.
However, there is enough "vacuum" beneath the closed throttle plate to "pull" the idle fuel through it's passages. Here's how it goes. Engine vacuum at the curb idle discharge port (located beneath the throttle plate) creates a fuel dilevery signal much like the one that activetes the main system at higher aiorflow rates. Vacuum "pulls" fuel out of the main well through an idle feed restriction, and then through a series of passages where it is eventually emulsified with air admitted at the idle air bleed. Once emulsified, the air/fuel mixture is drawn past the mixture adjusting needle and out of the idle port. (excerpt from from :Super Tuning and Modifying Holley Carb's by Dave Emanuel)
This is my favorite Holley book. It also has three pages describing the power enrichment circuit (power valve) and no where does it talk about it effecting the idle circuit in any way. It does speak to blown PV's and warped castings, and even then never mentions idle as an issue.
SO...in conclusion, I'm happy to announce that another myth has been put to rest. Thanks Ron, for bringing this up. It forced me to go back and refresh all my old Holley know-how, and even learn a few new things I didn't know. :D :D
THAT'S what these threads are all about!! :cool:

[steelcomp]

Steel, it is NOT differentiating between "idle" and "not idle". It is differentiating between LOAD and NO LOAD(or LIGHT LOAD). The vacuum that holds the PV closed is manifold vacuum. Under load, it drops below the preset value of the PV, opening the PV which then supplies the extra fuel to the main metering system. When the vacuum is high enough it stays shut.
So why is it there? As 058 said earlier it is sometimes called an "economizer" valve. It allows the main jets to be smaller allowing for better fuel economy at lighter loads(higher manifold vacuum). Picture driving down the road at 65. Flat road so you have light throttle opening, light load, but you are into the main fuel circuit. There is no reason to have the main jetted for max load demand under this condition. Plant your foot to pass somebody and manifold vacuum drops, opening the PV. Now you have fuel from the PV added to that of the main jets to provide your max load needs. If you were never going to have any throttle opening other than closed and WOT, you could easily eliminate the PV.
I'm gonna have to go stare at the Rochesters and Carter AFB out in the garage to answer about those, but they, too have a load- or demand-enrichment system.
I've heard this reference of the "economizer" valve, and agree with the statement that it's a backwards way of thinking about what the circuit does, since the name "economizer" is describing the operation of a system that's not functioning. It's not economizing anything, if it's doing nothing. It's an enrichment or "power" system, used to fill in that long duration drop in pressure, or any drop in pressure when the throttle plates are opened suddenly and the acc. pump is non functional, and the A/F leans out.
If you were never going to have any throttle opening other than closed and WOT, you could easily eliminate the PV.
If grandma had balls, she'd be grandpa! :D

[Oldsquirt]

I've heard this reference of the "economizer" valve, and agree with the statement that it's a backwards way of thinking about what the circuit does, since the name "economizer" is describing the operation of a system that's not functioning. It's not economizing anything, if it's doing nothing.
Calling it an "economizer valve" was probably a marketing ploy, but that is what it is intended to do.
Think about it this way, if the PV was removed and blocked, what would you have to do with the main jets? What would that do to light load, part throttle A/F ratio, and therefore, fuel consumption? Simple answers. You would jet UP. Fuel consumption at part throttle light loads would go UP as a result. The power valve provides economy by turning off a fuel circuit. I think that that IS doing something. Turning on that circuit when demanded for power is also doing something.
It's an enrichment or "power" system, used to fill in that long duration drop in pressure, or any drop in pressure when the throttle plates are opened suddenly and the acc. pump is non functional, and the A/F leans out.
This implies that it only works briefly during acceleration and that is incorrect. As long as you are under LOAD, defined by the manifold vacuum level the PV is designed to open at, the valve is open and supplying additional fuel to the main circuit.
Isn't the job of the accelerator pump to "fill in" while the main circuit catches up to demand under rapid throttle opening? Since the PV is a part of the main circuit, how can it possibly perform the function you describe?
Again the name "economizer" and "power valve" depends on one's perspective. On one hand you can say it is adding fuel when full power is demanded. On the other hand you can say it's reducing the fuel when power is not demanded. It's just "semantics". :)

