Line up your mixture disc to point at the line on the carb for your low speed as stated before. (You are running a tank pressure line from your muffler, right?)If you are unable to get "fat" (rich setting), perhaps your tank clunk and/or fuel lines are likely too small. With an engine this big, you may need large fuel lines instead of the standard medium size. You should be able to get it to run fine on bench/ground without plugging a muffler outlet pipe. Several people have problems keeping enough tank pressure on long up lines (hovering, etc) unless they run a pump or plug one of the outlet pipes to increase pressure, but you should still be able to get it to run fine on the ground just by running stock pitts muffler and no pump. Look at your fuel lines. How far out have you backed out the HS needle while it is running (turns from closed)?