I think Ive solved the Grubee gear box clanking and chatter at idle and take off. The centrifugal clutch springs engagement is so close to the Honda GXH 50 or Huasheng clone engines idle rpm that at idle the clutch is always partially trying to engage creating the chatter ang grabbing. Solution is to lighten your clutch shoes slightly by drilling holes in the metal portion of clutch shoe in an area that wont affect strength similar to swiss cheese or airplane wing spars. I was sucessful with several 5/16" holes. Just drill slowly cooling metal constantly with atf or light oil . Wash shoes after with paint thinner if your running dry clutch. Also carb idle issues. The carbs on these engines were designed to run wide open or nothing. A small slide carb from a pocket bike or scooter will vastly improve idle and drivability. Hope this helps!