[steelcomp]

oldsquuirt:This implies that it only works briefly during acceleration and that is incorrect. As long as you are under LOAD, defined by the manifold vacuum level the PV is designed to open at, the valve is open and supplying additional fuel to the main circuit. :boxingguy Isn't the job of the accelerator pump to "fill in" while the main circuit catches up to demand under rapid throttle opening? Since the PV is a part of the main circuit, how can it possibly perform the function you describe? :lightsabe
steelcomp:It's an enrichment or "power" system, used to fill in that long duration drop in pressure, or any drop in pressure when the throttle plates are opened suddenly and the acc. pump is non functional, and the A/F leans out.
I thought that was pretty clear. :notam:

[HP350SC]

Calling it an "economizer valve" was probably a marketing ploy, but that is what it is intended to do.
Think about it this way, if the PV was removed and blocked, what would you have to do with the main jets? What would that do to light load, part throttle A/F ratio, and therefore, fuel consumption? Simple answers. You would jet UP. Fuel consumption at part throttle light loads would go UP as a result. The power valve provides economy by turning off a fuel circuit. I think that that IS doing something. Turning on that circuit when demanded for power is also doing something.
This implies that it only works briefly during acceleration and that is incorrect. As long as you are under LOAD, defined by the manifold vacuum level the PV is designed to open at, the valve is open and supplying additional fuel to the main circuit.
Isn't the job of the accelerator pump to "fill in" while the main circuit catches up to demand under rapid throttle opening? Since the PV is a part of the main circuit, how can it possibly perform the function you describe?
Again the name "economizer" and "power valve" depends on one's perspective. On one hand you can say it is adding fuel when full power is demanded. On the other hand you can say it's reducing the fuel when power is not demanded. It's just "semantics". :)
Excellent explanation Oldsquirt.

[HP350SC]

Some more food for thought. Realistically how much time in a blown boating application would a power valve need to be closed ? Info. knows why I am asking this ;)

[Oldsquirt]

.....used to fill in that long duration drop in pressure, or any drop in pressure when the throttle plates are opened suddenly and the acc. pump is non functional, and the A/F leans out.
These are two separate conditions. The first is a long WOT, or near WOT, run. It can be an acceleration or steady speed(I'm picturing a long run across the lake at WOT). Classic periods for power circuit to be functioning.
The second case is the one that bothers me. I would expect that if the throttle plates are opened suddenly(which implies they were fully or partially closed first), the accelerator pump would activate to cover the lean spot.
I don't see how you can suddenly open the throttle WITHOUT activating the accelerator pump. At the same time if the throttle was closed sufficiently to turn off the power circuit, there will be the same "lag" in flow through that circuit when it is turned back on as there is in the rest of the main metering circuit. If the throttle WASN'T closed enough to turn off the power circuit, you are still under load, although maybe have gone from "heavy" to "medium", but the circuit is on because of the existing load not the added load from additional throttle opening.
I guess what I am trying to say is that I was taught, and believe, that the power circuit is for steady state enrichment, not for transitional enrichment which is the accelerator pump's job.

[Oldsquirt]

Some more food for thought. Realistically how much time in a blown boating application would a power valve need to be closed ? Info. knows why I am asking this ;)
Although its not blown, one of the things I plan to test, once I get my damn boat back in the water(upholstery guys sure can work slow), is manifold vacuum under varying throttle/speed conditions. Very curious to see just how heavily loaded the engine is and how much of the time the PV would be "on". Of course I've been planning to do this for a couple seasons and it seems everytime the boat hits the water that plan gets tossed out in favor of having fun. :D