This was one of those Aw Sh!t days. The plan was to go out for a biennial flight review in my newly annualed Cessna 170. We taxied over for some gas then went to runway 5 and did a run up. Bad mag check. Couldn't burn it out with running it lean. Clearly needed to go back into the hangar for some TLC. Now the thing that is bothering me is that just before the annual I ran the engine for doing compression checks. Had a perfect run up that time. So what gives? Back in the hangar I pulled and checked the top plugs. This can be done without removing the cowling. I had an even chance of finding the offending plug. Nope. Put it back together and ran it up with the same result, bad mag check on the left mag. By this time it was late afternoon so I parked in the hangar. Tomorrow is another day to dig deeper into the problem. It is the pits removing the cowling, but it must be done. We don't leave anything to chance in aviation maintenance.