Author Topic: 1967 GT500 Power Valve/ Carburetor Help.  (Read 238 times)

1967 eight barrel

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1967 GT500 Power Valve/ Carburetor Help.
« on: June 28, 2018, 03:16:03 AM »
If there is someone who is very well versed in dual quad set up and power valve-jetting and doesn't have an issue with a bit of phone time please PM me.
I fly for American, so I will make an appointment at your convenience to call. The Concours people are lost and being in NE Texas I have not found a single competent carburetion tech.
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shelbydoug

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Re: 1967 GT500 Power Valve/ Carburetor Help.
« Reply #1 on: June 28, 2018, 10:40:55 AM »
What is the problem you are experiencing? The original carbs have no power valve blow out protection at all.

The diaphragms on the power valves are showing an inordinate amount of failures. Something like 50% or 1 in 2.

You should add the power valve check valve to both carbs and I recommend that you purchase a check valve test kit. It's the ONLY accurate way to verify that the check valve is good and holds vacuum. If it won't, that means it is leaking fuel through it into the carb vacuum chamber and that in turn is draining into the intake manifold.

That scenario is VERY VERY common these days.


The ethanol added to the pump gas is the main suspect with this issue. Once the valve is exposed to it, it will begin to deteriorate around where it is crimped in the center to the plunger. You can't necessarily see that.

In addition, the ethanol IS affecting the Viton in the needle and seat. If the car is sitting for about 3-4 weeks with fuel in it, the needle will tend to stick. Usually open, but I've had then stick closed also.

What happens is the fuel is drying in the needle and seat assembly and the compound it is making acts like glue.


In order to start a Holley carbed engine that has been sitting several weeks, first you need to pressurized the fuel system, like with an electric fuel pump, and let it sit for an hour or so before you try to turn the engine over.

Gasoline is one of the best solvents there is but you need to give it time to work. Even then, you just pressurize the system with a couple of turns, then check to make sure the carbs aren't flooding like Niagara Falls.

If they are filling the bowls and you aren't flooding, you are set to fire it up.


IF you have experienced a back fire in the engine at ANY TIME the probability of blowing out the power valves goes up exponentially. One of the issues I am having with cold starting with the 2x4 set up is backfiring through the secondary because it has no choke on it.

It is entirely possible this is because I am just either entirely or partially retarded but I need outside evaluation to determine that and I'm afraid to ask?


I'm wondering exactly what the ethanol is doing to the diaphragm in the fuel pump too? It's basically the same materials.

You want to have someone run the engine for you while you are away. Probably once a week for 20 minutes or so. They need to be versed in this procedure.

You do not want the carbs going dry from sitting after having had fuel in them. The bowl gaskets will dry and shrivel up.

When you put a gasket kit in the carbs, you want to use the ones that say the gaskets are  "reuseable". They are probably going to be teflon coated and blue but at least you won't have a fire that causes $10,000 worth of cosmetic damage.


This is a Holley carb design issue. AFB's, Edelbrocks, Webers don't have this issue. I will point out also that the check valve just reduces the likeliness of blowing out the power valve. It doesn't eliminate the possibility. It can still blow out.
« Last Edit: June 28, 2018, 02:49:58 PM by shelbydoug »