First off, flexible metal cable like that is not recommended for gas engines due to the possibility of radio interference. Secondly, when you do use those types of cables, the outer sheath needs to be anchored as close to the servo as possible while still allowing enough throw, and at the firewall. Lastly, I have sometimes had to solder a length of solid pushrod to the flexible cable where it goes from the firewall to the carb to help eliminate unwanted flex. You can use a brass sleeve to facilitate soldering the cable to the pushrod. A couple of times I got away with just cleaning the flex cable with flux and wicking solder into that portion in front of the firewall to stiffen it.

You might want to just go to the regular plastic flexible pushrod. It is stiffer than the metal cable.

AV8TOR