Mower stops after a few seconds running?

Qualcast suffolk punch 35S

After cleaning the plug the machine always starts but will not continue to run - remove the plug and it is black signifying too rich. I can make it run by manually jigging the mixture control between start and middle settings but that does not last much longer. What are the initial settings to use when maintaining this machine? I don't have a manual.
Any ideas on a fix?