Hi again,
I have just taken the engine out of the airframe and noticed that the 'o' ring between the carb' and airbox is out of position on one side such that I can roll it away from the airbox with my finger nail The seat in the airbox is a 45 degree chamfer so there is nothing to stop the pressure from forcing the 'o' ring out of position, in fact the chamfer helps this happen. I cannot think of a reason why YS have not put a recess in the airbox to retain the ring properly rather than a chamfer. The 'o' ring that was fitted has a 2.5 mm section which seems too fat for the chamfer to hold ( it tends to try to squeeze out). I am wondering if the preveous owner replaced with the wrong size seal, I cannot check at the moment as there are no sizes given on the parts list. I would imagine that as the engine came up to full throttle (pressure) the seal became unseated, reducing the pressure in the chamber, and then reseated when the revs dropped. The 'o' ring would therefore act as a high pressure valve causing the rev instability. I will source a new 'O' ring and try again. If the seal becomes unseated again I will have to machine a proper recess location in the airbox. Thanks to all who have helped me I will let you know the results of the next test. Regards Tony