So if I understand you correctly, the choke pull off is kind of an unnecessary thing if the desired setting can be achieved by using the adjustment screw as you described?? The reason I ask is that I am buying a 68 GTO long distance and the guy who checked it out for me commented that it "needs a pull off choke". Can you explain for me please? Thanks in advance!!
OK, first you need to know what the seller is calling a "pull off choke." Is he referring to a linkage piece or the vacuum diaphragm (technically called the "vacuum break).
The secondary opening I have referenced is the removal of the linkage rod that goes from the vacuum break to the secondary flap lever.
The linkage that connects the vacuum break to the secondary air valve flap is bent in such a way that it is part of what is called the "choke unloader system." As the linkage rod is pulled forward once engine vacuum is developed when the car starts, the linkage rod contacts a tang connected to the choke. It pulls on the tang, which linked to the choke, pulls open the choke just slightly to let air in and prevent flooding.
In removing the linkage rod from the vacuum break & secondary air valve flap, this disables the use of the vacuum break and its function upon the secondary air flap which is to keep the air flap closed when engine vacuum is present or closes it shut after opening when you let off the throttle and engine vacuum returns. Under wide open throttle, there is no vacuum going to the vacuum break to keep it closed. The spring tension on the secondary flap along with the engine drawing in air to further aid in its opening, pulls on the vacuum break diaphragm arm to extend it out because there is no vacuum to pull the arm in and its linkage rod used to pull the secondary air flap closed. The vacuum break is essentially made inoperative without vacuum.
Remove the linkage rod altogether, and you have made the vacuum break useless, or inoperative. Without the vacuum break regulating the opening rate of the secondary air flap, or pulling the flap closed once you let off the gas, you now want to manually adjust the opening rate of the secondary air valve using spring tension only. You are not concerned about the closing rate because it will simply close shut with the spring. It is the opening rate that requires adjusting and this is where you fine tune the opening rate to your particular engine/car.
With the linkage rod removed, you have now disabled the function of the "choke unloader system" tang. So you would want to now manually adjust your choke so the flap is set slightly loose or even just a hair open to compensate for what the choke unloader does. Generally, when the engine is cold, the choke will close a bit on the tight side. The choke unloader system uses the linkage rod you just removed to put enough tension on the choke tang (as the vacuum break pulls closed) to crack the choke open just a little so air will enter the carb and the carb does not flood when first cranking the cold engine.
With all this said, using and keeping the function of the vacuum break, secondary air flap linkage rod, and choke unloader system won't hurt a thing and may be best for most people. The vacuum break can be made to respond faster by selecting a different canister or drilling out the inlet hole and additionally adjusting the tension spring on the secondary air flap - just may require a little trial and error to get it where you want it.
For myself, I have always removed the linkage rod and manually adjusted the secondary air valve spring tension to where I found it worked best - popped open with no bogging. It would take a few runs down the road and I would have my needed tools with me. Never had an issue with the choke as I would adjust it manually anyway with my concern being that it was fully open when the car was at full operating temperature.
Cliff Ruggles' book recommends using the vacuum break and try finding one that matches your engine/car with regards to opening rates. He also does not recommend using just the spring tension only to adjust the secondary air flap as it will/can weaken the spring. I like to adjust the carb my way as described which goes against Cliff's recommendations. I am no expert and respectfully feel Cliff is. I sometimes also run with scissors and Cliff probably would not.
The vacuum break or any missing linkages can be gotten, so not really a big deal in my book. Hope that answers your question and did not confuse you further.