[MetalMan]

To keep the spring from flinging out you just need to make sure the spool always stays within the pullstart housing. From the factory there should be a piece of tape or something holding the spool in. When you remove the pullstart from the engine, have a flat-blade screwdriver handy so you can pry the spool off the backplate instead of directly removing the pullstart housing (prying the spool off will effectively remove the pullstart). While you are holding the pullstart, keep a finger or thumb over the spool.

One of the biggest causes of pullstart failure is pulling the string too far. There's no need to pull it more than about 10in. from the base. Also, if the engine is really tight, use the flywheel to turn it over instead of the pullstart.