Sounds like I have the same problem. I replaced the plastic gears and sprocket assembly on my Sears garage door opener because the top sprocket sheared off. That repair went well but now the door only moves 6-12" up and down! We've tried all kinds of setting for the limiters but nothing changes. Can you give me more details on the part you got to fix this? I'd really appreciate it so I won't have to call in the repair man!
P.S. I don't see a speed indictor on the Sears parts list. Do you have a part #?