Spark and Burn
Spark plug tuning is one of the most under appreciated or miss understood sciences in motor sport, especially kart racing. The only certain measure of performance you can use to determine a spark plug's worth is how efficiently it burns in YOUR engine's combustion chamber. I put special emphasis on YOUR chamber because I learned two facts in racing: No engine is the same and everyone has an opinion on what works best. Save yourself a headache. Choose a plug that is the best application for your performance package and current environment. Always keep a pack of hot weather and cold weather plugs for those rainy days at the track.
Test and Tune:
Avoid getting wrapped into the cycle of swapping carburetors and mixing gas to boost RPM's without indexing your plugs first. Indexing plugs have a dramatic effect on performance. Before hitting the track, I test each plug to determine which offers the highest gain in RPM by revving the engine while the kart is on the stand. It does not matter if you have the same model plug or different brands, each plug will perform differently. While testing, you may notice a 200 to 600 RPM difference when swapping plug with the same gap and number of index rings. Be sure to mark the plug to record its orientation in regards to the carburetor after finding its optimum position.
The best burning plugs leave a even burn pattern from a flame that spans east and west of the center electrode. The mission is to burn as much fuel as possible each time the plug sparks. The black residue left behind is carbon buildup. Carbon buildup on the plug is used to measure the distance of flame. The plugs shown on this page are an excellent example. The plug on the left did not provide a even burn pattern, so less fuel was burned in the chamber. The plug on the right offered the best burn, providing 500+ RPM's more than the plug on the left.