My 318 w/850 Thermoquad never idles well in gear, and stumbles off the line

I have an 800 equipped LC teener, circa 1973, that runs absoutely great. That carb can be made to run gangbusters on just about anything. The primary side is just a small 2bbl and can be tuned as such.The secondary air door can be adjusted to satisfy various cfm requirements. That's part of what made this such a great carb, back in the day.
The difference between the 800 and the 850 is just the primary bores. Most oem 2bbls are bigger than the 800 primary(1.375, IIRC). The 850 primary was 1.5 IIRC, which is about the same as a lot of Carter 2bbls. I love that TQ.
I would check the bushings inthe baseplate where the throttle blades pass through. If the blades don't park in the same place every time, relative to the transition ports, this is what you get; intermittent idle and tip-in problems. A stiff return spring will aggravate the condition. So check that first.If its sloppy, you'll have to fix that.

Well-used TQs also have problems with the accelerator pump circuit. It seems the pump checks don't last forever, and that short plastic connector pipe looses it's grip over time as well.So if your baseplate is useable, then a re-kit is probably in your carbs future. There is a test for the pump circuit.