Okay Mooney friends, got some troubleshooting to do. On a long (2hr) cross country a couple days ago my '67 M20C, she began to develop an occasional stutter while flying through rain. Thinking it was carb ice, I applied it, but that didn't seem to be the case. I got to clear air, tried leaning, enrichening, and she still held a slight hesitation and occasional RPM flicker for the remainder of the flight. Over the airfield at 3,000, a mag check produced a MAJOR loss of power on the left mag, accompanied by backfiring. Ground run up produced the same result, barely staying running. A mechanic friend on the field looked things over and found that the plugs (with 500 hours) to be worn, with three of them showing signs of shorting "two completely dead, one on its way out" according to the mechanic. He replaced all 8 plugs with brand new Champion plugs, runup checked okay.
I just flew the plane, all checked perfect on the run up. I did a long taxi out to look things over, and leaned thoroughly while taxiing out. (OATs around 90F) Takeoff, climb, and cruise to the next airport over were perfect. On takeoff for the return, all appeared normal. Climbing through 1,500ft, I experienced a significant loss of power lasting about 5-10seconds. Mixture was full rich, All temps and pressures looked good. Thinking it was fuel related, I made a turn towards the airfield and switched fuel tanks, checking all normal emergency items. The issue cleared completely, and I spiraled up above the field. An airborne mag check above the field produced an extremely rough running mag, similar to before. I pulled power and decsended down to the field. After rollout, I did a mag check and 1,700 and 2,000, both of which were clear. After taxiing in, I did a full power run to try to replicate the symptoms. With the left mag selected, she slowly began to run rougher, eventually developing the same backfiring, which disappeared after switching back to both, but kept a hesitation while on both or left.
Thoughts? Mag, plug? Running too rich, fouling plugs repeatedly while operating at the higher temps? I've never run it in temps this warm before. I'm thinking that's the case.