Having been in the engine business for the last 31 years I can tell you that NGK sparkplugs were not the issue. A plug or plugs may have been damaged from the installation causing a misfire in the engine. I have and still use NGK's in alot of the engines I build and will take them over the other brands out there. As far as the convertor becoming plugged by using the wrong brand plug, Still trying to come to terms with that diagnosis. LOL
