Dear smart people,

I am attempting to rebuild a carburetor for the first time. It's a Holley 4150, 650 cfm double pumper. I got the whole thing taken apart, I think I have it cleaned (actually - I'll have a question about that later)...and am now trying to put it back together.

In the rebuild kit I purchased, there are three power valves. I believe two of them are the same, they have the numbers 6.5 and 19114 stamped on the top. The third one has the numbers 10.5 and 16014.

My carb only had one power valve, in the primary side. There are no numbers stamped on it. How do I know which valve I should put in? I suppose I could put the original back in...but I would rather put a new one in. I have no idea what this power valve does, or what the impact of one or the other is.

Background info...the car is a 1970 with 350/350. Previous owner raced it, so he put on a Torker intake, this Holley carb, a different cam (I have no details about the cam), headers, etc.

Any guidance would be greatly appreciated!

 

Good info from Lars here might help:

http://www.digitalcorvettes.com/foru...ad.php?t=86274

 

The 10.5 power valve might give you worse/terrible gas mileage because it will open any time moderate acceleration is used. The 6.5 power valve usually opens when the gas pedal is pressed more than halfway to allow more fuel into the intake for better acceleration (from what I understand) Which would give better acceleration and better fuel economy than the 10.5 power valve which would stay open most of the time while driving.

It is possible that you may even need a 3.5 power valve if your engine runs too rich with the 6.5 power valve ......

you really need to drive the car with a temporary vacuum hose connected to the intake and run into the cabin where a vacuum gage would be attached to it that coukd be watched while you drive the car so you can get a reading of how low the vacuum drops during moderate acceleration.

Post your readings if you do the vacuum gage test.

 

Holley usually recommends taking a vacuum ready at idle (auto in gear) and divide / 2 eg hg @ idle = 14 /2 you should use a 7 or next lower number 6.5.

 

This is the key.

I was running the stock valve on my double pumper and it was always very very rich. Then I learned about what the valve does. At idle, my engine only pulls about 6-7 hg of vacuum, so the #6.5 or whatever valve it was would just continuously dump fuel into the carb at idle. I'm running a #2.5 (125-25) now and its fine.

What are your engine specs? Stock cam/heads?

 

True! Holley carbs are set at the factory to deliver the most "Punch" which means "max-size" jets and power valve. My engine is stock and idles at 18"---which would require an 18/2=9" power valve.....the exception is----under moderate acceleration the vacuum generally drops to 7"....so I use a 6.5 power valve to keep it from running rich.

 

The power valve adds more fuel when the vacuum drops. So, I would put on the 6.5 - because you say the cam was changed. I am guessing you will have lower vacuum than stock because of the cam. Then when you adjust the carb with a vacuum gauge, when it is back on the car, you will be able to get a number. Because the Holley is easy to work on, (lets not get that started again), then you could change out the valve, if needed. Use the paper from Lars.

 

Yeah when I changed out my power valve, the only pain I endured was getting the fuel pipe to seal against the bowl. On my carb, it is a metal to metal seal so it needs to be clean clean clean

 

Thanks for the help guys!

So....unfortunately I don't know what the previous owner put in for a cam, I just know it's not stock. And I don't know if he changed heads...

Is my following understanding correct....when at idle, the car generates the most vacuum (represented by a larger number in in Hg). As you press on the accelerator, the level of vacuum reduces (represented by a smaller number in in Hg). The number on the power valve represents the vacuum number that it will start to dump in more fuel. So, the 6.5 will allow more fuel in when the vacuum is 6.5 or lower (such as 5.0, 4.5, etc). But if the vacuum is 8, then it will remain closed?

If that's the case, then I think I am best off putting in the 6.5 and putting it together...then get it on the car and see what it does as you have all said. But the 6.5 would be more conservative in terms of making it not drink so much gas during "normal" street driving....correct?

 

My understanding is that the number on the valve represents about half of the vacuum hg that indicates when it will start releasing fuel.

My 2.5 power valve starts releasing fuel when my vacuum reaches 5hg. Since I'm idling at 6-7hg, it won't dump fuel at idle like the old 6.5 or whatever it was did.

So, take your idle reading, divide it in half, and round down to the next available power valve number, and you should be good.

Just like you, I don't know what cam I have. Someone in the car's past life put in a long duration cam so I can't even get anywhere near 10hg at idle. On the bright side, it makes great power up to about 6,500 rpm and tops at nearly 7k

 

The # on the PV is the vacuum at which it opens, if its marked with 6.5" the it will open at 6.5" of vacuum.

The PV will not dump fuel into the motor, it is drawn in from the boosters. They will only add fuel when the booster are active.

