I just replaced the gear kit on my Sears garage door opener. However, now the chain only travels about 6" with or without the door attached. Initially the door was stuck in midair. I was able to adjust it somehow get it to go all the way down and by adjusting the "down limit" it now goes down and stays down. I can see that the "limits" operate by contacts which close a circuit. However, when I try to raise the door, it comes up about 6 inches and stops. No amount of adjustment of the "up limit" seems to help at all. Any suggestions?