Just so I'm not making any assumptions - when you adjust the low speed mixture, are you then retuning the high end needle to slightly below peak to the rich side? If not, you'll never get the low speed mixture set correctly. Make your low speed mixture adjustment is small increments - 1/8-1/4 turn MAX at a time. Its very easy to blow right past the sweet spot. Does your engine have the limiters on the needle valves? Although preset from the factory, they may not be correct for your elevation, humidity, temperature, and fuel combination.