Got me stumped. After cleaning plugs and re installed them ran the engine everything was fine. Flew it one hour, stop for gas, started then did my run up before take off. 1700 rpm check my cs prop, ok. Switched of right mag drop off 75 rpms , then the left drop of 125 rpm at started back firing, turned back on rechecked the right again a 75 rpm drop then after a second it back fired. Repeated process once again same results. Ran the engine on both Mags to 2400 no back firing back to 1700 no issue with both mags but does occure on one mag. Any suggestion were to start and what sequence to start. Thank you in advance for any help.