Someone should rename this thread "why not to jump to conclusion!"
Here is a copy of the instructions I pulled on adding the secondary tube. All of it makes sense. Pretty straight forward, except for the very last part. At the bottom it says to "Plug the passage where you removed the lead plug, either with another lead plug or tap the hole with 8-32 thread and use a setscrew
" Maybe it will make more sense when I start to do it, but aren't I adding the tube where the lead plug used to be? It sounds like i'm removing the plug and then drilling the hole where the plug is/was?
The ”715”, “735”, “750”, “780”, “800” and “850” (1&5/16” and larger primary venturi) vacuum secondary carbs will not function correctly without the little slash-cut brass tube in the primary which was in all the old carbs in the vacuum passage to the secondary diaphragm, to amplify the secondary opening signal. Holley quit putting that tube in the carbs more than 30 years ago. Dumbassery at it’s finest for 99¢. Without it, the carbs open the secondary late and not all the way, even with the lightest spring.
The secondary butterfly is offset with the larger area on the upstream side of the throttle shaft and the impact force of the air flow is strong enough to keep the throttle from going all the way open. The more the secondary opens, the less air flows through the primary and the less the opening signal becomes until the force of the diaphragm and the air impact on the offset butterfly balance.
The sharp edged venturi don't have the problem and the carbs smaller than 600 CFM don't have the problem, though I have added the tubes to some on occasion when the secondary is desired to function as one-to-one at low engine speed. When there is two carbs the air flow per each is halved and the secondary can be lazy sometimes, the little tube quickens things up and stronger springs can be used.
A lead plug seals the passage from the vacuum diaphragm mounting boss to the primary venturi. Center punch the lead plug and drill it out with a 1/8” drill. Drill only deep enough to remove the plug, about 3/16” or so. The drill bit will catch it and pull it out.
Drill through the passage into the venturi with .120” (#31), take care to not hit the bottom of the booster.
Use a smaller bit or toothpick and Sharpie mark to make a gauge of the length of the hole to the venturi wall, it will probably be a little over an inch. Use this gauge to Sharpie mark the succeeding drill bits so you can drill to the desired depth.
Drill with .125” to a depth which leaves about 3/16” of the .120”. Drill with .128” (#30) to a depth that leaves about 3/16” of the .125”.
From the venturi wall back to the diaphragm mounting boss the hole is stepped with 3/16” of .120”, 3/16” of .125” and the remainder .128”.
Brass tubing is available at most hardware stores. A foot of 1/8” ID x 3/32” ID is about $2.50. Cut a piece about ¾” long. File one end at a 45° angle and file a screwdriver slot in the other end. Align the slot with the angled tip so you can clock it in position before driving the tube in place. You need a 1/8” diameter screwdriver.
Put the tube in the passage, properly align the tip angle, and use a 1/8” punch and a small hammer to drive it in place with about 1/8” protruding into the venturi bore.
Plug the passage where you removed the lead plug, either with another lead plug or tap the hole with 8-32 thread and use a setscrew.
Do not plug the bleed hole in the secondary venturi.