I don't understand the power valve...

So I've spent both my large cups of coffee on search this morning looking at the difference between 94's and 97's. Since I now own four 94's, I'll run them on my Edmunds 2x2. Going on my 47 bone stock flathead. I will install some nice heads eventually, when I find some, or give up and buy new ones.
I've been studying the 94/97 systems so I can properly rebuild/tune them. I understand AFB's, and I'm starting from scratch on these.
The part I don't understand is...How and why does the power valve function?
I see all kinds of expert opinions on whether I should keep the valve, or screw in a dummy, and my logic says that before I make that decision I should understand the function of the valve in the first place. I still don't get it.
It appears to me the valve opens when it senses a low vacuum signal to richen up the mix to give the engine a "boost". I don't understand how taking that function out of the system will improve driveability.
To me, what should happen is to modify the "timing" of the valve to open later. Taking the valve out completely will cause a lean situation just after tip-off, correct? Now, that may not be a bad thing as two carbs will inherently be richer that one to begin with.
I'm pretty comfortable with the rest of the carb, it's not much different from the Rochester 2CG's that were on my 65 GTO. Accelerator pump, spray bars, etc. But after reading for a week or so, I'm still not getting the power valve thingie. Can someone dumb this down for me?
This has a lot to do with why I never warmed up to the Holley four barrel carb! Thanks, Mike

 

http://www.jalopyjournal.com/forum/showthread.php?t=709829&highlight=dick+york

Have Dick get you power valves and jets. I told him what I had for a set-up (same as yours) and he sent me the power valves and jets for a very reasonable price and without me having to experiment to find the right combination. It works great!

When you open the throttle plate the bigger hole decreases manifold vacuum. The power valve is set to open at a specific vacuum and add fuel to regulate the smoother flow of power/rpms. When you open two carbs, the vacuum drops faster so your power valves need to open at a slightly lowerer vacuum so you don't dump excess fuel into the engine at the wrong time. Along with different size jets to compensate for the extra carb feeding the system.

Then upgrade the ignition system to really make it purr nicely without black smoke and carboned pipes.

overspray

 

Not counting the choke and decel, there are 4 principle carburetor modes.

Idle; Idle fuel is drawn by manifold vacuum through holes under the closed throttle plates, adjusted with metering screws.

Part Throttle, Off Idle - Transition - Ports or slots above the throttle plate add fuel as the carb transitions from manifold vacuum to venturi vacuum to draw fuel.

Cruise, Main Metering Circuit - Airflow through the venturis draws fuel through the main metering circuit A/F ratios of about 15-1 for the leanest cylinder, can be leaner with good fuel distribution. You are limited by the lean misfire limit of the leanest cylinder.

Power Enrichment - Additional fuel is added to the main circuit through a vacuum operated (Vacuum is used as a load signal) bypass (Power valve), metering rods moving to a smaller step from vacuum, throttle linkage or a combination. There are other variations but we are richening the mixture to about 12.5 (12.5 pounds air/1 pound fuel, or .08 Pounds of fuel per pound of air).

The concept is for economy we want to waste no fuel so we have excess air, and for power we want to waste no air, so we have slightly excess fuel.

In a multicarb setup the secondary carbs may not operate in cruise so we may eliminate the enrichment circuit. In theory a multicarb might run leaner as most carbs get richer with higher air flow, that's why we have air correction jets which try to keep the A/F constant over a wide airflow range.

Lean spots cause driveability concerns hesitations and more often lead to complaints. So we tend to make carburetors rich in transition.

Overrichness could cause driveability but more often cause fuel economy concerns, unless the condition is gross rich.

So eliminating the power circuit makes for simpler adjustments. A properly set power circuit would deliver better performance and economy but may be more difficult to achieve.

At least, that's how I see it!

Hoop

 

Hoop!, overspray! Thanks! I understand now, thanks to the way you explained it. Twice the carb, half the vacuum, twice the fuel flow. I kinda did understand it. But, by that same token, I can see why folks give up trying to tune the power valves to open at the right time. Could get tedious!
Worst that can happen here is that I won't be able to tune the power valves correctly, probably not going to be a problem, as overspray indicates that I can order parts to get me in the ball park.
If I'm not successful at timing the power valve, I can remove them at the cost of some power and driveability.
Since I have four really nice 94's, I want to use them, even if it means I have to study how to make them work. I could rebuild/tune the Rochester 2CG, I can learn these.
Thanks again, Mike

 

Anyone running without has to set up with mains that are too rich, leading to poor performance at part throttle/cruise (obviously most of your driving) and rapid ring wear...which can get REALLY rapid on a rich engine.
On a 94 the valve itself is just the on/off valve and astual metering is by the apertures leaving the PV hole in main body and connectig to the main discharge bars...note that any gas from power enrichment, just like gas from the main jets, has to be pulled in by the venturi pressure drop which in turn requires substantial airflow. Car will actually be running below/at edge of this action with lots of input from off-idle circuit at low speeds and easy cruising.
So...when you accelerate with any vigor with duals, vacuum will drop dead and PV will open but it won't have much effect untill RPM builds enough to "excite" the venturis. Low speed bogs thus are caused by excess of air not being properly fueled, not excess of fuel...accel pump helps bridge this by actually dumping in fuel. Your thinking should focus on cruise vacuum and when that drops off as you choose a valve number.
Operation (and rebuild) of the 94 is very well explained in a circal 1939 Service bulletin, with copious cutaway illustration.
This club has put what you need to read, mark, and inwardly digest online:

http://www.norgv8club.org/norg/inde...rs-and-tech-tips/category/9-service-bulletins

That's the Bible on the 94...

