1973 Plymouth Scamp accelerating/braking issues, please help

This sounds as if engine is running lean at idle.
Check fuel bowl for dirt & blow out internal passages, adjust float for correct fuel level, look for vacuum leaks and fix as necessary. Adjust idle circuit using vacuum gage to highest vacuum, and idle speed, than adjust idle rpm by screw that moves throttle plate aka idle speed adjustment screw.

Secondly, if this unit is a remanufactured carburetor, chances are good you may never get it to work properly.