Well, conventional wisdom (and old recommendations from the radio mfg.'s) states a minimum of 12 inches, but in most modern radio installations that rarely happens. Certainly the grounding is critical as mentioned, and a resistor plug which you say you have. If you are running a 72 mhz system, you just might not be able to get away with having that throttle servo where it is so close to the ignition box and spark plug wire. A 2.4 gig radio system would probably be ok with it. Another thought, you could also try changing that throttle servo. Check the shielding on the spark plug wire, it's ground connection, and go from there.... For sure don't get frustrated and fly it unless it is perfect with no glitches on the range test at all.

AV8TOR