I went over to the airfield and took the exhaust off the plane so I could look up the ports. On the front cylinder, I saw this. Click to embiggen, and you'll see there's a nick on the cylinder wall (where you can see the honing marks, FFS!) with a little bit of dead metal above it. I'm thinking that's my problem right there.
Oh, I also discovered that the brake lights aren't working on the van. It never rains, does it?