 

Okay, I am far from a carburation expert, but I have gone throuigh this deal before. When running two 94s on an otherwise fairly stock engine, I came up with the use of 3.5 power valves as the best cure. This was in endlessearth's little '29 roadster. I ran the car around with a vacuum guage connected for the better part of two months making notes as to where the vacuum fell to it's lowest point and under what conditions. I did use modern Holley power valves in these carbs which required a bit of clearencing in the chambers for them and squaring off the seal boss on the power valves themselves on a drill press. Pretty simple really, just a bunch of figuring it out time. Also, if you don't a UNI-SYN - buy one right now!

 

Just to add to this Holley now has an easily installed kit that comes with a drill bit and a check ball to put into the PV vacuum circuit to prevent ruptures by intake back fire. Works really good. I believe all of the new ones come with it.

 

The Holley 4150-4160 four barrels are largely a very eccentric rearrangement of the parts and systems developed in 1937 by Ford for the AA-94 line. Jets are the same, basic circuits work the same, PV is identical except for that slightly jiggered gasket surface. Accel pump is perhaps biggest difference other than turning everything sideways...

 

OK!!
I just sat down at the kitchen table, where I do my best work, and disassembled the AA1 that is the mate to the running carb now on my engine.
I'm really understanding this little carb much better now. The only difficult step is going to be re-installing the ball check that resides under the accelerator pump. At the moment, it's under the kitchen table somewhere. But I'm betting a new one comes in the rebuild kit.
A set of cheap guitar strings to clean passages and I'm good to go.
Bruce, I understand your analogy of the Holley four being, in many ways, a double AA. The systems are the same, even if the components look different.
Also, I understand now why these two carbs need to be set up on the flathead with direct linkage instead of progressive. There would be no way to make a smooth transition from the primary carb to the secondary. They both have to be primarys.
I also understand why, in practice, even if the 94 isn't as sexy as the 97 it is superior in that it has "shooters" instead of the emushion (sp?) tube. When you need the shot of fuel to overcome the lean situation, you need it in the venturi, not dribbling in the airstream.
I'm beginning to get it! Yea the fun of learning!

 

Uni-Syn eh?
I always just used a vacuum gauge on the AFB's. How is the Uni-Syn superior?
Don't get me wrong, we're not talking a lot of money here, and I'm always up for more tools, I 'm just trying to understand why the Uni-Syn is the thing to have.

 

The uni-syn is an adjustable mechanical air flow meter to be sure each carburetor has the same airflow at idle.



Has anyone tried a 2stage power valve in a 94, looks like you might need to clearance the throttle body but might be better than an on-off power circuit?

 

I suspect that on a flathead when 2 carbs open it may go so low so quickly on the vac it would just go past both stages...but that is an interesting idea to test!
Mike, one of the key things to look at on a dual is fit of throttles to housing...usually checked by looking through at a light bulb. The staked screws are capable of very slight loosening if you need to jiggle the throttles a bit...
You will be idling with 4 holes, and so you need to be able to get air flow at idle really low to get reasonable settings.

 

Just like adjusting a Hilborn injector, right? We do that on the sprint car all the time. Can make a world of difference..

 

Hillborns, a magneto...and don't forget, your Ford has a handcrank in its original toolkit! No vacuum, no electrical crap! Life of Riley!

 

The word power valve is a bit misleading. Think of it as an economiser. You can get the power (fuel) for the airflow that you have, by simply installing bigger jets. BUT those jets will be waisting fuel all the time !! By using a power valve, it by-passes the jets and adds more fuel only when vacuum is so low that it requires richness. This allows you to use smaller jets for when the airflow increases to a point where the venturi signal is substantial. Those smaller jets will provide a more stoiciometric mixture for cruising speeds. Thus economic and efficient operation.

 

Ford vs speed lingo...both correct! Close, economiser. Open...power valve. And of course...it is necessary since engine has different needs at part throttle and full.

 

At part throttle we want, typically the best fuel economy, and at high throttle openings we want max power, ergo;



A"feel" point is that we like the feeling of power change with throttle opening to be linear, so we often prefer an earlier transition to a rich mixture, it "feels" better.

If we can tolerate a later transition it will definitely improve mileage.

Of course we are limited by the leaner cylinders, and an A/F spread of 2 or more between cylinders was fairly common in the years covered by this forum.

