OK, I'll try riding the bike before engaging the throttle to see if that is how it works. I actually doubt that this is the problem. In the ebay listing, the seller has a youtube video demonstrating the motor functioning from a dead stop with the front wheel raised (near the end of the video) - when I do the same thing, the motor does not run.
http://cgi.ebay.ca/ws/eBayISAPI.dll?...m=320376612090
http://www.youtube.com/watch?v=3X0zm...layer_embedded