I have a d6 9u and the pony motor is not running well. I took off the carburetor and cleaned all the all the passage ways. New gaskets as well. When I try to start it it will not run. However if I remove the needle assembly it will start right away. To keep it running I have to take the assembly in and out to regulate the air in the carb. Gas tank has been cleaned and new lines are installed. Any help would be much appreciated! 

take it apart again and have it sonic bathed. probably a microscopic bit of shit some where you cant see or get to.
did you clean the fuel tank and fuel lines? it also a good idea to fit a inline filter instead of the old glass bowl for better filtration and the filters are cheap to buy. to clean the tank get some light chain such as dog chain and put in the tank, add some rust converter, bung the openings and shake well(another member zip tied it to the wheel of his ride-on mower and mowed the yard tank was cleaned when he finished) empty out rinse with water and then fuel. its a good idea to use a tank sealer such as red coat or POR-15 to preserve the tank and stop the corrosion permanently. fuel has a bad habit of loosening old gunk that you thought was gone.

mixture setting could be wrong too.

2 cups 1/4" drive screws with pointy ends / Bake and shake 1 Acre - the points get into every little nock and cranny.

Muriatic acid works very well but is extremely dangerous. ⚠️⚠ USE PROPER PPE or Common sense ⚠️⚠. Use about a cup in a small 1 - 2 gal pony gas tank, USE OUTSIDE, and swish around for ten minutes DO NOT BREATH. The vapors well take your breath. Drain and water flush then dry quickly, I use a hair dryer or the like. Tank should be clean but repeat if necessary. It will flash rust instantly so spray WD-40, light oil or even Pam or diesel - something.
To dispose of wast properly add baking soda stir or aerate until the acidic solution stops bubbling and is neutralized.

muriatic acid also known as hydrochloric acid.

i prefer a phosphoric acid based rust converter as it changes the iron oxide to iron phosphate which is naturally anti corrosive.