Usually, making sure the ignition system is well-grounded is necessary. That means that the negative wire from the battery pack to the ignition unit also needs to be tied to the crankcase of the engine, along with making sure that the engine mount, engine, and battery pack have common grounding. Making sure that everything shares a common ground, including the R/C system is necessary. Also make sure the spark plug lead is shielded and well-grounded, too.