Good vacuum at high idle, NONE at warm idle.

1978 FORD F-250
100,000 MILES
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ROYSE
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I have a rough warm idle condition in a 1978 Ford 351M.
Hooked up a vacuum guage, and on high idle it sets steady at ~17 inches.
As soon as the carb warms up and it drops to normal idle the vacuum drops to ZERO and stays there.
Seems a little backwards to me!
Other than the rough idle and an occasional stall at stop signs it runs fine, has plenty of power, no smoke, etc.
Oct 20, 2011 at 11:31 PM
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RASMATAZ
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When was last tune-up?

You could have been hook-up to a ported vacuum instead of full manifold vacuum below throttle plate. Try adjusting the A/F mix and base/curb idle, Check the EGR and PVC valves. Have you check engine compression and fuel pressure?
Oct 26, 2011 at 2:09 PM