Hi All,
Really stupid question..... I have a Holley 870 SA carb and need to change the power valve for a different rating due to the amount of vacuum I am pulling. I have just been speaking to a supplier and asked if they come in pairs or individually, was advised that they come individually but there is only one in a carb. I was under the impression that there was two, one in each metering block? Or does this not apply with vac secondaries?
Am I loosing the plot?

Cheers all,
Lester

 

The power valve is only on the primary side of Your carb. Make sure You get one that opens about .5 inches of vacum above the moment you go to wide open or hard excelleration. Thats when You want it to open.

 

Thanks for the info.
I am working from memory, but I am sure there looked as though there was a power valve in the secondary metering block when I had it apart a few months ago. Am I just imagining it, or is there something that looks like a power valve in the secondary but actually isnt?
Cheers


Lester

 

some models do have a secondary pv....be sure to take a vac reading at idle and use a pv rated at half that much....

i did not look up the model #

 

Its a Holley 870 SA Carb (80870)

I am pulling about 16 in hg at idle.
At 2,500rpm or above on a constant throttle it is pulling 21in hg.
I have just purchased an 8.5in hg and 9.5in hg power valve.

Guess the easy answer is to pull it apart and see whats inside? It just would have been ideal to have the parts to put it back together before it came apart.

Cheers all,
Lester

 

either one will be ok...the lower # may give a little more mileage...dont use the 21 hg for calibration....a 10.5 pv will hurt mileage

 

Sounds good, if I pull it apart and do have two power valves shall I put the 9.5 in the primary and 8.5 in the secondary, or should they be equally matched? In which case I will buy another of each before I start.

Lester

 

If it has a visible metering block on the secondary side then You probably have a second power valve. Usually the secondary metering block on a vacum secondary carb is hidden inside the fuel bowl as a small plate on the side of carb housing.Rather than a complete section like the primary side. Some vacum secondary carbs do have them.

 

There is a lot of misinformation about power valves...

First (the biggest myth), the power valve doesn't affect idle mixture even if it is open at say 9.5" of vacuum and the idle vacuum is only 8 inches. The reason why is that it adds metering area through the power valve channel restrictions to the main circuit which flows upwards to the boosters. At idle, there isn't enough vacuum drop across the boosters to start the main circuit no matter how big the main jets or PVCR.

Second, and most importantly, the power valve can be thought of as a switch which richens the main circuit flow (the power circuit) if the main circuit is already flowing.

Power valve selection should be based on establishing a smooth application of power as the throttle is opened such as going up a hill or slowly adding throttle to accelerate. If it is noted that the engine feels flat when vacuum drops through, say 9", but doesn't pull hard until the vacuum drops further to 6", this is a good clue to use a power valve that opens at 8.5" rather than the one that opens at 6". Of course, this road testing needs to be done with a vacuum gauge.

In summary, it is the need for main circuit enrichment that determines the proper power valve and not the vacuum at idle.


__________________
1978 Corvette
406 SBC AFR heads Lunati roller Quadrajet TH-350 3.08
11.59 sec / 118.9 mph 1/4 mile