Courtesy of http://www.he-man.us
If the Backpack is already apart, then measure the string so that you can wind up one side 16 times. Then wind the other side once. (This may take some playing around to get 16 winds.)
Make sure you wound it the right way. You can tell it’s correct when you put the cog into the backpack. It has a cut away on the side of the backpack for the string to run though. It should run right off of the cog (spool) through the hole. If the string has to pass along the bottom of the back pack before coming out the hole in the bottom, then it’s wound the wrong way. In which case unwind it and do it the other way.
Now put it together (put the bushings in the top and bottom too) and test it while holding the backpack together. Make sure it runs up and down smoothly, (you may have to sand the pulley [spool]). If you do sand it, just sand it very lightly just to round it better or to knock off the burs.
If you have too much drag when going up the string, it may be wound the wrong way. (See above)
When it is working just the way you want, very carefully open the backpack and put a small drop of oil in the center of the spool. (Don’t put too much on or the glue won’t stick.)
Put it back together, recheck the backpack by running it up and down. Now very carefully put glue (model glue works great for this) on just the sides of the backpack. (Don’t put any on the top or bottom or you might get some on the string.) The glue should suck into the seam with a capillary action. Wipe off any excess glue.
Wait until the glue dries and then play with your fixed toy.