Craftsman 1/2 hp garage door opener only opens 2 ft. then closes

I have recently replaced gear and sprocket set on Craftsman 1/2 hp garage door opener. For some reason, even with chain (on top sprocket that connects to door) and limit gear and assembly disconnected, still only travels about 3 seconds in each direction. Tried by reconnecting all, applying correct tension, etc, and still doesn't work - opens a couple of feet, then stops. Something in limiting gear, but I can't tell what - no visible obstructions, lubed and well cleaned when replaced. Any thoughts?

The RPM sensor might be the issue here. The black cap that runs through the RPM sensor needs to be compressed completely as far as it will go on the motor shaft. Push motor shaft forward as far as it will go and tighten the collar with set screws. Put black cap back on RPM sensor. After that try adjusting up and down pressure. That that might solve your problem.

-Jeff