1974 Thermoquad, 360 HD Truck , Rebuild adding performance modifications

ch1ll said
Drilling holes in the primary plates is an old trick to add to the idle circuit when you have a cam that idles at too high an rpm that the idle circuit can feed it. Air bleeds add air to emulsion tube which controls the fuel curve through your rpm range in a Holley, not sure about a TQ.

Brooks James said
This is probably a dumb question. Is it a different concept, but would drilling 1/16 holes in the primary throttle plates do the same.as opening up the air bleed hole ?
I'd like to try something even call .095 good enough for now and move on
If I screw it up I've got 30 more in the shed

Here is the reason for drilling the plates. You have installed a bigger cam and now the engine idles with less manifold vacuum. To compensate for the lost vacuum you have to open the throttle up more to obtain sufficient idle rpm. This uncovers more of the transition slot that is cut into the throttle body causing the engine run excessively rich at idle. So drilling the plates introduces more air into the engine and leaning the air fuel ratio so the throttle can be closed to something closer to normal.

Roger