Allen.
For larger engines say anything bigger than a 15 large tubing is better, particularly for 4 strokes. The small tubing reduces the fuel draw of the engine which makes the needle touchy to set and can cause funky engine runs. As for the clunk, you want something with some weight to it with a 1/8" - 5/32" hole.
You can make a clunk from a piece of copper tube then wrap with ~1/2" of copper wire and solder.
Best, DennisT