This engine is all repaired now. The governor fault was caused by the loose throttle assembly - it could only move a few millimetres but it was enough to put tension on the governor spring so that the governor lever would not return to idle. Also the muffler was very close to the arm and might have been touching it.
It has been a very interesting learning experience for an amateur mechanic and the engine looks to be in very good shape now.
Thank you for your expert advice.
