The Instructor and pupil were conducting training circuits from Runway (RWY) 25 at Weston Airport when EI-SKL experienced a partial engine failure after take-off. The Instructor called an emergency, and turned back to land on the reciprocal runway. He successfully landed the aircraft on the grass to the left of that runway. EI-SKL rolled to a nearby taxiway and shut down. The aircraft sustained no damage and the pilots exited the aircraft in the normal way. Later, on engine examination, it was found that an induction pipe had worked itself loose.