Hi Keith,
I have a similar problem. I’m on a brand new carb, an OM for the MZ251 B30N1 and I have checked everything twice over but no sign of life. Not a splutter! The spark plug is dry and so I think it must be the carb. I’ve also disconnected the inlet stub from carb to combustion chamber and, kicking it over, the piston slides nicely up and down and there is compression, so no problem at that end. I noticed on disassembling the choke from the carb body that the rubber bung had dislodged from the end of the plunger as mentioned by Andrew (is there a way to stop this happening? – it is a new carb btw). I wait with interest to hear others’ views.