As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
RECENTLY INSTALLED SPARK PLUGS. RETURNING HOME, NOTICED ODOR OF FUEL VAPOR, ENGINE STARTED RUNNING VERY ROUGH & WOULD ONLY DEVELOP ABOUT 2000 RPM. MADE LANDING AND FOUND SPARK PLUG BROKEN AND THAT THE PART WITH THE CABLE WAS LAYING ON THE ENGINE AND WHAT WAS LEFT WAS STILL SCREWED INTO THE CYLINDER.