Re: 1959 Johnson 5.5 Dying at High speed
It is the path the fuel travels when in high speed operation.
Take the carb off, soak it in solvent or Pine Sol, wash with hot soapy water, blast with compressed air, then shoot aerosol carb cleaner into all passages/cavities, wash again, then blast dry with comrpessed air. Should now be spotless, time to rebuild.
Float needle could be sticking, float issue, clogged passage, etc...
Are you running on a fuel pump or pressure tank>>?