There are two popular versions of the K&B 40. The model 4011,and the older model 8011.If at the end of the prop shaft there is a bulge--that is a ball bearing equipped version,and more then likely,the bearings are shot. If its the other version (no bearings)then the bushing is bad. You can still get bearings through MECCOA engines.think its www.modelengine.com Option two--take it to your nearest hobby shop,and they will give you a evaluation. Good Luck--