Loren, there may be a faulty or out of adjustment safety switch, there could be 3 to 4. the seat, deck/clutch and brake/park,reverse. Look in your manual for there location. You may have to get a small engine tech. to check it out. Check the attached links,instruction and guides,manual, Good luck .
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How to test lawn mower electrical safety switches

http://support.mtdproducts.com/app/answers/detail/a_id/327/~/safety-interlock-switch-locations---riding-mowers

http://lawnandgarden.manualsonline.com/manuals/mfg/cub_cadet/rzt_series_tractor_model_rzt_50.html?p=2