holley carb power valves

grumpyvette

Administrator
Staff member
yes its a P.I.T.A. but reading thru the links holds a lot of info you'll miss if you don,t take the time
this is a power valve its screwed into a metering plate between the carb fuel bowl and the carb body in many carbs, it restricts
additional fuel flow until the vacuum drops to a known level stamped on the power valve rim,
What the power valve is supposed to do is allow additional fuel flow, that comes in as the velocity of air through the carbs venturi speeds up and the vacuum reading drops,it opens, as the vacuum drops and the accelerator pump has mostly run out of stroke and is in the process of supplying a shot of extra fuel to cover the rapid increase in air flow during the transition from part to fully open throttle that starts during acceleration. the diaphram on the power valve senses the throttle is open and opens a valve which allows a richer fuel/air ratio mixture to flow until you lift off the throttle .
as the vacume reading increases the power valve senses the increase and then it closes and the carbs fuel delivery internal fuel routeing goes back to standard jetting, Leaning its mixture out for only partial throttle open operation used for steady speed operation like durring low speed cruising.

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SO when you depress the throttle the accelerator pump cam forces fuel through the accelerator pump that sprays into the venturies, as the throttle blades open, and the vacuum drops away rappidly to much lower levels.
The power valve senses low vacuum and opens up and allows more fuel to flow through the carberator allowing a richer and better, less detonation prone power producing mixture, and thus you don,t get a big "BOG" or cough when the carb suddenly ingested more air and the carbs jetting alone can,t keep up with demand, the power valve fuel supply flows fuel until you lift off and the vacuum in the intake plenum increases, and shuts it off.

as with most subjects a few dollars spent on related reading material is money well spent
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http://www.amazon.com/Holley-Carburetor ... lley+carbs
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Perfect for naturally aspirated. It's sensing the engine and doing the right thing at the right time.


keep in mind the power valve supplys extra fuel as the vacuum drops as the throttle blades open in the transition process from idle to full throttle, the power valve allows additional fuel to flow, as the plenum vacuum drops.

Ive generally found youll want to START the tune with the power valve used being selected too be close to 1/2 the vacume/plenum reading at a steady cruise, at minimum throttle NOT at idle in neutral

POWER ENRICHMENT SYSTEM
The power enrichment system supplies additional fuel to the
main system during heavy load or full power situations.

KEEP IN MIND that you TUNE based on the results youll see in actual operation, and you don,t assume anything here, your almost certainly going to need to change the power valve, jets and accelerator pump cam based on the results youll see under actual operating conditions.
Holley carburetors utilize a vacuum operated power enrichment system
and a selection of power valves is available to “time” this sys-
tem’s operation to your specific needs. Each Holley power
valve is stamped with a number to indicate the vacuum opening
point. For example, the number “65” indicates that the power
valve will open when the engine vacuum drops to 6.5" or
below. An accurate vacuum gauge, such as Holley P/N
26-501,
should be used when determining the correct power
valve to use. A
competition or race engine which has a long
duration high overlap camshaft will have low manifold vacuum
at idle speeds. If the vehicle has a manual transmission, take
the vacuum reading with the engine thoroughly warmed up and
at idle. If the vehicle is equipped with an automatic transmis-
sion, take the vacuum reading with the engine thoroughly
warmed up and idling in gear. In either case, the power valve
selected should be 1/2 the intake manifold vacuum reading
taken. EXAMPLE: 13” Hg vacuum reading divided by 2 = 6.5
power valve. If your reading divided by 2 lands on an even
number you should select the next lowest power valve. EXAM-
PLE: 8” Hg vacuum reading divided by 2 = 4 power valve.
Since there is no #4 power valve you should use a 3.5.
Most of the popular Holley “Street Legal” and “Street
Performance” carburetors incorporate a power valve blow-out
protection system. A special check valve is located in the throt-
tle body expressly for this purpose. This check valve is
designed to be normally open but will quickly seat to close off
the internal vacuum passage when a backfire occurs. Once
closed, the check valve interrupts the pressure wave caused by
the backfire, thus protecting the power valve.
If you have a carburetor older than 1992 (or you have experi-
enced an extreme backfire) and expect a blown power valve,
use this simple test. TEST: At idle turn your idle mixture screws
(found on the side of the metering block) all the way in. If your
engine dies the power valve is not blown
The purpose of a power valve is for it to add the needed extra fuel the engine needs to transition and maintain the correct fuel/air ratio,when you are under wide open throttle. It is not intended to be an enrichment circuit during normal throttle cruising.
If you are lean or rich during normal driving conditions, you need to tune the jets, not the power valve.
The reason you check the vacuum at idle and take that reading and divide it in half, is so you do not have a power valve that will open at idle as this will tend too cause an excessively rich mixture at idle, or just off idle. the power valve opens to flow extra fuel during the transition period and while the accelerator pump is also adding fuel, . Under WOT the vacuum of the engine will drop to a vacuum reading of 1.5 or even lower near zero and the power valve will then open adding the extra fuel needed during WOT.
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keep in mind theres lots of small passages that can get clogged and theres several different fuel bowl gaskets, these vary by manufacturer and carb model, so be sure you use the correct gasket on the meter block and fuel bowl, if you leave debris in the fuel bowl or use the wrong gaskets they can block passages and the carb won,t function
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AS ALWAYS YOUR GOING TO MISS A TON OF INFO IF YOU SKIP THE SUB LINKS
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GET A FUEL PRESSURE GAUGE AND MEASURE DON,T GUESS
how can you possibly set up your fuel system unless you know the pressure and flow rates required and what currently exist's
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READ LINKS

