Power Valves Explained: How they work, how to tune 

love the break down on power valves , a lot harder to change when you have multiple carbs like a six pack.

For all of the young people watching this, try to imagine growing up in the 70's and 80's, being 15 or 16 years old and trying to figure out this stuff before the internet and smart phones. My current carb (Since 2006) is a 1095CFM Demon. My cousin, (A younger, LS guy) refers to it as "A semi controlled fuel leak". Fuel injection is awesome, carbs are old school, I enjoy them both.

I love your Carburetor tech videos. When I get stuck or question something I search them

I was surprised you didnt go over how the early holleys didnt have power valve savers and when the engine backfired it would blow the power valve diaphragm out and it would just mess up everything. I used to drill out the power valve channel and install check valves but holley does that now..!! Lots of good info ...!!

They were known as economy valves many years ago. Two stage power valves were great. The problem is that most people did know anything about power valves to begin with. This valve was for those who understood on how to use them.

I use a air fuel meter and vacuum gage to tune mine. Main jets for air fuel ratio of 14-1 at 2500 driving down the road and the power valve vacuum setting for when it leans out. Right now running 69 jets and 2 inch power valve on a 500 cfm 2 barrel .

From my understanding and studies, the purpose of the power valve is that it allows you to cruise (pretty much at any speed) and you want it to remain closed. You only want to open the flow and enrich when under larger power demands like spirited acceleration and/or WOT. PV's allow the carb to downsize the main jet about 7 numbers vs no power valve so that you can cruise and sip fuel. So where I get a little lost in this video is the various cruise tests. I would be reading those vacuum readings and making sure I chose a PV which is closed under all of those cruise conditions. I am learning all I can and working hard trying to tune multiple holleys to get one working well on my hot 327SBC, but I can't say I'm doing so well yet. Using vacuum, wideband, tach, and video recording gauges and trying..... Have a horrible lean condition on throttle tip in.

I understand the PV number is when the PV starts to open, but I'm having a hard time understanding the spring rates since the spring collapses to keep the PV closed

All good information.... of course we have more questions. With a Manual transmission, the vacuum varies. Add in a draw through turbo setup, and im scratching my head as usual. I see a rich condition under load for sure...and sometimes a lean condition depending on gear im in at the time. and cant seem to jet it down (or up) far enough to fix this. The power valve is my next direction. I assume you will see the power valve open on your AFR gauge. I need to pay more attention to this. Thanks for the tips

Hey you put out some very informative videos. Just a question or two if you don't mind. What would the 30, 40, 50 and 60 MPH show me on my vacuum gauge and my wideband? I understand the vacuum will drop but at what steady speed should the PV kick in? Or am I misunderstood something? Maybe you could do a video (I'm sure it would help a lot of us) on exactly how and when the jets work, how and when the squirters kick in and for about how long. How and when the PV kicks in and when should the secondaries kick in on the jets and/or the PV. Or do the jets along with the PV on the secondary side kick in exactly at the same time? Sorry if I'm going overboard but that would explain a lot, especially to me.

Great breakdown on power valves, and dialing them in! I subbed!

Great video best one out there easy enough to try two different ones if you don't have AF readings then you find what it likes depend on camshaft overlap selection bottom line more cam less vacuum

If your engine is cammed up, don’t be surprised if you have to use a 105 Power Valve and install one size larger front jets than what came in the carburetor stock when using Premium gasoline that has the 15% ethanol.

throttle position and inches of vacuum [load ] tell u when the mixture occurs . An Air/fuel gauge tells u how much [quanity] fuel. Advanced tuning methods a vacuum gauge can do both

I run a vac gauge inside my cowl. I got 15 inches at idle had to go with a 9.5 pv to get right afr at like 1/3rd throttle

First of all you explained it very well and understandable. I have a question though You said if you are getting a stumble after selecting a power valve some may increase accelerator pump swifter or speed and this will only mask the problem My question is then when is it appropriate to change accelerator pump speed and or squirter? Thanks

