Mountfield rv40 model sp474 type els464tr hunting?

Hi just purchased an rv40 from eBay (my first petrol lawnmower ever) when it was demonstrated to me it was hunting and he advised it will stop when it warms up. However when warm with the throttle all the way up in the choke position it runs fine but if I lower the throttle it starts hunting again. When starting it hunts until warm even in the choke position. Any advise what could be at fault, it's just been serviced, new plug, new oil and a clean air filter.
Thanks in advance for any replays