As Blumbly says Bumps, broken rings will cause running problems in a two stroke because they will cause a loss in compression. Depending on the sealing of the piston in the bore, the symtoms can vary often making it hard to work out, From what i understand, the biggest difference between the 125 and 160 motors was the hardness of the bore and the thickness of the pistons, this resulted in 125's still running , and often really well, with next to no rings left as the sealing was so good, where as the 160 barrel was a lot softer and piston much lighter resulting in wear that affected the running much earlier. combine that with worn crank seals etc, etc, it's mostly why it's the 160 that causes headaches.....
