455 olds quadrajet

[Ed Wright]

It's been a few years, but I used to rebuild at least one Quadrajets a day. I don't remember seeing primary throttle blades drilled from the factory. It's getting air from somewhere to idle at 900 RPM. Has to be a vacuum leak, throttle blade has moved so they don't seat, or holes somebody drilled. A stock engine should idle at 8 deg BTDC, of course vacuum will be higher with more spark advance. As mentioned before, try spraying something like Berryman's B12 in the top of the carb. If it's lean the idle will pick up. If not too lean, it won't like that stuff. It will like it just better than not near enough fuel. Normally, 2 1/2 turns out is close to where the idle mixture screws end up.
Most of the time, if the carb is too lean with no vacuum leaks, you will only find one side with something in the idle fuel restriction. Turning the mixture screw on that side will have very little effect.
Good luck!