http://garage.grumpysperformance.com/index.php?threads/setting-up-your-fuel-system.211/

http://garage.grumpysperformance.com/index.php?threads/fuel-pressure-regulators.635/

http://garage.grumpysperformance.com/index.php?threads/how-big-a-fuel-pump-do-you-need.1939/

http://garage.grumpysperformance.co...ss-fuel-pressure-regulators.12776/#post-65998


keep in mind a few basic concepts, you can select a 30cc or 50 cc accelerator pump volume ,
the accelerator pump cam will control how fast the carbs linkager starts to cover the transition between idle and fully open venturies when the engine could go lean, remember that a larger squirter size does not give you more total fuel, it only allows more volume to move through the squirter faster, but this only tends to result in a shorter duration squirt, of fuel befor and during the transition,as the power valves and secondary jetting add fuel and air flow, THE Only way to truly know how the combo of power valve,booster type, accelerator pump cam ,squirter, accelerator pump size and carb jets will work on your combo, is to try a couple different combos and see how the 60' times and the engine feels as you accelerate, and both experience and knowledge of how to read spark plugs and change ignition advance curves will help a great deal.



http://forums.holley.com/entry.php?430- ... lve-Tuning

http://www.holley.com/types/Power Valves.asp


http://garage.grumpysperformance.com/index.php?threads/locating-vacume-leaks.882/#post-45944

http://www.enginebuildermag.com/Article ... etors.aspx

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THE O-RING ON THE NEEDLE AND SEAT, AND THE DIAPHRAGM IN THE ACCELERATOR PUMP, CAN BE DAMAGED BY ETHANOL IN FUEL UNLESS AN ETHANOL COMPATIBLE MATERIAL IS USED
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http://www.streetperformance.com/m/cats ... valve.html

the first thing you need to understand is what a power valve does, the simple explanation is that there's a number stamped on the power valve (EXAMPLE 6.5), that relates to the VACUUM reading the carb will need to drop below (in that case 6.5" of vacuum) before the power valve opens and adds additional fuel to what the carbs jets already allow
the higher the number the faster the power valve opens as the vacuum drops as the throttle opens, they really should be called, vacuum activated fuel enrichment valves
at idle your car produces more vacuum because the throttle blades are almost closed restricting flow into the intake, as you open the carb throttle blades vacuum drops off and flow increases, at some point the carbs increased air flow requires additional fuel to maintain the correct fuel/air ratio, the power valve controls the point where that fuel starts to be added using the drop in intake vacuum as the trigger to release more fuel.
power valves have a diaphragm that can be ruptured, a BLOWN or busted power valve will leak fuel in an irregular manor,and not regulate flow, so if it does not test correctly, you'll need to replace it, the most common cause, for a busted power valve is carb back fires,

