A common source for a misfire is arcing under the spark plug boot. Air can ionize and become more conductive. This can cause an arc adjacent to the spark plug insulator and eventual carbon tracking that makes the progression of the failure get worse.

This misfire feels like a stumble when the throttle is opened at light engine load. Usually going beyond 50% throttle will make the misfire go away because the arc voltage requirement of the spark plug gap goes down as load increases.

The carbon track looks like dirt. However it is really carbon burned into the ceramic insulator. The first picture below shows a spark plug that was causing a misfire. The second picture shows the spark plug after the insulator was cleaned with solvent. The mark that remains is carbon burned into the ceramic material. This can't be removed by wiping with a solvent.

                

It is not necessary to replace the spark plug wires when resolving this problem. Replace the spark plugs. Clean the inside of the boot with a towel and mild solvent. Apply dielectric grease on the inside of the wire connector boot. It will displace the air and prevent the problem.