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Good vacuum at high idle, NONE at warm idle.

Tiny
ROYSE
  • MEMBER
  • 1978 FORD F-250
  • 100,000 MILES
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.
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Thursday, October 20th, 2011 AT 11:31 PM

1 Reply

Tiny
RASMATAZ
  • MECHANIC
  • 75,992 POSTS
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?
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Wednesday, October 26th, 2011 AT 2:09 PM

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