If 7" will pull it up and hold it up, that's actually surprising. I found somewhere between 9 up to 11 or so was common. Any drop in vacuum below 7 will open the power valve and immediately make your mixture richer. That could happen when leaving a stop sign, climbing a hill, shifting gears, etc. Warping of the castings will cause vacuum leaks and also damage to the castings can exacerbate that...please see where I used brass tubing to try to help restore damaged casting pieces knocked off by "professional" rebuilders along the way. The power circuit isn't the worst idea I've ever seen...that is to say it is an okay idea on paper and then only if you have real honest gasoline. If you have unleaded gas it will not work as well. If you have ethanol-laced unleaded, it will gum up rapidly and the piston will get stuck in the down position...constantly from what I've seen. In a couple of the Rochesters I studied, I had trouble removing the piston things were gummed up and stuck so badly.
If your power circuit is (currently) working, then I'd wonder what size main jet you had? Float level is important. I mentioned why in another post. Float condition is important...for obvious reasons. Years ago service stations had an in-exhaust pipe monitor they used for state inspection tickets. Those were really just a modified air/fuel ratio monitor. If you could find one of those, you could get a good idea what was coming out of the engine. One can be running rich and seem like it is running ok...at times, that is.
