My YS91AC runs good above half trottle but below it gets mighty rough down to an idle. It will actually stop if I let it run at this speed. At idle it runs OK. All the jets are clean, one way valve is clean, no leaks, no restructions, new plug, have not touched the regulator adjustment except to clean it, diaphragm has a little bulge in it and I have one on order. Tried diffenernt jug of fuel. This engine has been performing flawlessly in my U-CAN-DO 3D for many weeks and suddenly this.