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Super Moderator
24,653 Posts
Yo 79broncoowner,

Normal engine
Steady gauge 18"-20"
at all speeds.
Throttle is
released and engine
quickly drops from over 2000 RPM to idle.
Needle jumps 2" - 5" above normal, then drops to normal without pause or hesitation.
Needle wavers rapidlybetween 10"-20" at idle, becomes worse at higher RPM. Weak or broken valve spring causing valve to open slowly. Needle wavers irregularlyNeedle wavers at idle, fluctuates rapidly in smaller range at higher RPM. Manifold leak at intake port.
Steady lowLess than 2" down from normal.
Retarded ignition timing.
Drifts at idle,stabilizes at higher RPMBurnt valve; combustion chamber leak.
Steady very low
Late valve timing
Wavers irregularlyin one range despite engine speed.
Unbalanced carburetion; improper spark plug gap; ignition timing; poor valve seating.
Steady Extremely low
Up to 15" drop
Warped or cracked intake manifold. Bad carburator-to-manifold gasket.
Vacuum averages lower than normal at idleneedle fluctuates almost 3" on both sides of normal.
Worn valve guides admitting air, and upsetting carburetion.
Needle rhythmically drops1"-7" below normal
Leaking combustion chamber or valve; ignition or plug failure effecting one cylinder Needle drops rapidlybut intermittently and then recovers. Valve sticking at times, or won't close properly.
Needle jumps 2"- 5" above normalupon quick deceleration, but hesitates at a higher pressure before returning to normal. Restricted exhaust system causing back pressure.
Drops to 0" then risesunder full throttle with increased RPM. Dirty air cleaner; Undersized carburetor; restriction in exhaust system.
Needle does not jump much beyond normal when throttle is quickly closed, and engine speed is cut from above 2000RPM to idle. Piston rings may be worn and allowing blow-by into the crankcase.

by Craig U

880 Posts
Discussion Starter #7
My new engine is using oil a quart about 600 miles. Thought maybe the vacuum gauge might say something
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