The engine starts easily but will run only with partial choke and then unsteadily, switching erratically from two to four stroking It will not idle. If the choke is opened the engine two strokes well for a few seconds then cuts dead. The fuel lines are good, oil ratio is correct. Filter is the right one and clean. Problem is same cold or warm. The mixture screws are set as per the operators manual.

Hi,
First thing I would check is the tightness of the four screws attaching the reed plate to the crankcase. That is the roughly circular piece that the carb attaches to. They sometimes come loose. I will attach the parts breakdown.
Peace,
Clarke

Thank you and apologies for the delay in reply. All OK there, properly seated amd four screws good and firm (9, 10 and 11 in the diagram. Right?). Thank you for your advice.