Garages and Carports - Garage Door Opener Rack Replacement

I have a Chamberlain model 7902K screw drive garage door opener and the rack is toast (rack chatters and door does not move) if I unhook the trolley, the rack moves back and forth smoothly. I have a new rack, can I replace the rack and how do I do this, it looks complicated, like separating the tracks and trying to get the old rack in a position where it just drops out. All answers greatly appreciated.

Is the rack chattering due to no lubrication? I repaired one like that. The owner had clipped the door track with his car. After I put everything back in place, the door wouldn't move or would move a few inches & stop. If I disconnected it, it moved smoothly. I lubricated the rail & all was well.

Good question , I will hit the rail with some 20W oil and see what happens.

Well I lubed the rail and ran the rack up and down (with trolley detached) several times then attached the trolley and she still chatters. I am of the opinion that the teeth on the rack (plastic of course) are worn down and I need to replace the rack (not the first time, this will be the second replacement in eight years). I just do not know how to replace the rack, I assume I must drop the motor assembley and then separate the track at mid-point to remove and then install a new rack, sound about right?

It sounds right but it's one of those things that I would have to see first hand to be sure. If you're handy at all, I'm sure you'll see what to do once you start.

They call that thing a carriage.

To install carriage, remove the door arm from the existing carriage. Remove the nuts to the bolts on the screw drive rail at the front header bracket and slide the rail off to one side, giving room for the old carriage to be removed by sliding it off the end of the rail. Slide the new carriage onto the rail and replace the rail on the header backet bolts and secure. Reinstall the door arm and engage the new carriage. Adjust limits, if necessary.
Other Things to Consider

The Genie screw should be lubricated every couple of years, using only Aero or Low Temp brands of Lubriplate, shown on the side panel. Any other lubricant can cause the door to lock-up in cold weather.

Thanks, the new rack came yesterday along with a How To. The How To solution is to run rack to full up position with trolley disconnected, then turn down travel screw 8 turns counter clockwise. Next, disconnect the opener arm (the one that looks like a boomerang) and then take the retainer pin off at end of the rail (at front garage wall) and lower to a support. Then take the two rail clamps off of forward end of the rail Next, using a remote, run the rack all the way off of the screw, and then put new rack in place and again using remote, run the rack back up and on to the screw drive. Now, reassemble, turn down liimit screw 8 turns clockwise, and attach the trolley and you should be in business (film at eleven).