Drilling a hole for ported vacuum advance

[J.Peter]

I need help with my Quadrajet on my big block 70 Vette.
My carb is not the original part.
 I need  to drill the hole in the drivers side primary throttle bore for a ported vacuum advance setup.  The outer fitting is already there but it is not drilled through to the throttle bore.
I know the hole should be slightly above the throttle blade  - but how far above and what diameter should it be?
Even a photo with no dimensions would be very helpful.
Thanks
J.Peter

[Cliff Ruggles]

You'll need a donor baseplate to get the exact measurement.  Make sure you are on the drivers side as some carbs had a higher location on the passengers side for ported vacuum which is meant for EGR.

I use a very sharp punch to make a starting point for the drill bit.  Using a pin vise to hold the bit helps as it lessens the angle some.

The factory actually used a slot instead of a hole so that there would be more area uncovered.  Since making a "slot" is difficult I'll drill a small hole and very carefully work the bit back and forth to widen it as much as possible. 

I use a .046" drill bit, USA made.  If you try to drill that small of a hole with a Chinese bit good luck, they break if you stare at them too long!......

[J.Peter]

Thanks for the reply,
I like your idea about the slot - I will use a small Dremel bit.
Thanks also for writing that lovely rebuild guide book.
Last fall when I took this carb apart I added some threaded idle tubes - they work nice.
J.Peter