Question? Am I correct is saying that if the rating of the Power Valve is lets say a 6.5 and that's higher than the idle vacuum at 5 inches of vacuum at 900rpm the engine will run too rich because the Power Valve is Open, and if the Power Valve is a 2.5 the engine could run lean until the engine vacuum is below 2.5 inches of vacuum? On this new build I have a new AED 750dp on a Chevy 302ci with 10.5 compression, Comp Cams custom grind solid flat tappet with .516 intake and .537 exh lift with 242 intake and 250 duration @ .050 and with a 110 lobe separation, valves set to .012 cold, new MSD ignition and pro-billet distributer, Victor Jr intake, AFR 180 Street Heads bolted to 1-5/8" headers to 3" flowmasters. It mated to a M21 trans and 3.90 gears on a 28" tall tire. I had the timing at 19* initial and 36* with the same problem but my Brother told me it was too much for 91 pump gas so I lowered it to 13* btdc initial at idle and 32* total. Plugs are all consistant light tan color as well. The problem I'm having is an intermittent lean pop and low power through the carburetor when under load accelerating in 1st gear under 3k rpm and inconsistent idle rpm when warm to operating temperature. I'm at 1-1/2 turns out on the Air/Fuel screws, accelerator pumps adjusted properly with cams in the #2 position. I just opened the secondary butterflies because too much of the idle slot was showing from the underside of the primary butterflies (this did help the idle but is lower idle at operating temperature than when warm). The standard squirters are 31's as well. The vacuum at 900 rpm is at 5 inches so I put a 2.5 power valve in it and didn't think about it much until I watched your videos on this subject. I'm thinking I should have a 4.5 power valve. Am I on to something here or what else can I check or adjust? BTW, above 4k rpm it's an ANIMAL for a 302! Thoughts?

Wowzers! Excellent explanation and detailed instructions for achieving the optimum efficiency and power!! Thanks for the cutaway and your knowledge!!

Great vid as always, this may be a stupid question, but I've looked around a bit and haven't found a good answer. Power valve in secondaries, is there an advatage if so how do you tune it, or should i just block it and jet up. Thanks for any insight anyone has.

I just bought a mercury cougar with a turbo charged 302ci in it. It’s running very lean. I don’t know anything about it and I’m lost.

Thank you, sir.

so when you are checking engine vacuum at 30, 40, 50 and 60 mph - do you take that number and use it for the power valve, or do you still divide it by 2? Thanks!

I have bought a 750 Slayer for my 454 and it ran great but only 7 lbs of oil pressure, a little long in the tooth. I had the engine completely rebuilt and now it's a 461, it had no problems at all on the dyno and even tried an 850 CFM race carburetor and only picked up a few horsepower and torque. Put the engine in the car and I can't figure out what is going on? It will melt the tires! But I can't figure the stumble out for a life of me?

I put 780 meatering plates on my 650 carb with a 5 power valve with jets for a 850 carb and I got about 21 miles per gallon of gas as long as I didn't open the 4 bearel but when I stopped the gas pedal it would rip the rear wheels was wicked I still have the car the same engine and the same carb runs awasom

Many people think engines bog because they have too much fuel. Such is rarely the case. A little rich rarely causes bog/hesitation.

I used to get 21mpg in a 77 Pontiac g/p with a 280 duration cam in a 400. Go holley.

I have a zz502 with a holley 950 cfm when im takeing off it has a hesatation and low power and cruiseing its surging up 2000 then after that its a rocket but cruiesing from take of to 2000 cruiseing its a dog

Can a pv go bad and cause a backfire??

I've tested new power valves with a mityvac and some leak, so they're not fool proof.

I afr gauge

Vacuum is inches not lbs.

There is no such thing as a carburetor that is too large. The emulsion well is the least understood area of ALL carbs, not just holley's. You described power valve operation precisely backwards. Engine vacuum keeps the valve CLOSED, not open. The power valve channel restrictor under the valve should be used to tune WOT operation and the main jets used to tune cruising A/F ratio.

I have a feeling that a carb tuned by you would be just as good as a Holley sniper unit without worrying about electronics leaving you on the side of the road, keep the videos coming

Holley used to recommend a different process for PV. **Automatic timing & idle set, engine operating temp, “In Gear”: Vacuum hg minus 2hg= base PV. **Manual transmission timing & idle set, operating temperature: Vacuum hg divided by 2= PV.

Ford carbs used the same valves on 2 barrel and 4 barrel flat top carbs in the 60s and 70s. Worked on a lot of them. Ford called it an economists valve. Found below the float bowl. Turn the carb over and under a cover held on with 4 screws holding the cover.

Too bad about the two stage power valve. I tuned my 650 DP using one. It allows a leaner cruise mixture for economy and the first stage takes the flat spot out when pulling hills and keeps the full metering block restrictions from opening. Better drivability, better control of the mixture. Std equip on an 85 GT 5.0L mustang. Not looking forward to the day it fails.

