Trying to SMOKE
If possible more separation would be beneficial, I would try for 12" or more. But shoot for as much separation as you can get. It may not be the problem though if the pump is not running when you have gotten the hits. Under that case you would want to check for outside interference on your frequency, or model generated RF, be it the ignition system, servo leads, etc. Be sure to do a thorough range test before flying.