Selecting a power valve?

Bassman/NZ

Bassman/NZ

Well-known member
I have rebuilt my Holley 94. Completely stripped, surfaced the mating surfaces, blown it all out with air, new leather accel pump, new quality gaskets, new 48 jets (the 50s made the plugs a bit sooty), stainless screws etc. On assembly, I had a choice of three power valves.... but none of them had any markings on them. With the scientific method of sucking on them I was able to determine that two were quite stiff, and one moved a lot easier. I went with the easy one, and the carb performs well at low and medium revs, but gets a little ragged at the top end. I wonder if that sounds like a result of using the incorrect power valve? If it matters, it's a C59A with new Bubbas crab, new plugs and leads, 45 thou piston clearance. How does a person determine the value of a pv?
 
One way is to put a vacuum gauge on your engine, warmed up and read the vacuum level at idle. Then divide the vacuum reading by 2 and select a PV at that level. I have 3 94's on my Tbird 312 and that worked for me. Holley power valves are available marked by the vacuum level they open at. Example would be 17 inches of vacuum at idle divided by 2 equals 8.5, so you would purchase a Holley 8.5 power valve. You will find in some cases the Holley power valves of today will not seat in the old 94's carb bowl all the way. The new repro 94's may have accommodated that. If needed, some chamfering and carefully enlarging the area around the old 94's carb bowl wall near the power valve orifice will work. Kind of like being a dentist, just a little at a time until it fits, no more. You don't want to thin the carb wall too much and cause it to leak. Dremel tool is good with a light touch and be sure to flush out the debris. Charlie Price at Vintage Speed has everything for 94's and Strombergs. I had good service from him when doing the 3 deuces on my Y block.

http://www.parts123.com/parts123/Pa...HOME=www.vintagespeed.com&TITLE=VINTAGE_SPEED
 
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I think the Bassman is asking how check the power valves for vacuum. Also pretty sure that vacuum test to determine the correct power valve should be done while at cruise.
 
My experience was that 48 jets would be too lean. try 50s or 51s. There may have been a different reason for the sootiness. Ford fitted 50 or 51 jets to millions of cars. Would they have done that if 48s were better?

If your motor is relatively stock, Having the carb set to stock spec is the best setup. If it's not right (too rich for example), it's probably due so something being wrong rather than the jets etc being incorrect.

Problems with fuel pressure, needle valve, float, power valve and flatness of mating surfaces can all lead to an over rich condition.

I have personally had to contend with all these problems, so am speaking from experience rather than just guessing.

I acknowledge that you have addressed some of the points mentioned, but I would advise you to keep looking for a fault rather than trying to recalibrate the carb to compensate for a fault.

Mart.
 
How can I determine the value of a PV when there are no markings? Do I assume that the stiffer ones are likely to be up in the 6.5 or higher range, and the softer one would be in the 3.5 or so range, and therefore unsuitable for a stockish setup?
 
If they're not marked they should be stock equivalents, straight out of a normal rebuild kit. That would put them in the 7.5 to 8.5 range.

Mart.
 
If it was out of the long hidden storage you cant tell if they still in specs without testing them.
They dry up from age, sometimes when exposed to the fuel they get better sometimes not.
One of them little testfixtures is a great device.
The older ones sometimes was adjustable ( yes im a cheapskate) so you could tweak them to the range you needed.
 
To answer your question:

A carb has different circuits each providing fuel to the engine.
The idle circuits allow the engine to idle.

The accel pump circuit adds additional fuel when throttle is opened.

The main ( jets) circuit meters the correct fuel to the intake.

The power circuit adds fuel under load. On a 94 this is using a power valve thats adjusted to a vacuum load rating. If you drive the car with a vacuum gauge you should be able to feel the power added at the setting. Ex: A 7.4 power fuel would add fuel at 7.4 inches of manifold vacuum. A sta morning spent checking can make the circuit almost perfect for driveability.

Note : Some block the power valve on multiple carb set ups and in this case the additional fuel is made up for using a larger jet, or smaller with three carbs etc.......
 
NZ,
I sure agree that # 48 jets are too small.....best results using 51's. The soot
is the result of , in my opinion , unregulated fuel being drawn thru the carb caused
usually by an internal vac leak. Be very careful of any PV's from before corn gas ,
they will die real quick. The only reliable source again in my opinion for power valves
is Daytona Products in Florida. These valves are designed very similar to the originals
and are to this point in time very reliable. Occasionally the parting line left from
the manufacturing process can upset a good seal. By now most know I machine
the parting line off and install an interference fit nylon gasket which most always seals things up properly.
Charlie ny
 
I agree with CharlieNY. The soot on the plugs could even be from choking to start when cold. The black on the plugs takes a long time to disappear. Also agree 48 jets are too small, and may be why it doesn't run good under heavy loads. The power valve is just a vacuum timed switch to turn on the extra fuel needed for heavier loads. Once it opens, it does not control the amount of extra fuel you get. That is determined by two orifices after the power valve opens.

Without a vacuum pump, guage and special fixture, you can determine which power valve you have by comparing the spring tension to a known marked new power valve. Just butt the two small ends together and slowly push until both valves are seated. If they completely close at the same time, they are the same. (or very close).

Sal
 
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