Have you followed the directions in the manual? On page 12 of the Zenith manual it explains how to adjust the marker to optimize it's performance with CO2.
FOR CO2 OPERATION:
NOTE: Zenith is optimized for use with HPA, in order to
optimize the Zenith for CO2 use, please follow the instructions
1. Insert the 3mm allen wrench into the ZeRO Adjustment Screw
(Z001). Turn clockwise two full revolutions.
2. To optimize the velocity, make sure the rear velocity adjustment
is backed all the way out. Adjust the Inline Regulator (refer to
Regulator Adjustment) until the marker is shooting at 250 feet per
second over a chronograph.
3. To fine-tune the marker velocity do the following:
To increase your velocity FPS (Feet Per Second), use the allen
wrench to turn the Velocity Adjuster (P023) clockwise.
To decrease your velocity FPS (Feet Per Second), use the allen
wrench to turn the Velocity Adjuster (P023) counterclockwise.