Probably a stupid question, but there might be other reasons, this is the first I've found thus far.
Since I got my '86 Mustang GT out of storage a few weeks ago, sometimes (only when in gear), the engine stutters, misfires, something. Any RPM, from barely pushing the gas to WOT. There seems to be no rhyme or reason. My gas mileage is horrible as well, I used to get about 19 in it, it was down into the 16's. So today I'm doing a total maintenance day on all the cars, and when I got to the '86, I pulled the first plug and it's cracked. Around the white part. Not sure if I did it when pulling it or if it's been like that, but I'm guessing that might be a reason for the misfire. I didn't even get to the other plugs, after I saw that, I went about the other stuff I was going to do, and I'll swap the plugs tomorrow with new ones.
I don't think I've ever changed the fuel filter, I've been through a few tanks of gas since I got it out of storage, the wires are fairly new (did them about 4 years ago, along with the plugs, and I've only put 10K miles on it since then), and I can't think of anything else off of the top of my head that might cause it. What about bad timing? I haven't touched that since we added a little bit when it was on the dyno, and that was 4 years ago as well. Any other thoughts on why it might be acting this way?