800 RPM drop while doing ignition check Rotax 582

When I did my ignition check prior to takeoff I got a 800RPM drop when I switched the rear ignition switch off. The front only dropped about 175 RPM's. Changed plugs and plug caps but same problem still exists. any idea as to what it may be. Usually when CDI boxes go out they just quite working all together right? Ignition check was done at 3200 RPM.

It might be wires. When you replace the caps you're supposed to trim 1/8" off the end of the wire then screw the cap onto the newly trimmed wire.