lars

Holley Power Valve: Effect of Open PV on Idle

I see many comments and ideas about tuning Holley carbs. One of the most prevalent myths about Holley tuning is that if the Power Valve (PV) is not fully closed at idle, the carb will run massively rich, and will want to flood out at idle. An open power valve, due to improper selection or rating, will flood the engine at idle…

The Holley PV, like other carb power enrichment systems, feeds additional fuel into the main discharge passages. These main passages discharge their fuel through the main discharge nozzles in the booster venturies of the carb. These main discharge nozzles do not operate, and they do not discharge fuel, at idle. Only the idle circuit and transition circuit flow fuel at idle, and these circuits discharge fuel below the throttle plates at idle. Therefore, I have contended, an open PV has no effect at all on idle.

This topic came up again not long ago in a Forum post, and the flooding issue due to an open PV was again promoted. I conceded that it is possible that an open PV could cause a slight enrichment due to elimination of pressure drop through the main metering circuit (where the idle circuit draws its fuel from), but I still did not believe that an open PV would cause any noticeable problem.

So it’s time to test the theory and find out for sure: Will an open PV screw up your idle?

Here’s the test article: A 780 Holley 4150 bolted up to my test engine. The carb is up and running, and the engine has reached operating temperature:



Carb has been set up and tuned to idle at 14.4 air/fuel ratio:



Time to “fix” the Power Valve. The PV in the carb was a new 6.5 in good working order. The engine idles at 15 inches of vacuum, so the valve was closed in the above photos:



Here’s the test valve: I took a spare PV and jammed a piece of wire into the seating area to positively lock it in the full open position. This valve is never closing:



These are the discharge holes from the power valve: When the PV opens, additional fuel, equivalent to 8 jet sizes, passes through these drilled holes:



Those drilled holes intersect the main fuel passages, seen as the vertical columns rising upwards from the main metering jets. The PV fuel mixes with, and supplements, primary jet fuel, and this is then discharged through the main discharge orifice in the booster venturi.



Here are the results. With the PV jammed open, and no other settings altered on the carb, the idle air/fuel ratio remains virtually unchanged: Ratio is now 14.1:1, and I could not detect any difference whatsoever in idle quality or throttle response off idle.



Conclusion: An open Power Valve has no effect at all on idle. The fuel from the PV is not being discharged at idle, since it must discharge from the main discharge nozzles. As long as you have a correct throttle opening set on your carb, and as long as your Holley is running on the idle circuit, and not on the main discharge circuit, at idle, an open PV will not affect idle mixture or idle quality.

NOTE: An open power valve is not the same as a ruptured power valve. If the PV diaphragm ruptures, fuel will enter into the vacuum chamber area behind the power valve, and fuel will be sucked directly down into the intake manifold through the vacuum supply hole for the PV. A ruptured PV will grossly affect idle. An open PV will not. Also - this test only deals with idle quality. As soon as the carb comes off idle, the open PV will have a significant effect on mixture.

Lars

MelWff

thanks for doing the test and presenting the results

dochorsepower

Very good information, thanks for presenting results. I would imagine a lot of people confuse open power valve with ruptured diaphragm.

mikem350

ajrothm

Great post! Really good info! Thanks for doing the test!

Very interesting how the carbs actually work...

zuendler

Thx, now you could explain what other effects the open locked power valve would have.
From my understanding, it would allow more fuel flow in medium load conditions than the engine would need.
So it´s more or less a fuel saving device.
You would generate some dark smoke maybe, but other than that, would you recognize a constand open PV?

lars

This posts deals with the effects of an open power valve at idle, and the effects at idle are negligible - it would not be noticeable at all without an accurate exhaust gas analyzer.

As noted in my original thread, an open PV will have an 8-jet size enrichment effect under all conditions employing the main metering circuit with fuel being drawing from the main discharge nozzles. This will normally put the air/fuel mixture down in the 11.5:1 to 12.5:1 range. The desired A/F ratio at cruise with a closed PV is in the 14:1 - 14.7:1 range. A/F in the 12:1 range is not likely to emit black smoke, and the engine will run fine. But fuel economy would be lousy...

Lars

zuendler

Ok, mainly what I was expecting.
When you talk about cruising: doesn´t this happen on the transistion circuit? I know the main discharge nozzles come in with more air flow (and therefore the effect of the powervalve on them) but usually: is cruising a transition circuit thing, or a mix of transistion and main nozzles?

(Talking about my engine - I need only a little bit more throttle over idle for constant cruise of 35mph, and let´s say a 50% open primary throttle for 70mph. I assume at this speed the main nozzles are in use too and the PV would haven an effect).

lars

That's correct: Most fuel used at light throttle cruise is supplied by the transition and idle circuits, but it is supplemented by fuel from the main discharge circuit. With an open PV, you'll see cruise mixture go rich, even though not all of the fuel is coming from the main circuit.

Lars