Quick question, after setting the engine up it ran great, now on the high rpms it backfires, only now and then, only high rpm. It's a sachs 290 with a HR carb, could the new points need resetting, or is it most likely the high RPM mixture screw on the carb. At the moment it does not matter if it's inder load or on the flat, it's only at full throttle and as soon as you come off the gas its fine and revs fine except for the occassional backfire. Any 2 stroke guru's shed a light without me pulling the covers off again.