A few things:
1) Where is the battery wire connecting to the ignition box routed? Is it parallel to any servo wires/extensions, or metal push rods?
If there is any RF leaking from the ignition box it will be traveling down the battery lead and the battery lead will act as an antenna as well. Keep the ignition battery as close to the ignition as you can and make sure the battery lead is away from and not running parallel to any wires or control rods.
2) What type of control do you have between the throttle servo and the throttle arm. Is there any metal to metal connection from the servo arm to the throttle arm? If you have a plastic servo and throttle arm you should be OK. If the arms are metal you would need nylon clevis or ball link,

Get a package of these chokes and take the throttle servo lead or extension and at the receiver end wrap the lead around the choke three times and secure getting the choke as close to the receiver as possible.www.horizonhobby.com/products/rf-filter-ring-style-2JRPA029

Also take that tachometer off as it's only to check/test the engine. You don't leave it in to fly.