One needs to realize that the idle mixture (AFR) will be unaffected by whether the PV is open or closed. This is because the fuel admitted from the PV feeds into the PVCRs at bottom of the main fuel wells; at idle, the main circuit has not started as evidenced by the fact that there should be no fuel flowing from the boosters. When the PV should open is when the engine load increases decreasing the engine vacuum such as going up an ever-increasingly steep hill or opening the throttle under load. In other words, the PV should open when the mixture starts to become lean (or 2" Hg vacuum prior to this point) under load to enrichen the mixture. Although the guidance of "idle vacuum divided by 2" may work in many cases, it is better to establish when the mixture starts to go lean and then add 2" to this point to select which PV to use.

I'll just correct one thing you said at around 5.30 ...you mentioned the vacuum hits the diaphragm which overcomes the spring letting fuel to pass...actually the other way around...vacuum holds the valve closed then when vacuum drops under load the spring opens the valve allowing fuel to flow...when vacuum recovers it overcomes the spring and closes the valve again. I'm guessing that was what you might have meant?

I remember the days long before AFR gauge availability... the way to tune any carburetor was to read the spark plugs, the use of a vacuum guage, seat of the pants feel(butt dyno) and to get a better handle on tuning for max power, was to take your car to the 1/4 mile track and adjust accordingly.

hi Ideally power valve engage at just above cruise ie vacuum reading at 45-50-55 mph . Then set the PV mixture thru the PVR power valve restrictor [PV screw in jet ] . Around 13 to 1 AFR The accelerator pump should be changed for stumble prior to that vacuum reading . Primary main jets RPM range 30---50mph set for around 14 --15 to 1 Afr readings at light throttle . Power valves are NOT unique to Holley

i just testdrove a 68 chevy c20 with 4.56 rear gear ratio 4 SPD Manual. it idled in neutral fine and ran good at speed, although not very fast, it started screaming the engine to get to 60 mph. BUT maneuvering around corners and slowspeed driving WAS TERRIBLE. is that a malfunctioning accel pump or power valve or just a qwirk of 4.56 rearend? i couldnt buy if for this reason alone. and 2 nearby salvage yards have nothing for C10 rear axles to swap.

GREAT VIDEO . Thank you for your knowledge and video's USA 🇺🇸 TRUMP

I watched this 2/3's of this video and gave up. Should have been a 5 minute video on tuning a power value not all the other crap.

What is your opinion on a PV choice when the cam has a lot of overlap down low with low vacuum? Cruising speed has good vacuum tho. I get 8 to 9 inches of vac when in park and drops to 6 when in drive. I'll get over 12 at cruise.

This is good general information it would be good to pinpoint some common scenarios on Holley carburetors. The problem I found with most Holly's is cruising down the road they are usually too rich like 11 to 1 but under wide open throttle they are very close 12.8 to maybe 1 3. To get that cruising air fuel ratio 14 to 1 and then have a good tip in plus wide open and then idling is very difficult to achieve

Ohhhhh, yes! Thank you for another great and informative vid. It seems like the power valve is often overlooked. Great tips, especially with the right size carb. Be it car, truck or motorcycle, you're right, it's often a setup with a carb that is too big. Thanks!

using the rpm might be easier , drive at 30 or 40mph on the right gear, check rpm, check vaccum at that rpm when stopped and get a reading

Inspection plugs in the exhaust... and chaining the front down and putting it in gear accelerating in a pull to set the timing.... Crazy as you may,,, it works.😜 I always loved a Quadrajet ....but it's what I knew but once I started messing Holley I was impressed....Fuel injection is much more Simple.... once you learn... After all my rambling..... Very informative Video content... thanks

This is very good. if you have a "fully" adjustable carb as the new Edelbrock 4150 or one like it and you have an dual ARF gauges (v8, 1 for left and 1 for right & dual plane manifold), vacuum gauge and a human data logger. How do you know what to change? power valve, PV restricter, high speed, low speed air bleeds, air bleed restrictor, etc. I get if AFR reads lean at X RPM I need to add fuel to a circuit that affect that RPM. I've read the Holley red book on 4150/4160 and David Vizard's Super Tune and Modify Holley Carbs book. I'm having trouble turning words and pictures into practice. I've probably read what I need to know and didn't realize it, so maybe hearing it another way would turn the light bulb on.

I’ve always gone 2 sizes below manifold vacuum in drive. It gives instant response and no stumble.