youll see most places advise you start with a power valve rated at 1/2 the vacume reading, the engine has at idle, if its a manual take the reading in neutral, if its an auto take the reading at idle in gear with your foot on the brake,
EXAMPLE if the vacuum reads, at 15-16" then youll want about a 6.5- 8.5 power valve,in many cases, your generally going to find that a power valve thats marked at about 1/2 the vacuum at idle is a good starting place IE, if your pulling 17" of vacuum the 7.5-8.5 power valves about correct,if your pulling 11" of vacuum the 5.5- 6.5 power valves about correct
Holley's instructions, to use a PV that's half of your idle vacuum, are some what miss leading. They will ALWAYS lead you to choose too low a PV number, which in turn will force you to use too large jets, too much pump, etc. youll generally want to select a slightly higher number power valve if your getting a flat spot,. Keep raising the PV number until that flat spot clears up.
you take the vacuum reading at idle and cut it in 1/2 then jump up or down one -two numbers depending on how the plugs look, the engine runs and what your fuel/air ratio meter indicates example if your pulling 16" at idle that would be a power valve in the 6.5-9.5 range
youll also see advise saying that Basing the PV selection on idle vacuum is BAD advise that has floated around for a long time. If you take CRUISE vacuum, divide it in half and add about 2" you will be close.
read what holley says
http://www.holley.com/data/TechService/ ... 20Info.pdf

http://www.holley.com/data/TechService/ ... valves.pdf


http://www.onallcylinders.com/2013/03/07/how-to-tune-the-power-valves-on-a-holley-carburetor/

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read thru these threads also

viewtopic.php?f=55&t=1790

viewtopic.php?f=55&t=1442

http://www.holley.com/types/Power Valves.asp

viewtopic.php?f=55&t=8126&p=28205#p28205

http://www.chevyhiperformance.com/tech/ ... index.html

http://www.chevyhiperformance.com/techa ... index.html

http://www.jegs.com/p/Holley/Holley-Pow ... 1/10002/-1

http://www.holley.com/data/TechService/ ... valves.pdf

http://video.google.com/videosearch?hl= ... &ct=title#

http://www.classictrucks.com/tech/0806c ... alves.html

http://www.jegs.com/p/Holley/743661/10002/-1

http://www.summitracing.com/parts/MOR-65222/?rtype=10

viewtopic.php?f=44&t=773&p=1123#p1123

http://www.wonderhowto.com/how-to/video ... ve-267173/

http://store.summitracing.com/partdetai ... toview=sku

Ive had a vacuum pump tool for years to test Holley power valves with this adapter, pictured below and I recommend using the tester as it quickly locates leaky power valves, which can make you 1/2 crazy wondering why the carbs f/a ratio won,t be consistent
http://www.summitracing.com/parts/MOR-62295/?rtype=10
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http://www.harborfreight.com/mityvac-va ... 39522.html



http://www.mityvac.com/pages/products_hvpo.asp#

http://www.mityvac.com/pages/products_hvp.asp

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http://www.harborfreight.com/cpi/ctaf/d ... mber=93547

http://www.harborfreight.com/cpi/ctaf/d ... mber=92474

https://www.homedepot.com/p/Powerbuilt-Vacuum-Pump-Kit-648744/203120693
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with a few cheap components you can test a power valve

read this also

viewtopic.php?f=55&t=1790

viewtopic.php?f=70&t=202

viewtopic.php?f=70&t=1411

viewtopic.php?f=70&t=875

viewtopic.php?f=70&t=294

keep in mind there's many guys chasing carb problems that are eventually traced to the ignition, or fuel system, verify the damper and timing tab really do reflect TDC and your ignition advance is working correctly, and verify you have 5 psi of steady fuel pressure at the carb and the floats are set correctly and the cars valves are adjusted and there's no vacuum leaks before you go swearing its solely a carb problem

BTW someone is bound to ask eventually ,what is the difference between "standard flow".
SEE WHAT READING THE PART DESCRIPTION RESULTS IN???
http://www.summitracing.com/parts/hly-125-105/overview/
and "high flow"? in a power valve?
http://www.summitracing.com/parts/hl...1005/overview/
theres both a standard and a high flow rate version in each power valve, generally like with carburetor jets youll want to match the fuel flow rate of the power valve to the engines needs , I've generally found the standard flows work well on anything up to 350 cubic inch and the high flows work better on the big blocks,396 and up, and yes matching the carb jetting and reading the spark plugs and having a FUEL/AIR RATIO METER and a bit of testing is mandatory
 
