1974 Ford F-350 Pick-up Other - Engine

Question: I have a '74 F-350, 390 bored .40 over, Edelbrock Performer intake, 1850 Holley 4bbl carb. Recent repairs/improvements include, chaged points to Pertronix elec. ign., rebuilt carb. I continue to have trouble with extremely rough idle, acceleration and power are fine. While searching for vacum leaks, I discovered that if I disconnect PCV valve from carb. the idle smooths right out, but it sounds like a Hoover vacum cleaner. I replaced the PCV valve but it made zero difference. It still idles very rough unless I pull the vacum line loose from the carb. or the PCV valve. Can you help?

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Technican: Hi; Sounds like your power valve in the carb is leaking. Try turning your air mix screws all the way in. This should kill the engine. If it does not than most likely the power valve in the carb is leaking. On holley carbs all it takes is just one backfire and it will blow the power valve. .

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