Sounds like she's runnin' rich, and maybe burning a little oil. Carbon fouling will make a sparkplug not fire. My dad's old John Deere "B" tractors will do this sometimes. Even if I clean the plug, it still won't fire. I tuned the tractor up, and used a sparkplug that is one heat range hotter. Helped out a lot.
Check air filter and carb adjustments, ignition timing and plug heat range.
hope this helps,
Chip