The formula mentioned above is the recommended method for PV selection. From the two mentioned the 6.5" would more than likely be the best selection if you don't have the means to check the vacuum at idle in gear.

Neal

 

Run the 6.5" power valve, it will work just fine

 

Hmm, maybe I read wrong somewhere. Either way, here is a good paper on it:

http://documents.holley.com/techlibr...wer_valves.pdf

 

Unless you have an unusually radical cam, the stock power valve that the carb came with will almost always work just fine - Holley specs those so the carb will run pretty well on a wide range of engines. If you post your carb list number, I can tell you the correct, stock valve for the carb. In most cases it will be a 6.5, which should run just great on your engine - 6.5 is a safe PV spec for most modest street engines.

Contrary to common belief, an open power valve will not dump fuel into the engine at idle and has no effect on idle mixture, whether it's open or closed. The enrichment from the PV (equivalent to an 8-jet-size increase) is discharged through the main discharge nozzles, which are inactive at idle. The only fuel the engine draws at idle is the fuel drawn from the idle and transition circuits - these ports are located below the throttle plates, and are exposed to manifold vacuum. If your carb is discharging fuel from the main discharge nozzles (which are located in the venturi area of the carb) at idle, you have a serious setup problem that needs to be fixed.

Lars

 

Well there goes everything I thought I knew about my carb.

I took a brand new out of the box double pumper and installed it on my car when I rescued it from sitting for 10 years. No matter how much I tried to lean out the idle mixture, I could never get it to idle quite right without dumping fuel. Changing the power valve made a huge difference for me. Maybe it is in my head, I'm not a carb expert but I've had to learn over the past couple of years.

 

Lars.....................

thanks for the info...I can parrot info to try and help someone, but...now I understand how it operates!

 

If the carb is not set up right (which is common with out-of-the-box carbs), it is possible (and probable) that the primary side throttle opening was excessive, causing the carb to be running partially on the main metering circuit at idle - I see this all the time. When this occurs, fuel going through the PV circuit will, in fact, affect idle. You solved the problem by installing a PV with a numerically lower value, which solved the PV portion of the problem. The second element to your problem would be to increase secondary idle speed so that the primary throttle opening could be closed down, thereby eliminating the fuel flow from the main metering circuit at idle.

Lars

 

That makes perfect sense, I'll check that out. Thanks for clearing that up, Lars!

 

Some notes and clarifications on the Power Valve operating system:

The Power Valve (PV) is part of the main metering circuit, and provides enrichment (by approximately 8 equivalent jet sizes) to the fuel flowing through the main metering system. This fuel is discharged through the main discharge nozzles in the venturi of the carb. Thus, an open PV cannot pour fuel into the engine at idle unless the float level is grossly high or if there are other issues causing fuel to discharge from the main discharge nozzles at idle.

However, to make this statement more technically accurate…

Both Holley and Q-Jet carbs have the same basic fuel flow schematic and layout: Fuel supplied to the idle and transition circuits comes from the main metering circuit, downstream of the main metering jets and the PV. The metering orifices in the idle & transition circuits are much smaller than the restrictions defined by the main jets and the PV, so the jets are normally regarded as irrelevant to fuel flow at idle. In fluid flow dynamics, however, any orifice, even if it’s larger than the downstream orifice, will cause a slight pressure drop as the fluid passes through it, thus having a slight effect on flow through the smaller, downstream orifice. Opening and closing the PV will slightly change the pressure drop that the fuel sees on its way to the Idle Fuel Restrictor (IFR) and the idle mixture screws, and can, in fact, have a slight effect on idle mixture. If the mixture has been set at one extreme or the other of the optimum ratio, this change can have an effect on idle quality: Normally, an engine will idle without noticeable difference in idle quality in a range of about 13.5:1 – 15.0:1. Often, an open PV will not move the ratio outside this range at idle, so no idle quality change can be noticed unless a CO or A/F meter is used to detect it. But a non-seated Power Piston on a Q-Jet or an open PV on a Holley can, in fact, produce a measurable difference in Air/Fuel ratio at idle, which may or may not be detectable "by ear," depending on how far the event shifts the A/F ratio. It will not cause massive flooding, and it won’t pour fuel into the engine. But it can cause a change in fuel flow upstream of the idle metering orifices resulting in slight changes to idle mixture. This change is more noticeable on big-cammed engines pulling poor manifold vacuum, since upstream pressure changes in the idle fuel will have a greater effect on the idle fuel discharge rate.

Lars

 

Thanks for the explanation, although I had to read it a few times.

What do you consider poor manifold vacuum? Under 5hg? I'm at 6-7hg at idle and I think that is fairly poor. No idea what cam I have, but it definitely isn't stock and there is power still made above 6k rpm.