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

Most 318s that had Carter Thermoquads as stock units were the 800 CFM. The 800 CFM has slightly smaller primaries than the 850 CFM. All secondaries have 2.25" diameter butterflies.
Was the J/Y Tquad rebuilt before installation? check the Thermoquad phenolic section, inspect to see if it is leaking fuel at the epoxied sections of the bowl. This will require disassembly of the air horn/phenolic/butterfly base. Are the round seals that go inside the phenolic section in good condition(item # 49 in pic)? These are a specially designed seal that is flat on the top and bottom to prevent gas leaking in the main well. These are critical components. The bottom of the wells are the epoxied part that tends to leak gas. If either the seals or the epoxied parts are not in good shape, additional fuel will be going into your engine. This will cause rough idle/dying/poor initial acceleration. See item #49 (seals) and check the bottom of the phenolic bowl (item #50).
Good luck. I would suggest finding an 800 CFM TQ or 600 Carter/Eddy carb for your application.