how can i fix my craftsman eager-1 lawnmower Ser #143.996510?

That is one of two things.
The flywheel key may be partially sheared. That is usually caused by hitting something hard with the blade.
The second possibility is the blade could be loose. The weight of the blade is part of the flywheel action for the engine. If the blade bolt is loose, or if the blade adapter is worn it will do exactly what you describe. In either case, do not try to start the engine until you get it fixed. It can kick back hard enough to break bones.

Rick
October 2006