What type of spark plugs are you running ?

I have also used NGK plugs in the past with great success, they make a good plug. However, NGK was the plug that was recommended by the manufacturer. I run bosch plugs in my motorcycle, which is also the plug recommended by the manufacturer and I run champions in my lawn equipment (2 and 4 cycle) because they are the same specs as OE and lots cheaper. Going with manufacturer specs is always the best way to get a starting point on the type of plug, shell type, heat range, projected nose... then, as you make modifications, you can make changes to the spark plug. If I were to change the manufacturer recommended brand of plug, I would make sure that it still retained most of the characteristics of the original plug.