This morning mixed up some 2 pack tank sealant which is impervious to petrol. Pushed it into the end of the nozzle and after a few hours it had cured enough to run a 0.3 mm drill through. Because it was still a bit soft the hole ended up about 0.5mm. Carefully picked out any swarf and re-assembled the carb (with the original brass butterfly plate), a few pushes on the primer and gave the cord a couple of pulls and the motor fired up nicely and was able to be throttled down to a lovely smooth idle.
QED - the main jet was far too large for this motor.
The governor mechanism on these mowers pulls the throttle further open when the load slows the motor down and the cooling airflow drops.
It does nothing to the fuel flow through the carb or the air/fuel mix.
Thanks again AVB, your advice on how to remove the main nozzle was extremely helpful.
