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Old 06-24-2007, 04:50 PM
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Vacuum gauge needle jumps

Hello. Just reinstalled freshly rebuild 350 into 1980 Corvette (top and bottom end rebuild). Auto tranny. Idle is rough -moves up and down. I did a vacuum test using a manifold port: the needle moves very quickly (like a fast windshield wiper!) BETWEEN 13 AND 17. Could this be an internal problem? I did spray carb cleaner around the carb base - nothing. When i cover up the top of the carb with my hands the idle increases and smooths out. Would a bad egr valve do this? I'm stumped and need advice. Thanks in advance!

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Old 06-24-2007, 05:02 PM
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Doc here,

The Vacuum is right on the high side, too low on the low side...

A Rapid Fluctuating needle is indicative Of: bad / Burned Valves, Leakey Valve guides, leaky Valve seals, or OVER~ADJUSTED valves..(along with rough running..)

I'd do a re~Adjust on the valves and try and find any tight ones, then go from there.

If that won't cure it, you may be sending those heads BACK to the machine shop..

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Old 06-24-2007, 06:41 PM
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Here is a link to a website that shows and explains what the needle movements mean. Hope this link works. I look back at this site alot when tuning our vehicles.
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