Riding mower wont start?

14.5 horse b&s engine, starter wont engauge, solenoid clicks, has new fully charged battery, replaced starter with used one, still nothing, replaced starter again with used starter that i tested before installation and worked perfectly, fuse is not blown, removed solenoid cleaned and sanded behind it, checked all connections throughout, hooked jumper cable from battery, to solenoid, still no start, hooked cable to all 3 starters still no start, starter wont even spin, jumped mower with car, still no start, jumped terminals on solenoid with screwdrivers, nothing, What could be the problem?