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

How do you have the primary piston adjusted? An offline stumble usually indicates too lean of a mixture right off the bat.

The 850 TQ will not be too big. The last thing I would do is swap metering rods or jets YET. You "should" by all accounts be able to get what you have running very acceptably by simple adjustments.

I recommend this:

http://www.ebay.com/itm/Carter-Ther...ual-/181768176540?hash=item2a523a439c&vxp=mtr

Get it. Read it. Adjust yours accordingly and I just "bet" you will see a huge improvement.

As a starting point for your primary piston adjustment.

Push the primary piston tower and bottom it in its travel lightly with your finger and hold it there.

With a small flat blade screwdriver, turn the adjusting screw in the center counter clockwise and watch the tower and metering rods drop.

Continue slowly turning counter clockwise until you see the tower and rods stop dropping.

Now, adjust the screw clockwise until you see the tower and rods begin to rise, then go one more full turn clockwise. This is your baseline starting point.

From there you can adjust the primary piston screw clockwise to raise the rods (richen the mixture) or counter clockwise to lower the rods (lean the mixture).

I would no more than one half a turn at the time, recording the adjustments so that you can easily return it to its original position if you need to.

Every adjustment on that carburetor has a baseline starting point and until you get all the adjustments there, you will be shooting in the dark.

That manual will help.