Not sure what the troubleshooting protocol is for this, on my 66 dart 270 running a rebuilt engine, recently swapped over to the Super Six setup Carter BBD.
Idle is good. never backfires. Always starts up. running around town without issue. Doesn't overheat. Gets pretty bad gas mileage. I have started to take it on the freeway for about 7 miles each way. I can get up to 65, no issue , but once it gets up to speed, and I keep it at that speed I start to feel misses or stutters. If I push more gas, it doesn't stutter. However if I really push more gas I will get stutters at hi speed.
None of this happens when I drive around town between 40 - 45 mph.
I have no stutter or hesitation off idle.

I read the Carter manual and it does mention the StepUp Piston ? Not sure of the adjustment but??? Ideas?

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66 dodge dart 270, 225ci, 3.7l, L6

Had the same issue on a single throat. But noticed if I backed off very slightly, it would settle down & run smoothly. Mental troubleshooting while driving, figured a leak due to suction somewhere in the carb. Took carb apart, and found the retaining ring holding the economizer in position had fallen out, allowing leakage past the seal at high throttle settings. That's my story. Mebbe it'll help?

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"Louise", a 1976 Dart Custom project, (now sadly reverted to being just an "organ donor" to our other project Darts.)

Strange update: I just swapped out my old 1963 Alternator for a new reman single field alternator and now I am getting a real solid idle, strong, and when I get on the freeway, no stutters. ??? Does that make any sense? Maybe my gas mileage will get better?

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66 dodge dart 270, 225ci, 3.7l, L6

Are you running EI, if so then low amps or voltage dips will cause misfires...

Not sure on the requirements for GM HEI, but Mopar EI prefers the solid electronic voltage regulation to stay stable, and will work down to about 10 volts on the battery if alternator no longer will charge, just under that ignition gets salty...and the OEM starter may not crank over....