magneto c182 o-470r bendix

Went to go fly on Sunday, started the 182 up, and it struggled a bit to start up, back-fired once, and then ran good out to the run up area. Got to the mag check, set RPMs at 1700, switched to Left and RPM stayed at 1700, back to Both, stayed at 1700. Switched to the "R" on the switch and...