I hope that was for a single cylinder engine and not your stingyness refusing to tune up the engine with a full set of plugs.
That was one plug (of 4) for my two-wheeled rice burner.
I assumed it was lack of use (again) carburetion troubles, but it turned out not to be. Cleanup/rebuild last year:
Went through the same routine (no parts, though), but it didn't fix my problem. The plug sparked when hanging against the head. Dumping some fuel in the vacuum port didn't make that cylinder fire, so, tried an old standard plug, it worked, and bought a new one - Iridium tipped, not stingy.
Similar plugs in the 4-wheeled rice burning car went 130,000 miles (19 years) before I changed them (four years ago), just for good luck, during a major maintenance period.