A percussion drilling assembly for drilling through earthen formations and forming a borehole. In an embodiment, the percussion drilling assembly comprises a fluid conduit including a tubular body having a first end, a second end, a through passage extending between the first end and the second end, and an inlet port in fluid communication with the through passage. In addition, the percussion drilling assembly comprises an adjustable choke at least partially disposed in the through passage and including a first bypass port. The adjustable choke is adapted to decrease the volumetric flow rate of a compressed fluid through the first bypass port.