How or where can I secure a manual w/diagrams for my 12 HP 38" cut riding lawn tractor. Purchased in 1990. 6 speed transmatic transaxle. Runs good. Model #139-651. Briggs & Stratton synchro-balanced engine. Approximately 18 years old. Replaced drive belt then had trouble with speed staying in 1& 2 levels. Need diagrams for belt replacements, etc. Thanks for your help.

Truman Christian, March 2009