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grumpyvette

Administrator
Staff member
yes its a good idea to wade thru the linked info, yes it takes some time! but simply because there's a great deal of that information posted that will help you understand exactly how and why things work, its worth the effort
I think youll find that these related threads will provide a good deal of info you might find rather useful

http://garage.grumpysperformance.com/index.php?threads/infrared-temp-guns.11444/

http://garage.grumpysperformance.com/index.php?threads/spark-plug-info.202/

http://garage.grumpysperformance.com/index.php?threads/timing-tabs-and-indicators.1015/

http://garage.grumpysperformance.com/index.php?threads/verifying-your-real-advance-curve.4683/
why not use the correct test tools?
as a general rule you take engines cruise vacuum at about 2000 rpm average it with the idle vacume , lets say ones 17 " of vacuum and the others 19" you take the average which in this case would be 18" add about 2" to the average so you have 20" divide that in 1/2 and youll pick the next higher power valve or in this case a #10.5
lets say the readings are 9" and 11" average is 10" add 2" get 12" , 1/2 thats 6" so you start with a #6.5
http://forums.holley.com/entry.php?430-Holley-Power-Valve-Tuning
theres a vacuum number on the valve test it!
http://forums.holley.com/attachment.php?attachmentid=264&d=1340225776
http://documents.holley.com/techlibrary_power_valves.pdf
http://www.summitracing.com/parts/MOR-62295/?rtype=10
powervalvetester.jpg

http://www.mityvac.com/pages/products_hvp.asp

vacpump1a.jpg

http://www.chevyhiperformance.com/techa ... index.html
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afrchart

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" It Does (QUOTE)
IN the VAST majority of cases a street driven car benefits more from use of VACUUM secondary throttle bores, of course knowing how to tune a carb helps a great deal

viewtopic.php?f=55&t=211

viewtopic.php?f=55&t=635

viewtopic.php?f=55&t=5229

viewtopic.php?f=55&t=1639

viewtopic.php?f=55&t=1790

viewtopic.php?f=55&t=8126
Most carburetors employ what is generally called a power valve circuit. This circuit enriches the air-fuel mixture when the carburetor goes to wide-open throttle (WOT). At WOT, intake manifold vacuum drops to almost zero. When this occurs, the power valve opens and directs more fuel into the main power circuit, in addition to fuel delivered by the main jets. The Holley power valve employs a small rubber diaphragm that is opened by a small coil spring. The valve is held closed whenever sufficient engine vacuum is present. At WOT, engine vacuum disappears and the power valve spring opens the valve, directing fuel through a small, precise orifice in the metering block called the power valve channel restriction. This restriction determines the amount of additional fuel delivered to the engine.

Power valves are used most frequently on the primary side of a Holley carburetor. They allow the carburetor to operate with much leaner main circuit jetting for part-throttle fuel economy. Then, when the throttle is slammed open, the power valve adds additional fuel, creating the rich air-fuel ratio needed for WOT operation.
Most Holley power valve circuits are designed to add the equivalent of 8 to 10 jet sizes of additional fuel. Holley does offer a power valve block-off part that closes the power valve circuit, but this means the jet size must be increased in order to compensate for the lost power valve circuit fuel. Imagine how bad your fuel mileage would be if you had to add 10 jet sizes to the primary side of your carburetor!"
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this is a power valve its screwed into a metering plate between the carb fuel bowl and the carb body in many carbs, it restricts
additional fuel flow until the vacuum drops to a known level stamped on the power valve rim,
keep in mind the HIGHER the number the earlier in the transition process from idle to full throttle, the power valve allows fuel to flow


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jet sizes on the jets can be determined in most cases,
by simply measuring the internal hole diameter
( be wall aware that, some jets are not properly marked, others have been drilled larger than the marked size)
years ago I purchased two of these assortment packs when they cost about 1/2 the current price , Ive tried hard to keep 4 jets in each size in inventory as I use those, 72 jets in assorted sizes for $52 is still a good deal , and it gets better if you buy the pack or two of them, when summit posts a discount code
http://www.summitracing.com/parts/hly-36-181
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http://www.mortec.com/carbs.htm
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if you have a complete set of drill bits they can be used to get a good idea on jet size


