Solving ignition noise in the radio requires detective skills. Here are
some general tips:

1. Sometimes resistance is used in the high voltage circuit to reduce the
radio interference. This may be accomplished by using special resistor
plugs, or special suppressor type high-tension cable or resistor
suppressors. Remember, however, that reliability is paramount in the
aircraft engine. Adoption of the factory specified parts is recommended.
Make sure that any resistance parts have not been replaced with
non-resistance parts.

2. Determine how the interference is arriving at your radio. Unplug the
antenna at the transceiver. If the noise goes away, then the interference
is arriving through the antenna input. Now, unscrew the antenna at the
antenna end, so the cable is still there, but no antenna. If the
interference goes away, then it is arriving through the antenna itself. If
you can still hear interference, then the shield on the antenna may not be
fastened correctly and should be addressed.

3. The "ignition" noise may be from the high voltage spark or it could be
from the alternator. Alternators "whine", while ignition noise "ticks".

4. If you pick up ignition noise with the antenna disconnected, then the
interference is coming into the radio through the speaker leads or the power
leads and those must be filtered.

5. You can gain a bit by moving the antenna far away from the source of
interference.

6. Some radios are more subject to radio interference than others. Pulse
type interference can be reduced by using a noise limiter or silencer.

When everything else fails - turn the engine off and enjoy the silence of
gliding.

Colin