Being off one heat range in plugs shouldn't foul them in 30 minutes.
I suspect a carburetor issue, such as a needle valve not sealing.[/QUOTE
Thought that myself at one time. Had a Vanguard on a customers MTD blowing black smoke and generally running rough. Went through the carb 3 times. and wondering why it was running so rich, and pulled the RC12YC plugs and found them black with suet and replaced them with the correct for that engine RC14YC plugs and the engine cleared up immediately and ran smooth with no black smoke, and found out after returning the mower to the customer they had replaced the plugs less then 2 hours before the issues started.