Every engine manufacturer recommends at least 8" distance from any ignition component, including battery, to any direct connected RC component. The modern all shielded ignitions are no exception. Also no wire or low electrical resistance pushrod that can pick up interference, and guide it to near the RC components, like pull cables or throttle control rods
This is not always possible, so either use chokes in your servo wiring, or tightly twist the servo wires. If you then with a paint stripper heat them up to the insulating sleeve melting point, the flat cables will melt together, and the twist will be permanent, thus forming a choke of some sort. This will prevent HF glitches from reaching the receiver.