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If your tuning a holley carb having the 50cc accelerator pump kit and a selection of accelerator pump cams, a set of jets, squirters,and power valves in your tool box and a decent vacuum gauge, to tune with is a big help at times




https://www.holley.com/products/fuel_systems/carburetor_components/power_valves/

the power valve opens at low vacuum, such as at wide open throttle, and directs more fuel into the main power circuit. The valve itself is a small rubber diaphragm with a small coil spring. When opened, it allows fuel to flow through a calibrated opening in the metering block called the power valve channel restrictor.
think about the power valve as a trigger or valve that opens,
that flushes extra fuel through the carb once the plenum vacuum drops too or below,
the rated vacuum listed on the power valve,
if the power valve number is lets say, 6.5 vacume in the plenum must drop to or below that number for the additional fuel to flow.
put a bit differently lower numbers on power valves tend to kick in the additional fuel later in the power curve or later in the carb throttle blade opening sequence, as the lower vacuum tends to happen a bit later in the throttle transition phase


heres
bits of reference material.
Id try to get as close to the basic listed parameters below as a start point as you can, and the slight hesitation on acceleration might require a larger accelerator pump cam curve , or a power valve that kicks in at a bit higher vacuum level, example swapping from a 7.5 to and 8.5, make only a single change at a time and keep good detailed notes on results, and don,t discard parts, label them, keep in zip lock bags with labels
yeah I might not be up on the latest electronic data loggers but I have a good idea what an engine wants as far as a fuel/air ratio and combustion conditions and ignition advance curves and can read spark plugs, and while each engines a bit different you may be amazed at what consistent condition to start from does to help isolate any tuning problems


http://garage.grumpysperformance.com/index.php?threads/holley-accelerator-pumps-cams.1790/#post-4544

http://garage.grumpysperformance.co...annular-vs-down-leg-boosters.5229/#post-15353

set the float levels and verify the fuel pressure consistent at about 5 psi entering the carb inlet port.
your fuel/air ratio should be in these ranges
Idle- up too about 2500 rpm try for 14.7:1-15:1 f/a ratio
from about 2500 rpm- too about 4500 rpm try to smoothly and predictably transition the fuel/air ratio mix richer to about 13.5:1
from about 4500 rpm- too about 6500 rpm and higher ,try to smoothly and predictably transition the fuel/air ratio mix richer to about 12.5:1
this is only a starting point on the tune but it generally gets you in the ball park and tends to reduce the chances of the engine reaching detonation conditions.
the ignition advance curve needs to be checked, the chart below is a very good starting point to work from

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set the plug gaps at about .045, make sure the valves are adjusted correctly
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http://www.mortec.com/carbtip1.htm

viewtopic.php?f=44&t=1116

http://www.bob2000.com/carb.htm

http://www.valvoline.com/carcare/articl ... 4&scccid=1

http://www.holley.com/TechService/Library.asp

High Flow Power Valves are intended for Alcohol applications while Standard are intended for Gas applications and a Gas application will not benefit from the use of High Flow Power Valves and in fact would run worse not better. So for gas applications stay with the Standard Flow Power Valve.

http://www.nationaltbucketalliance.com/ ... /carbs.asp
 
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grumpyvette

Administrator
Staff member
The Truth About Power Valves Used with Holley Carburetors


The power enrichment system supplies additional fuel to the main system during heavy load or full power situations.


There still seems to be a lot of misconception about Holley carburetors blowing power valves. Nothing could be further from the truth, according to the company. Holley performance carburetors since 1992 have utilized a power valve check system that effectively eliminated this infrequent problem. Consisting of a spring, brass seat and check ball, the check ball system is 100% effective protecting the power valve diaphragm from damage due to engine backfire.

The power valve check ball is designed to be normally open but quickly seals to close off the internal vacuum passage when a backfire occurs. Once closed, the check valve interrupts the pressure wave generated by the backfire, thus protecting the power valve diaphragm. There is no way that the power valve’s diaphragm can rupture due to an engine backfire, according to the company.

