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You might try paralleling the electrolytic cap with a lower valued (say, .1 uf) non-electrolytic capacitor. While the large value of the electrolytic will smooth out large variations in the power at lower frequencies, electrolytics sometimes have poor high frequency response. A non-electrolytic capacitor often works better at higher frequencies.
This is a result of the very way in which electrolytics are fabricated. A .1 uf non-electrolytic is cheap to try, and causes no harm, even if it doesn't happen to solve your problem.
Levent, I used the 22mF from Mouser and that worked well. And I installed ferrite cores on just about everything coming through the firewall, including throttle cables, etc. The radio is now crystal clear. Mouser also sells ferrite cores. I also use ferrite cores on the headset jacks. I believe this is also recommended by Xcom, could be a Jabiru recommendation. Regardless, this is a simple solution, and worth the extra money.
Another issue was the crimps on the alternator wire connectors, the crimping tool crushed the protective coating and they basically fused together, no damage to the alternator. Now I make sure I wrap crimps with electrical tape or heat shrink, just in case.