The zook seems to have 31s???? if so the gearing is too high for attempting anything at a slow speed. I have two zooks in the family, we put reduction gears in the locked Zook and it made a world of difference.
Sounds like a loose fan belt as well.
They are also prone to low speed carby blocks (idlejet) change the leads and give it a good backfire, (bushmans way of clearing the idle jet) make sure you put them back.
This is my sons Sierra in action
http://youtu.be/MTUzEHPPyiY