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i have a holley truck avenger 670 carb and im havin trouble understanding what the power valve does. there in a power valve block off in the carb right now, but i havnt installed it on the engine. will this cause a problem and if so which power valve should i try? thanks alot
 

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taken from chevy high performance. http://www.chevyhiperformance.com/techarticles/74058_carburetor_valve_problems/index.html

What It Does

Most carburetors employ what is generally called a power valve circuit. This circuit enriches the air-fuel mixture when the carburetor goes to wide-open throttle (WOT). At WOT, intake manifold vacuum drops to almost zero. When this occurs, the power valve opens and directs more fuel into the main power circuit, in addition to fuel delivered by the main jets. The Holley power valve employs a small rubber diaphragm that is opened by a small coil spring. The valve is held closed whenever sufficient engine vacuum is present. At WOT, engine vacuum disappears and the power valvespring opens the valve, directing fuel through a small, precise orifice in the metering block called the power valve channel restrictor. This restrictor determines the amount of additional fuel delivered to the engine.

Power valves are used most frequently on the primary side of a Holley carburetor. They allow the carburetor to operate with much leaner main circuit jetting for part-throttle fuel economy. Then, when the throttle is slammed open, the power valve adds additional fuel, creating the rich air-fuel ratio needed for WOT operation. Most Holley power valve circuits are designed to add the equivalent of 8 to 10 jet sizes of additional fuel. Holley does offer a power valve block-off part that closes the power valve circuit, but this means the jet size must be increased in order to compensate for the lost power valve circuit fuel. Imagine how bad your fuel mileage would be if you had to add 10 jet sizes to the primary side of your carburetor!
 

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i'd start off with something in the range of 6" of Hg to see how it reacts first. since it had a blocked power valve you have to look at the jetting and see if it has fatter main jets since it didn't have a power valve. you want the power valve so that when you crack the throttle to wot there isn't a bog from lack of fuel. a wideband O2 sensor would be most helpful for tuning. that way you can see what the engines A/f ratio is during part throttle and wot operation.
 

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forgot to mention. you'll want the power valve to come in sooner if you have a heavy bog so a higher number valve would need to be put in. if it goes way to rich then you'll want a lower number so it comes in later. hope this helps
 
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