Holley's Power Enrichment System
Holley carburetors utilize a vacuum operated power enrichment system and a selection of power valves is available to “time” this system’s operation to your specific needs. Each Holley power valve is stamped with a number to indicate the vacuum opening point. For example, the number “65” indicates that the power valve will open when the engine vacuum drops to 6.5˝ or below.

An accurate vacuum gauge, such as Holley P/N 26-501, should be used when determining the correct power valve to use. A competition or race engine which has a long duration high overlap camshaft will have low manifold vacuum at idle speeds. If the vehicle has a manual transmission, take the vacuum reading with the engine thoroughly warmed up and at idle.

If the vehicle is equipped with an automatic transmission, take the vacuum reading with the engine thoroughly warmed up and idling in gear. In either case, the power valve selected should be 1/2 the intake manifold vacuum reading, or higher taken. EXAMPLE: 13˝ Hg vacuum reading divided by 2 = 6.5 power valve. If your reading divided by 2 lands on an even number you should select the next higher power valve. EXAMPLE: 8˝ Hg vacuum reading divided by 2 = 4 power valve. Since there is no #4 power valve you should use a 4.5. as a start point then normally work higher untill it runs correctly

Most of the popular Holley “Street Legal” and “Street Performance” carburetors incorporate a power valve blow-out protection system. A special check valve is located in the throttle body expressly for this purpose. This check valve is designed to be normally open but will quickly seat to close off the internal vacuum passage when a backfire occurs. Once closed, the check valve interrupts the pressure wave caused by the backfire, thus protecting the power valve.
holleypowe.jpg

If you have a carburetor older than 1992 (or you have experienced an extreme backfire) and expect a blown power valve, use this simple test. TEST: At idle turn your idle mixture screws (found on the side of the metering block) all the way in. If your engine dies the power valve is not blown.

– Tech Tip courtesy of Holley Performance

http://wiki.answers.com/Q/How_does_the_ ... retor_work



Main Jet: Controls the amount of fuel that enters the Main Well of the metering block.
Main Well: The passage that carries fuel from the jets to the Boosters.
Booster: The ring that is visible when looking down the throttle bore.
Power Valve: A vacuum operated device that controls the activation of the power valve system (more on this later).
Power Valve Channel Restrictions (PVCRs from now on): Two small holes, one for each Main Well, that act like jets to control the amount of fuel delivered from the Power Valve System.


First let's start with the Power Valve itself.

A Power Valve is little more than a vacuum operated on/off switch for fuel. It operates using a vacuum diaghram and a spring, which determines when the valve opens. I believe this is best explained using an example. Let's use an engine that makes 13" manifold vacuum at idle, a typical lightly modified performer. Knowing that value, we know that a 6.5" rated power valve would be used (typical power valve selection is 1/2 the amount of vacuum at idle, in our case 13/2=6.5). So, during idle and light cruising (above 6.5" vacuum) the power valve will be in the closed position ("off"), but during moderate to full throttle operation the vacuum will drop to below 6.5" and will cause the valve to be open ("on"). The power valve itself doesn't control the amount of fuel delivered, that's the PVCR's job.


Now to the Power Valve System (PVS from now on).

The easiest way to explain the PVS is that it functions like a vacuum actuated set of jets built inside the metering block. What this means is that when the power valve is activated ("on") it will allow fuel to flow through the PVCRs into each main well (one for each main jet), on it's way to the boosters, thus making the fuel mixture "richer". This system is one of the reasons Holley carburetors are so tuneable. It allows the carb to be jetted for maximum economy / driveability, while at the same time making it possible to tune for full throttle performance without having to rely soley on the secondaries. Remember the PVCRs control the amount of fuel delivered, not the power valve.


Why does the carburetor need a PVS?

It all comes down to economy vs driveability vs performance. For the sake of simplicity, I'll leave out the secondary system and focus solely on the primary side. If there were no PVS, all of the fuel required would need to be delivered by the main jets. This is a problem because as engine vacuum decreases the need far a larger jet arises since the booster signal weakens. What this means is that if a carburetor was jetted for idle / cruising situations, the moderate / full throttle performance would suffer, and be too lean. On the other hand, if the carburetor was jetted for maximum full throttle performance it would run too rich at idle / cruise, since the higher vacuum draws a stronger signal. The PVS was designed to combat this very problem, and does it very well.



