rapid fluctuation on vacuum gauge

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1971demonH

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Jul 13, 2014
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hamburg ny
trying to tune a fresh rebuild and the engine feels like its misfiring. smooth idle but rough acceleration. vacuum fluctuates rapidly between 0-10. vacuum increases with rpm and still fluctuates.
 
Tell us about your build.....engine specs? Could be timing, could be ignition, could be carb? More detailed info would help.
 
one cyl (maybe 2) ain't pulling its' load like the other seven are so when it fires or attempts to fire the vac drops significantly which is the split second drop you see as a fluctuation. do a basic compression test (always a good idea) & check plugs/wires/firing order. rotor phasing/radial gap wouldn't hurt either. On the fuel side it might be a vac leak. Mild cam? (vac sounds low) problem started after breakin on a new rebuild? #5/7 plug wires are correct. I'm thinking ign. Holler back
 
I like the Early Valiant/ Barracuda link. I printed off a copy and intend to keep it in my glove box. I elected to install the Rallye dash vacuum gauge in my Duster 340. I figured it would come in more handy as a diagnostic tool more so than a tach.
 
all 8 cylinder have good equal compression around 155 + or - 5. converted to electronic ignition new holley 750, rpm air gap intake, and xe274 cam. 14* btdc initial timing. fires right up every time! firing order is correct
 
according to the reference chart, it looks closest to weak valve springs but the machine shop measured the spring rate and said it was plenty for my cam.
 
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