Frank, a couple of years ago I bought a Moki .61 Long Stroke, it did exactly what you describe but not all the time. I found that the banjo fitting that you attach the fuel line to on the carb was leaking air in around the clip that holds the high speed needle steady, the holes in both the clip and the banjo fitting are a larger diameter than the stud they are mounted on. I cut both ends off of a SuperTigre muffler gasket and used them as gaskets between the carb body and the banjo fitting and the clip and the banjo fitting. Problem solved.