440/6 Power Valve Question

[MoparHonda]

My '70 Plymouth 440/6 runs rich, too rich.

My engine is stock except for the camshaft, due to the non-stock cam, I pull about 6 inches of vacuum at idle. (I would like more vacuum, but it is what it is.)

I suspect I have an incorrect power valve. I read that divide the vacuum by 2, then step down .5. That works out to 2.5.

Is the Single-Stage Power Valve Standard Flow 2.5 in. Hg, Holley Part# 125-25 the correct power valve?

 

[68 HEMI GTS]

Power valve is not causing your rich condition unless it’s blown or is leaking around the gasket.

 

[lewtot184]

stock power valve would be 6.5; therefore you have a power valve issue. try a 3.5. proper ignition timing curve is a must with increased valve timing and will increase idle engine vacuum.

 

[68 HEMI GTS]

stock power valve would be 6.5; therefore you have a power valve issue. try a 3.5. proper ignition timing curve is a must with increased valve timing and will increase idle engine vacuum.

Care to explain how he has a power valve issue?

Hell he didn’t even say where it’s rich at.

 

[MoparHonda]

Thank you, all.

I don't know what's causing the rich issue, process of elimination points to the power valve, I don't know what valve I have now, the stock valve is too rich for the vacuum I have.

Agreed, timing will affect vacuum, I believe I have the timing set for the maximum vacuum.

LewTot184, thank you for your help; why do you suggest the 3.5 valve? I've read several articles and posts and they all say divide vacuum by 2 and if the result is even, subtract .5. That gives me 2.5.

LewTot184, are you recommending Single-Stage Power Valve Standard Flow 3.5 in. Hg, Holley
Part# 125-35 ?

 

[68 HEMI GTS]

Let’s here some engine specs and what the actual problem is. Rich at idle, cruise, wot, all of the above? What’s timing, initial & total? Rest of the combo

How about the condition of the carbs? Rebuilt, old, new, if they were rebuilt were Holley kits used?

 

[MoparHonda]

Thank you, 68 Hemi GTS, engine is stock, except for the cam. I don't know the cam specs, it was in the car when I bought it.

Rich at cruise and under acceleration.

Timing as of now is 10 degrees advance at 900 RPM idle. The overall advance should be stock, the distributor is stock.

I had the carbs rebuilt by National Carburetor and they used Holley parts/kit. I don't know what power valve it has without removing and taking the carburetor apart. (Until I do take the carb apart, I'm going to assume National used the stock power valve, which is too big..)

(Please bear in mind, I'm not a professional mechanic and I'm not an expert on all this.)

 

[68 HEMI GTS]

Your gonna need some more initial. The curve in the distributor will need shortened to limit the total. Probably needs to be up around 16-18. What’s the camshaft specs?

Reason I’m saying it’s not your power valve is, the power valve is part of the main system. In other words it’s enriching the mixture when your on the main circuit (using the boosters) and your vacuum would be below the set point of the power valve. At idle your not on the main circuit. Depending where your cruising at you may or may not be on the the mains. I guarantee if your on the mains at cruise it’s high vacuum (way above the rating of the power valve). Now if your power valve is damaged (blown or leaking) that would cause an extremely rich condition at all times.

To give you an idea, my 511 idles at 5.5 in of vacuum @ 1100 rpm. I run a 6.5 power valve. I want the early enrichment in the transition to wot. My plugs stay nice and clean because the PV does not affect my idle or cruise. If you look at a metering block, the PV feed holes go to the same main wells as your main jets. Those main wells feed the boosters.

 

[lewtot184]

Care to explain how he has a power valve issue?

Hell he didn’t even say where it’s rich at.

i was thinking maybe you explain how he doesn't have a power valve issue with 6" of vacuum and a 6.5 power valve and I did mention the timing issue if you'll re-read my reply.

 

[MoparHonda]

Thank you, 68 Hemi GTS, I'll bump that advance up to 16-18 next time we have a warm afternoon.

