Originally Posted by
Rich
The Mystery Cylinder mounted to the firewall might be a vacuum servo for a vintage '80's aftermarket cruise control. That's where most of them were located when folks retrofitted it. Look for a vac hose to one side or the other, probably on the left, where a black hose is visible in the photo, plus a secondary linkage/cable from the right side of the cylinder around the back of the engine up to the front-mounted throttle spool. The linkage might be gone or not.
If that's what the Myst.Cyl. is then it suggests other recommendations relative to the throttle's reluctance to idle without some help. Let us know.