Corrections for Fuels and Environment;


Temp/Alt Factor * Fuel Factor = Correction Factor



Hoop

 

Wow, I feel like I'm setting at the "grownups' table trying to understand just what the #*&% they are talking about!
Hoop,....Whatever you just said, I guess that's what I'm after.
I e-mailed Dickster27, and he has made recommendations on which power valves and jets for the two carbs I've chosen to use. I'm starting to understand why the auto manufactures have gone to EFI. You can only tune a carb, or for that matter, mechanical fuel injection, for a specific window in the engine's power band. With EFI, those adjustments can be made on the fly. Everything else is a compromise.

 

The simplest explanation, power circuit takes you from green to orange by adding 22.2 Pct more fuel to the same amount of air.

Hard to beat experience for the easy answer, so go with Dickster27s recs, all the rest is only required if you want to have 40 years of hot rodding experience, vs 1 year 40 times

 

Ya think this is fun? Try setting up three Weber side drafts where you CAN tune them all across the range! When I got back into the world of Holley carbs and AFB's and such, my first reaction was "that's it? That's all there is to play with?"...

 

Now you know how Peggy Lee felt,

Is that all there is?
If that's all there is, my friends, then let's keep dancing
Let's break out the booze and have a ball

 

Not much to add here but on my stock 21 stud w/ 2 94's, I run 3.5 PV with 51 jets which I believe are the stock size....

 

dang it Mike....you stirred up some good stuff....thanks all for the postings and edumacation....learned alot on this post....

lwf

 

When I was 15 we had a 289-2bbl mustang and thankfully the carbs don't come much simpler than that 2100.

My dad "dualed" it out and told me it would need to be rejetted. Since I read every Hot Rod article I could there was an article about enlarging the power valve restriction .003 instead of going up on jets.

We had ran it on a measured 1/4 mile before the duals and it took 16.7. After the duals it was 16.3. With the power valve restrictions drilled out .003 it went 16.1 ish.

I read about flipping the advance plate over from the 15 slot to the 10 and setting the initial up 10 more degrees and using the lighter springs.

My dad drove it 50 miles each way and he said it was using less gas.

It was now easy in the high 15s.

I read that certain 390 2bbls were the biggest and my friends mom had a 66 Mercury with the 390 2bbl, well soon she had the 289 carb and we had the 390.

There was a problem now with wheelspin so I made one traction bar in metalshop. By preloading the right rear with the one bar it would now spin both tires.

It was now in the low 15s and would beat the 289 4bbls easy.

The moral, not having money to spend on parts and a big appetite for learning taught me at a very early age that tuning to get the most out of a combination was very rewarding.

So power valve operation was my start into the world of engine tuning .

PS be sure to move the carb tag over to the new carb, the guys at Houston International knew enough to check the tag, just not the 1.33 venturi ID.

That's my start to "hot rodding"....

 

Although it has been alluded to here, I think it would help to point out that not all Holley power valves are created equal. I think just about everoine knows about the differences between the ones in 94's and the 4-BBLS, but there is also a difference between the ones for 2100/2110's and 94's. A lot of the generic kits come with the later model power valves, which are a bit higher in profile than the ones originally supplied with the 94's. I spent a couple of weeks last summer chasing a problem caused by this difference. I obtained a couple of power valves from an online source. They weren't for the 4150/4160's, so I thought they were OK. Turns out that they are a little higher, and given the way tolerances stacked up between the power valves, gaskets, and catb bodies, the opening on the power valve was pressing on the carb base casting, so it had no source of vacuum.

I finally figured out what the problem was when I set the main carb body (wity power valve installed) on the carb base without a gasket and noticed it wobbled. I obtained the proper power valves and all my problems cleared up. It's probably not a common problem but it can occur.

Bottom line, get your power valves from a reliable source. I have heard that Vintage Speed has an extra thick base gasket to address just this problem. A word to the wise.

 

Like the EFI guys always say "a carb is nuthin but a calibrated fuel leak), my first experience with power valves, was when I noticed my hotrod was nosing over sooner than it should in each gear. As vacuum came back up pv was closing. Changed pv to a higher #, problem solved.

 

This was one of the best threads that I've read in a long time. Great stuff about carbs and how they work. I could even understand it all. Very well explained.
Thanks to all.

 

Great thread. I'm doing the same routine with my dual 94's. Had 7.5 PV's and 50 mains. I went to 48 mains and 5.5 and fixed most of the problem. Got some 3.5's to install and try now. Thanks to the HAMBers tech pages!

 

I agree with everyone's comments about this thread. I've struggled with the power valve issue since I started researching the 94 carb, and have only now had it explained to me in a language I can understand.
If you are like me, where technology has as much a place as ascetics, the 94 seems, on the surface, to be the superior carb over the 97. Interesting comment from the 75 year old hot rodder I bought the car from. " If you want that Edmunds manifold to be right, you'll throw those 94's away and get some 97's!" Even though the carb on the engine now is a generic replacement AA1 that is as nearly perfect as one can expect from a carburetor!
If you guys lurking out there have more comments, or information to add to this thread, please feel free to add. I think this is the best part of the HAMB, sharing knowledge.
Thanks to all, Mike

 

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I almost forgot! This is the purchase that started it all... Edmunds 2x2 with two carbs that I picked up at the Pate swap meet. If you guys ever get the opportunity to go to "Pate" take a lot of money! If you can't find it there, you're looking for the wrong stuff!