Im assuming this is for your 68 with 351W?.
I wish I had 16" of vacuum at idle, the best I can get is around 10", but mine purrs like a kitten.
Dont blame your vacuum, because thats not your problem.
Put a new pcv valve (if you haven't yet)
set your timing, then adjust your carb.
when you're setting your air/fuel mixture screws , a good place to start is 1 1/2 turns out and adjust from there you want the highest vacuum reading (highest idle) turn each screw in or out little by little to achieve highest idle/vacuum reading.
then adjust your main idle screw, dont open to far where allot of the transfer slot is exposed too much, you want to end up with just a tad of that slot exposed passed the butterflys (when looking at the carb from the bottom side).
If for some reason, it needs more idle but you dont want to open more on the primarys because of too much slot exposed, you can crack open the secondaries a little (same, only exposing a tad of that slot)
If engine still needs more air and the butterflies are cracked to the max (.020" ? slot exposed) you can drill small holes in the butterflies for more air, then re-adjust.
If none of this makes sense to you, take your car to a carb guy or a good old school tune up place.
good luck
I wish I had 16" of vacuum at idle, the best I can get is around 10", but mine purrs like a kitten.
Dont blame your vacuum, because thats not your problem.
Put a new pcv valve (if you haven't yet)
set your timing, then adjust your carb.
when you're setting your air/fuel mixture screws , a good place to start is 1 1/2 turns out and adjust from there you want the highest vacuum reading (highest idle) turn each screw in or out little by little to achieve highest idle/vacuum reading.
then adjust your main idle screw, dont open to far where allot of the transfer slot is exposed too much, you want to end up with just a tad of that slot exposed passed the butterflys (when looking at the carb from the bottom side).
If for some reason, it needs more idle but you dont want to open more on the primarys because of too much slot exposed, you can crack open the secondaries a little (same, only exposing a tad of that slot)
If engine still needs more air and the butterflies are cracked to the max (.020" ? slot exposed) you can drill small holes in the butterflies for more air, then re-adjust.
If none of this makes sense to you, take your car to a carb guy or a good old school tune up place.
good luck