You should definitely be sure about your timing and firing order.
The 75 would have had a catalytic convertor built into the exhaust manifold. If the car has had a running problem for long I wouldn't be surprised to see the cat plugged. There may have been a test port to read engine mixture ahead of the cat on that model. If so hook a pressure gauge on a long piece of MB hard vacuum line and drive the car watching the pressure. Two to three pounds of backpressure should be tops at cruise or 60mph. Doing this for any length of time can melt the hose.
Another method of checking cat backpressure would be with a vacuum gauge.
The vacuum should be higher at a 2000rpm idle than at normal idle. It the pressure is less at a stable 2000rpm then suspect the cat.
Another problem the Solex 4A1 carb had was the secondary air valve spring tention. I doubt this would be the problem at sixty, but, anytime you enter the secondaries (the back two barrels of the 4 barrel carb) this problem can put you right on your face.
To check, stop motor and depress what looks like a choke on the rear barrels with a finger. This is the secondary air valve. It should be free moving and spring loaded closed. This can't be checked running as the metal diaphram at drivers side rear of carb will hold the plate closed with manifold vacuum.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician