I recently developed an ignition discrepancy issue and can't seem to conquer.

After performing initial engine start on the O-360 A3A, Itypically wait until engine is no less than 100 degrees before making a runupfor magnetos and other systems checks.
This particular morning, while performing mag checks at 1800rpm and making sure all systems were in good working order, the right mag hadit’s usual 30 to 50 rpm drop. Returningselector switch back to both mag allowing rpm to stabilize and subsequentlyswitching to left mag, I experienced a continued backfire. Switch back to both and was experiencing arough running engine. Cylinderscompression check all at 78 and 79 while timing set at 25 º BTDC and both mags sync within 1/2 ºor less.

My first thought was that this mag just went south on me andsince I had a recently 500 hr inspection done on another set of mags, decidedto replace it. After engine start, leftmag continued to run rough (no backfire) and new set of spark plugs installed. Another start and engine continued to runrough. Check the p-leads and all seemed to have the proper resistance,continuity and no short. I purchased a new set ofspark plug leads and engine continued to run rough. Replaced the ignition switch with no changeto the discrepancy.

Because the engine rough running is isolated to left mag, I’veruled out induction system issue. Idecided to remove the newly installed left mag and reinstalled originalmag. Engine start once again resulted incontinued backfire.

Could the issues of left mag backfire be a coincidence and newmag rough running be related to bad condenser and or bad e-gap set? Any suggestions?

In CowTown USA,
DonDDay