I'm just posting this as a reference for anyone attempting to remove the idle mixture screw caps on their Quadrajet. This particular carb is model #17059226 from a 1979 G20 Van with a SBC400. The idle mixture screw caps on this carb are hardened so it's not possible to drill through them.
This rotary accessory kit can be purchased from Harbor Freight for $20
It contains a number of different abrasive cutting wheels. This one is the smallest and it cuts right through the hardened steel cap.
I chucked it in a cordless drill and made 3 horizontal cuts through the cap and 2 light vertical scores. Then used a small punch and hammer to break a hole in the cap.
You can now hook the cap with the thread of a drywall screw. Grab the screw under the head with a locking pliers and drive the cap out by tapping up on the pliers with a hammer. It takes some force.
Voila!
This method leaves very light score marks on the front of the base plate by the mixture screw holes. These marks can easily be removed with a file.
Online I couldn't find any images or dimensions of the cap or it's depth in the baseplate in relation to the mixture screw. So here's that info.
Different model/year Quadrajets may have different types of caps. That I don't know for sure. But this cap actually fully encapsulated the screw head inside the baseplate.
