My Cont TSIO 520 was a little rough on start up from cold in about 45* weather, but seemed to smooth out as it warmed up. After about 10 minutes the mag check was good on the right, but decidedily rough and missing on the left magneto. I didn't have any tools with me nor a spare plug as I have had such good service with this plane that I don't carry either. I used to carry both with a another plane. I have not had any times when this engine fouled a plug which then cleared on run up, so I didn't think that was going to be the cure. I am at Front Range airport, and was glad to find mechanics there on Sunday morning. Its not easy to trace the plug wires from one mag to the plugs since they are bundled together, but the mech pulled all the top ones which have easy access. and of course no problem found. Getting to the bottom ones is harder but sure enough the front cylinder, think it is 5 or 6 on the right side, had a bad plug, half the insulator was broken off of the center electrode, and the shop didn't have a new replacement plug since these are 32 S,fine wire ones. Any we hope to find one locally in the morning. Glad I didn't try to fly, and just hope it would be better in the air. It would have probably run ok on the good plug ,but I would have been given up my dual ignition backup. There is a lot of pr for Tempest plugs these days, but I have always used Champions and seemed to work well, this only the 2nd cracked plug Ive had in over 10 years or about 1000 hours. New ones are about $85 a piece, so its good that they last a long time. The outside of mine look old but the electrodes still look good, even on the broken one. Not sure why it broke, bad plug or backfire starting or just age and chance, but it was no doubt about rough. The FBO has a big screen tv where I got to watch the Longhorns smite the purple toads just as it used to be and should be once again.