The nomenclature of high speed needle valve and low speed needle valve is misleading. The low speed needle valve is acutaully a mixture adjustment. If this adjustment is way out of wack then it effects the high speed adjustment. Check the low speed adjustment compared to the recommended primary setting. If it is not even close to the intitial factory recommendeded then adjust it back to the recomemnded setting. Is this engine brand new or was it picked up used?