I have no idea as to the cam specs, it was in the car when I bought it. (The gentleman I bought it from was not a car person.) I suppose I could get a dial indicator and record the lift.

 

[MoparHonda]

Also, 68 Hemi GTS,

1) At what point does / should the main circuit come into play? The car has a 3.54 rear end, at what cruise speed would the main circuits come into play? I turn 3,500 RPM at 70 MPH on the interstate.

2) At what point does / should the boosters come into play?

 

[68 HEMI GTS]

At 3500 rpm on that small carb they will be on. It would be hard to give an approximate. What jets does it have?

Distributor will probably need a shorter curve also to get the total advance all in earlier.

I’m guessing if it has a lumpy cam it probably has a lot of idle fuel so that may contribute to your rich condition. Increasing the timing may allow you to dial the idle screws in a bit. Center and outboards. That’ll also lean the mixture throughout.

If your really into getting it dialed I would recommend an A/F meter. They are invaluable and remove the guess work. I have an inovative I take in and out as needed.

That’ll guide you much better than people guessing over the internet.

 

[moper]

Here's what I see.... A stock ignition with a non-stock camshaft. Internet or not, you can't touch something that major and not touch the other systems to make it all work. So to an extent I agree with 68. You need more initial, and that will mean making some major adjustments to the internals of the distributor, or outright replacement with something that can be properly adjusted because you also have to adjust when the centrifugal comes in. Timing always has to be done before the carb(s). You should have 16-18* initial, and 18* centrifugal, plus the vacuum advance which should be connected to a ported vacuum source. At that point, you can reset the idle speed and mixture using a vacuum gage to the smoothest lean idle. After that - you can look at the power valve. In terms of valve sizing, opinions abound. I tend to go 2 numbers lower than the vacuum gage shows. Some at Holley say divide the vacuum level by 2 and use that. Others say go 1 or 2 lower than the reading. Regardless, it usually helps to hook up the gage so you can read it while driving, and then drive around a bit and note the levels at various throttle openings. You may find that you need something else and the "guidelines" aren't right.
BTW - if it's an automatic, you need to take the reading in gear.

 

[MoparHonda]

Thank you, everyone, I'll work on this as soon as we get a warm afternoon.

 

[ckessel]

The 6.5 is what usually came on the carbs. One thing to know, anytime you get a cough or worse, backfire, through the carb, you kill the power valve. They can still hold a vacuum but will not operate correctly until you toss in a new one. There is an easy fix/update you can do on the throttle plate to install a check valve that will block the vacum passage in the event it gets pressure, cough/backfire, instead. For me, anytime I work on a Holley, I do that mod. As you are finding out, it gets a little tedious removing the primary unit to get at the valve, jets etc.

 

[MoparHonda]

Thank you, CKessel.

That's a good idea, I'll do that when I have the carburetor apart.

 

[ckessel]

Let me know if you need info on what to do for the check valve. Easy peezy.

 

[MoparHonda]

Thank you, CKessel, will do.

 

[383man]

The 6.5 is what usually came on the carbs. One thing to know, anytime you get a cough or worse, backfire, through the carb, you kill the power valve. They can still hold a vacuum but will not operate correctly until you toss in a new one. There is an easy fix/update you can do on the throttle plate to install a check valve that will block the vacum passage in the event it gets pressure, cough/backfire, instead. For me, anytime I work on a Holley, I do that mod. As you are finding out, it gets a little tedious removing the primary unit to get at the valve, jets etc.

One thing is if the power valve gets blown from a backfire then the eng will idle to rich since it would have to blow the diaphragm out which has intake vacuum on the other side and it would be pulling extra fuel right into the intake at idle if the power valve diaphragm gets blown out. And I would think you would notice it being much to rich at idle then. But as you said its good to install the backfire protection check valve in the carb to prevent that. And most new Holleys should have that but I don't know just when Holley started putting the P/V protection valve in them. Ron

 

[MoparHonda]

Thank you, 383Man.

Is there a way to visually tell if a power valve is bad/blown?