Power valve question - PV leak when engine is off?

62Ranchero200

62Ranchero200

Famous Member
Greetings Ford Six Fans,

This may be a stupid question...

As I understand it, the spring on a Holley power valve (such as on a Holley 4412, "500 CFM" 2 BBL) is always trying to open the PV, and engine vacuum holds the PV closed. For example, if my engine idles at 11" of vacuum and I have a #55 PV, the PV should open when engine vacuum falls to 5.5".

So far so good?

If that's the case, shouldn't be PV open and remain open when the engine is shut off? Wouldn't that cause a continuous fuel leak until the fuel bowl was drained?

Thanks
Bob
 
Howdy Bob:

YEs. Something is definitely wrong with the spring that holds the PV closed until Vacuum drops bellow about 5.5" of vacuum. Check it out, or better yet, replace and reassess.

Adios, David
 
This may be a stupid question.

Bob,
I get your question and you got me stumped. 'Never thought about that, I keep thinking of the fuel bowl - jets - PV \cavity and ... ? . Hope someone willl simply explain.

have fun
 
Probably a pin hole in the power valve diaphragm. Is your float level too high??
Bob, your metering block has the o'ring seal to block not the thin gasket Holley uses & the valve loosens up.
If you have a slight hesitation you might want to jump up to a 6.5" or a 7.5" valve to cover a lean hole before the main jets come in fully?? Bill
 
OK, I think I understand now ...

@CZLN6, if I'm not mistaken the spring is always trying to open the power valve, and vacuum keeps it closed. I want to be sure I understand this correctly.

As @Powerband mentioned, the emulsion tubes connected to the PV channel restrictors extend only above the fuel level. Yes the PV is open when the engine is off (with no vacuum to hold it closed), but without venturi vacuum to pull the fuel up above the fuel level, none leaks out into the manifold.

However, as @WSA11 mentioned, if there's a pinhole in the PV diaphragm, or if the diaphragm is completely ruptured (such as after a severe backfire in an early carb without a blowout preventer), fuel will leak out with the engine off until the fuel bowl is completely drained.

All correct?

@WSA11, this leak is relatively recent and I have not changed the adjustment on the float, so I don't think the float level is too high. Thanks for reminding me about the O-ring gasket, I had forgotten about that. :eek:

If anything, I still have a too rich condition occasionally and I will probably go down to a PV that kicks in at lower vaccum (like a #35). Keep in mind that I'm going to upgrade from the 274 cam to the 280 cam, so my vacuum will be even lower than it is now, unless the higher compression from milling the head compensates for the longer duration cam.

Thanks
Bob
 
I agree with everything in your above post.......But I do not understand the leak,is it internal leaking into the manifold or external where you can see it ???
 
@drag-200stang:

Apparently I had two problems.

First, the Quick Fuel metering block was 1/8" thicker than the Holley metering block, and the fuel bowl bolts barely engaged the threads in the carb body. As a result, I think the bowl was slowly leaking out onto the headers and the ground. When the engine was running, the fuel evaporated on the headers, but after shutdown, the headers cooled and fuel puddled on the ground until the fuel bowl was empty.

Second, as @wsa111 reminded me, the Quick Fuel metering block uses an o-ring for the power valve; over time the PV can loosen. When I removed the PV to test it (with a Mityvac), I noticed that it came out easily; I think it may have been a bit loose. As a result, fuel was probably continuously leaking (engine running or not). It must have been a slow leak though, because otherwise the wideband would have shown a pig rich A/F (10.0), and the engine would have run badly with black soot from the tailpipe.

On a side note, the fuel bowl bolt holes, although probably 5/8" deep, were only properly threaded maybe 1/8" - 3/16" from the factory; so I chased the holes with a standard 12-24 tap. Now I can use longer bolts which can bite all the way down, giving me a much more secure clamp on the metering block and bowl.

Thanks
Bob
 
Bob, you sent me a link on longer bowl screws for the QFT thicker metering blocks.
Holley has bought out QFT & they now offer a 2 3/4" stainless bowl screw kit with nylon washers. The cost from Summit is $5.90 for a set of 4. The part # is 5-18 QFT.
https://www.summitracing.com/parts/qft-5-18qft
 
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