I have several MGS including a 50 TD. If your piston doesn’t move freely up and down you will encounter fuel starvation. SU reports in their manual most carbs that came into the factory for rebuild didn’t have a piston that traveled freely because the jet was not centered. It is simple to center the jet but you need to get an SU manual for a more detailed explanation. You may also be experiencing bad spark plugs that loose spark under compression.
Rod Knox is correct. The Carbs on an early MG did not have diaphragms. This was introduced in 1974 on SU Carbs, not Strombergs
oldwrench, diaphragms were used on stromberg, not on su crbs, other wise you are right on. Dave from steam boat springs, moss motors or classic motorbooks carry manuals for these carbs. do not convert to down draught webers. this is often recommended, but usually more trouble than the former.