Shakey vacuum reading?

[AMXguy]

While trying to tune the fresh 383 in my buddy's Charger we notice about a 3 in quiver in the vac reading, is this a sign of a vac leak on an intake port? it idles with about 18 in. another thing I was wondering is does it matter where you take the reading? it's hooked to the brake vac tree.

 It runs fine but he doesn't want a lean cylinder causing a problem.

[moper]

at 18" the thing should be rock steady IMO. I like to use the carb manifild vacuum source. The PB one only really "see" one cylinder.

[Robb]

sticky valve ?

[Chryco Psycho]

is the brake booster leaking internally ?

[wally426ci]

 

[moparmaniac59]

A three inch fluctuation (between 12 & 21 inches) may indicate that one or more valve springs are weak or broken. Does the fluctuation change as engine speed is increased? if so this might indicate worn intake valve guides. A slightly smaller fluctuation but with a normal vacuum condition (which seems to be what you have) may indicate an ignition problem. Check your coil, dist cap, plugs & plug wires ect. Let us know what you find. I hope it's something simple!!


                                                                     Matt B.

[AMXguy]

We'll try to mess with it again today, he's got a new everything, booster,ignition( Mopar electronic ), everything new in the engine including valves , guides and springs.  it runs real nice and probably this never would have been noticed other than because we were trying to set the idle mixture with the vac gauge.

  we'll move vac gauge to the carb port and see if that makes a difference. my first though was a manifold leak, but a rag over the carb slows the idle so I don't know.

[BIGSHCLUNK]

Just a thought?/  Lopey cam/lopey idle /lopey vac reading??