Removing (blocking) the Power Valve from the secondary block.

This is something that is very common practice among racers. The reason it works in the secondaries and not in the primary is because you never use the secondary circuit while just idling or cruising around, so the problem listed above doesn't exist. Just remember that if the secondary power valve is removed (blocked) the jets must be increased to compensate of the lack of fuel. Usually an increase anywhere from 6-10 sizes is needed, depending on PVCR size and carburetor cfm.


http://www.angelfire.com/fl4/pontiacdud ... Info2.html


There is a lot of misunderstanding concerning power valves in Holley carburetors. Many 4-barrels come with a particular power valve depending on the carb list # and application. Some carburetors have two power valves, while others only have one. The power valves are numbered by the amount of engine vacuum in inches at which they will open and add additional fuel to the power circuit. In other words a 6.5 power valve will open when the vacuum signal on the engine drops below 6.5" of vacuum and will remain closed above. One of the misconceptions is that they can't be trusted to work because an engine backfire can "blow out" the power valve. Many of the newer Holley performance carburetors now come with a built in power valve "blow out" protection which eliminates this problem. If you have an older model carb you can purchase a small, inexpensive, easy to install Holley kit #PN - 125-500 that will protect the power valves in case of an engine backfire in the carb. I use the CENTEK, "Power Valve Shield". See their website at www.powervalveshield.com , which takes about two minutes to install and does not require any drilling.

Many tuners will automatically remove the power valves and use a "plug" thinking this is the "hot" ticket. However, if the power valve is removed and plugged, the main jet size must be increased 6-10 jet sizes to make up the required fuel amount lost by the removal of the power valve. When the power valve circuit is plugged, part throttle fuel economy is lost and may become overly rich. Plug fouling may become a problem at part throttle.

Stock engines have high vacuum readings (10-18 inches at idle) and the Holley power valves with higher readings like 6.5 to 10.5 will work. Longer duration non-stock camshafts and other performance related parts can cause a problem, because engine manifold vacuum may be lower. The power valve, if incorrect, will always be open, even at part throttle, leading to an overly rich air/fuel mixture. The solution is to choose the correct power valve. To determine this, you need a vacuum gauge. On a manual transmission vehicle, hook up the vacuum gauge and take the reading with the engine at idle. Then use a power valve that is rated 1-2 inches below that amount. For example, a motor that shows 7" of vacuum at idle should use a 6.5 or 5.5 rated power valve. If you have an automatic transmission, take the vacuum reading at idle in "DRIVE" (with the emergency brake on and the wheels blocked) and chose the power valve 1-2 inches below that figure. You can get a little more detailed information by driving the car with a vacuum gauge hooked up with a longer hose so you can read it while driving. Drive the car at medium loads and while cruising and note the various vacuum readings. Then chose the appropriate power valve rating.

Holley makes a performance style "standard" flow or the "high" flow power valve. The latter has larger openings for more fuel flow with bigger engines. "Single stage" power valves are available in 1" increment sizes from 2.5" thru 10.5". Holley also makes a "two stage" power valve that is more for "economy" minded users rather than "performance" enthusiasts.
 

grumpyvette

Administrator
Staff member
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heres a good example of what plugs look like when the idles a bit rich but the power valve is not kicking in to richen the mix during cruse or transition, as air flow increases. notice the black plug body but the obviously very lean condition the plugs running under once the rpms increase
now you might also need to increase the jets size, change the accelerator pump cam, change the ignition advance curve, or set the float levels or check fuel pressure but it looks like increasing the power valve about 2-3 stages is called for, ie, if your running a 6.5 the power valve should be an 8.5-or -9.5, you generally look at idle vacuum and select the power valve that just above 1/2 the manifold vacuum then work up as required, example if you had 16" of vacuum at idle you start with a 8.5 power valve if you get a lean miss, during transition go up two primary gets sizes and swap to a 9.5-or-10.5 power valve, untill the plugs get a faint tan or grey look
similar to this
14pl.jpg


theres a great deal more info here in the links below

viewtopic.php?f=70&t=202

viewtopic.php?f=55&t=109

viewtopic.php?f=70&t=4683
 
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