I agree w/ the other fellas about the low speed mixture being set too rich. Be careful to not go too lean, it's easy to go to far w/this screw as the sweet spot on this needle is quite small. Run the engine between each adjustment you make and bring the throttle up to full power quickly and consistently. If you get into trouble, OS does provide a default setting for the low speed needle in their engine manuals and I think you can get these manuals in PDF either at tower or the osengine home page. Good luck and mind your fingers.