I used to build Q-jets as part of a job I had years ago. The one thing that trashes them is the primary shafts, they wear out the bores in the throttle body. Grab the linkage side and see if there is play in the shaft, if there is it will suck air and it is near impossible to get the idle right. These can be re-bushed. At one time I have a special drill bit to do this, it has a pilot shaft before as part of the bit to guide it straight through the bore. Then a bronze bushing could be pressed in. A few other things, if you put in a larger can you can't make it idle because the idle circuit is not adjustable, you have to use a letter drill to open these up, I'm thinking .021-.025 if i remember right. The video guy was right about the secondary spring. On the rear bores there are fuel wells that hold a shot so when the air door opens if feeds extra fuel to help prevent bog. This does a similar thing to a double pumper. The port for this is usually just above the air door, you can epoxy that hole and drill another just below where the air door meets the body so it draws that shot when the secondary throttle opens, makes the opening of the rear barrels a bit stronger hit. You can also bore the primary throttle bore and install larger blades from a 2GC and use a Dremel to control the body bore to make a smooth transition. This gives a big signal to the primary boosters and when you crack the throttle from idle the engine jumps harder. Mos of these floating around are 750cfm but there are a few 800cfm and those have a larger primary. I mostly saw these on the likes of Olds 455, Chevy 454, etc.. You can run these on a smaller engine but the jetting will be wrong across the entire range. I learned all of this from old articles years ago in Hot Rod, Popular Hot Rod and most were written by the Carb Shop in California. They also years ago had the parts and tools to do all this. Been so long since a Q-jet has been built my guess is the tools would be hard to find. I have seen the shaft bores drilled on a mill though so the bushings could be installed without needing the special drill.
Hope this helps someone.
DC
LOL! My past experiences with Furds is why I drive nothing but Chevy and my past experiences with quadrajets is why I run nothing but HOLLEY!! Now where did that big boobed serving wench go and I'll get us another round!!!!
CHEERS!
