'67 Holley #4160 idle screw adjustment...

[Patrick T.]

There is an idle adjustment screw (needle valve) on either side of the #4160 carb. I was checking the intake manifold vacuum at the screw in plug hole at the rear of the carb with a vacuum gauge.

So with the engine at idle speed, I adjusted the idle screw on the driver's side and it was very sensitive; the maximum manifold vacuum I could get was 16" of vacuum.

Then I go to adjust the idle screw on the passenger side and I couldn't get any change in the vacuum reading when I adjusted the idle screw. I completely closed the needle valve and nothing changed. Opened it way up and still no change.

Have I forgot something here? How come I am getting no change in idle speed or vacuum reading on the passenger side, but the drivers side is responding? The carb was just rebuilt. Thanks, PT

[Gerard F.]

Re: '67 Holley #4160 idle screw adjustment...

Patrick,

It sounds like you are adjusting the idle mixture screws on the metering block instead of the idle screw on the throttle shaft. Try this:

Turn both idle mixture screws in to lightly seat, the 1-1/2 turns out. Then set your idle with the idle screw on the throttle shaft. Then with engine hot adjust the idle mixture in slightly on both sides until you get a lean roll (20 to 30 rpm drop) then back out 1/4 turn. Or adjust each side the same amount for highest vacuum. I think you have to keep switching sides, rather then adjust one and not the other.

Jerry Fuccillo
#42179

[John H.]

Re: '67 Holley #4160 idle screw adjustment...

Pat -

Either the idle passage in the metering block or main body is blocked on that side, or the idle air bleed orifice is blocked. Idle mixture is adjusted by working back and forth incrementally between the two idle mixture screws to achieve highest steady vacuum; if you get no response at all to one of the screws and good response from the other one, a passage is blocked somewhere.