!952 Mercury with 255 Flathead V-8 Will Idle But Not Gain RPM.

[Ggmac]

Agree , check for a blocked exhaust, next I would broken or frayed wire inside the distributor, the one that goes from the coil to the points . Its a 2 wire setup. I just yesterday did 2 replacement wires on 8 ba motors. Both wires were still attached but had frayed coating that when the point plate moved it shorted the ignition.
Its easier to remove the distributor to do this .
Do you have a vacuum gauge , if so use it and post the results. Good luck