If you ever have a backfire or a sneeze in the intake manifold it usually busts a hole in the rubber diaphragm inside the power valve and causes an excessively rich mixture flooding the engine out and making it run like crap

I have a 351 Cleveland with 224 232 @ 50 cam 108 LSA with Holley 750 VS. When accelerating from cruise I note Vacuum goes from 17-10 Hg but not much lower. Afr goes from 13 to 17 and starts pinging. It currently has 6.5 power valve which I figure is useless. I think I need to go to a 10.5 power valve to get any enrichment. Does that sound right?

Power valves are NOT exclusive to Holley carburetors.

What about dual carburetors for power valve ?

If you're still responding to this video. It sounds like the higher the power valve number the sooner it opens?? Maybe it sounded like the lower the rpm the more vacuum so a higher rated power vacuum is needed. Something sounded backwards. I think where you said the engine will see 4.5 vac before it sees 3.5 is where I got confused.

I have an '86 Honda Prelude in the UK and it shows symptoms that point to a malfunctioning PV based on what you are explaining. I know someone else who has the same symptoms on their Honda. Engine runs well from cold but then once heated up runs rough and revs drop down suddenly. I am thinking PV may be stuck?

Hey Brian I have been trying to tune the 1812 on the 71 Corvette. I believe the spring on the AVS is bad. It binds at 1/12 turns. I have to tighten it to 3 turns to make the AVS adjustable. Do you know if this is a orderable part from Edelbrock?

So help needed. I have a ATM Innovations XRSB-750. I'm running really rich 11.2 at idle and 14.0 at cruising speed. The fuel is pushing out of the exhaust at Idle and wanting to die upon any acceleration. For context I have pump nozzle of 31 primary and secondary 35 high speed bleeds 70 idle air bleeds .78 primary main jet and .93 secondary, 33 idle feed restriction, 2.5 power valve and 110v needle and seat. Timing locked out at 38 with 7.5 psi of fuel pressure that can be regulated down if needed. My question is what changes do i need to make to fix the off idle stumble/car completely dyeing or choaking out and the extremely rich low end. Thanks in Advance

Hello, good video, very good explanation, you touched on a topic where you mention a carburetor with very high cfm for a very small engine. I will be waiting when you explain the physical difference from an 850 cfm to 650 cfm for the street and how to try to correct it. Thank you.

But it in the street application, you don’t have to fool with her about size or anything like that it’s grueling on putting his fab shop Hannah yes through anything else Carberater, to look structure in Lott called it out yet

I thought you were talking about 2 stroke power valves from the title. Lol😂

Would like tuning tips on a edelbrock 2.o street tunnel ram with two holly 600 cfms vacuum secondary, 454 bb chev , 400 turbo , 3.50 gear set

Awesome video man thanks for the tips. I love the buy a little 2 1/2 inch vacuum gauge for my car.

Hi! I am a new subscriber. I am very new to this and found out I know nothing! Lol! I am going to lean as much as I can. Thank you for sharing the knowledge

How do you adjust with 2 power valves? Do you run a smaller one on the primary and a bigger one on the secondary if its vacuum secondary or run them the same size?

Does any of the vacuum gauge tuning advice change for an airboat application since it is always in gear and under load due to the prop spinning at idle?

I size my PV based on cruising vacuum and how much the vacuum drops as I accelerate. Example, if I have 12" cruising and it drop to 8" on accel, then I'll chose a 8.5. Never base PV size on idle, its not used there since the PV channels feed from the main well to the boosters.

Excellent!

I remember the 67 Chevy Caprice with the Big block 427 had a factory vacuum gauge and there was others in a long them year's also had factory vacuum gauges....A crucial measurement.. I also installed inserts in the manifold and or Header's back in the day to read the color of the exhaust Flame..... other little tricks putting the car under load while setting timing and there are many other recurve Kit's and on and on..... Very good content here😎

I fought the concept of fuel injection about as hard as I fought the concept of buying CD's instead of vinyl records, but with fuel injection, I didn't have all the trial and error, test runs, driving conditions, and removing the float bowl assembly to get to the metering block and jets every time it needed adjustment. You just plug in the laptop and tell it where to add/remove fuel at a given load or RPM. I love the simplicity of carburetors, but the tuning takes skill and patience that most old school rodders don't bother with, and just settle for a close, off the shelf carb.

