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Old 07-30-2013, 21:26   #1
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Which Holley power valves to use?

We have a 3310-1 on a 1970 Chevelle with a 396. The carb has two power valves. It makes 5.5 inches of vac at idle in neutral and 3 inches at idle in gear. Is has a very big cam in it and 4.56 gears. Holley lists a 1 inch and 2.5 inch power valve. Not sure whats in there now but it smokes up the garage with gas fumes when it idles so I bet the wrong power valve is in there. My question is which power valve would you use for the secondaries? I read you get the vac reading in gear and cut that number in half. If there isn't a part made for that number, use the next one smaller which in this case would be the 1 inch.....but what about the one for the secondaries? Since this engine makes practically no low speed vac, should I put a 1 inch in for the secondaries? I read about getting rid of the secondary power valve and going up 8 jet sizes but thats the way it came from the factory and it was driven around for years that way.
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Old 08-01-2013, 09:06   #2
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The power valve shouldn't have anything to do with the engine idling. If the vacuum is a steady low reading, I would think you have a vacuum leak somewhere. The metering jets on the carburetor is the only thing that effects fuel delivery at idle. The power valves purpose is to deliver that extra boost of fuel when you stomp on the accelerator so you don't have that hesitation in acceleration.
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Old 08-01-2013, 11:07   #3
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power valve

The power valve does have a lot to do with the idle. For one the 3310 is a vacuum secondary carb, the secondaries do not have a power valve. The power valve should be at least on number smaller than the vacuum reading at idle with the car in gear at operating temp. So if the car has 6.5 inches of vacuum then I would install a 5.5 power valve, hope that answers your question.
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Old 08-01-2013, 13:43   #4
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If the power valve has a torn diaphragm, it will leak excess gas into the intake at idle, causing the engine to idle too rich.

To determine which power valve your car needs, you need an accurate vacuum gauge that you can read while driving. When you accelerate, watch the gauge to see what the reading is when the engine bogs or hesitates during acceleration. Then, select a power valve that is numerically higher than the reading when it bogs. So, if the engine bogs at a reading of "6", you need a 6.5 power valve.
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