What if your fighting the Power valve opening at idel due to long duration camshaft, persay 5" at idle

CARB LIFE 🚗🔧💪🇺🇸👊🙌💯

Thanks for sharing the knowledge. My question cant be answered by anyone but the guy that installed a Holley Street Avenger 670 cfm on this 1983 GM 305 ci. That is, why is this 670 on this engine? I did not check the numbers on the heads, knowing that people like to install 350 heads on the 305, I should check that. That may answer that question. However, , specs. call for a 550 or 600 cfm on typical stock engines of the period. So, it idles great, it runs smooth through all driving ranges, but, it smells rich at idle. Primary jets are 65, power valve is 6.5. I intend to do the vacuum gauge trick you showedto determine engine to power valve requirements after I get the carb back together and installed. Mostly, my question is this, what is your take regarding the overall functionality of installing a 670 on a tired 305 ? The engine is installed in my 48 Plymouth, a back road cruiser car, all it needs is a 2 bbl. for that, but right now that option isnt available to me. What say you ?

Isn't CFM intelligent? It's not gonna use what the engine doesn't need, fuel or air. Recently saw intake manifold dyno tests with single and dual plane intakes. The 406 didn't need anything more than 650cfm making 560hp @ 7k rpm. A 830 CFM carb was used. The jetting was a little rich when looking at the plugs. Isn't going a little big better than not enough?

First time rebuilding my Quick Fuel carb. I noticed on the Holly web sight that there are three brands of power valves, Demon, Holly, and Quick Fuel. Are these universal..? Can a Demon power valve be used in a Quick Fuel carburetor..? I'm obviously a novice at this...

This video got my sub 👍 Look forward to checking more once I get running and gleaning all I can, thank you for them !

I think I understood if my car makes five pounds of vacuum which I got a pretty radical cam that would mean a 2.5 power valve but your suggestion for me to go to a 3.5 just trying to get clarification

I use to hear about something being off in the engine... The result was a backfire thru the Carb. When this happened it was said that the Power Valve can be blown due to this..... Any truth to that? Good Vid by the way.... I was thinking while watching, how hard it use to be to get this info..... Pre computer days. :)

What about blocked off power valves… saw a carb opened up and heard “blocked off power valve hmmmm” we did discuss the power valve in the other side though

Yeah, but you can tune it take to get to 4150 into it down to 500 if you use the right car valve in the right jet size and spring size and metering rod

In all honesty the only thing I know about a power valve, or rather what I hear, is that a backfire through the intake will kill them. On a side not is it possible to tune a Holley to compare to a Q-Jet in terms of fuel economy?

😊😊😊

I dont understand how a higher vacuum rating power valve opens sooner. Why doesn't the smaller rating see that number sooner and therefore open up sooner?

I need that Chev K-20 @ 0:37

Having EGT's will also help you tune your carburetor when it comes to putting in the right jet sizes

Im56 years old! Just buy a new one! Make sure you're at top dead-center! A vacuum secondary, smaller is better for overall air to fuel

Looking forward to your reports about tuning the edelbrock 4150. I will tune right along with you since I have my own edelbrock 4150 that I will try out on my current engine build. And I have a lot to learn.

Excellent Video ! Simple to understand ..

Would the power valve have any effect off-idle? My 351 with 600 vac sec has on off idle stumble that I had attributed to the accelerator pump. By the way, it's worse when col, better when warm, so it's a lean mixture problem.

GUILTY. THATS HOW I HAVE SET MINE. ONCE AND FORGET IT! GUESS THERE WAS MORE TO DO GOOD INFO BRO

So I have a big overlap cam, 107 LSA. At idle, in drive I have about 4-5hg. At cruise it’s at 12(30mph, 14(40mph) 15.5(50MPH). I’ve tried a 6.5, 4.5, 3.5PV. The only issue I really see, is if I’m driving at 40-60 mph and hit the gas pretty good, it seems to break up, then goes. Could that be the PV? I know, I need an afr gauge. At the track, at wot it runs pretty much fine. It’s a 750 Proform race carb. I think 74 primaries, 80 secondaries( I think). Any input? TIA

Then we get into accelerator pump size and curve!

Does power valves come as a adjustability options? Like a flat screw driver head on the power valves?

What about a bigger cammed car? I get about 7 at idle but when cruising it’s around 14. Am I setting the power valve half of 7 or half of 14?

Great video 👍🇦🇺

AWESOME VID !!!! A++++

There is no such thing as too much cfm.

Interesting stuff.

What about reading spark plugs?

Good stuff thanks