a resistor plug is *usually* enough by itself with no other shielding but using a non-resistor plug with a resistor added in before it like is done with the CM-6 isn't the same. i assume it has something to do with the long unshielded path between the resistor and ground as compared to a real resistor plug. since the little zenoah 20 uses the CM-6 it needs the metal cap. i don't know if the larger EI zenoahs use the same plug or the 14mm plugs but they may have also just used the metal caps as added insurance or maybe even found that they had too much RFI and actually needed it. the magneto zenoahs with the little separate coils are also a true CDI system but seem to get by just fine on the resistor plug alone.... having played with them a bit i believe they are putting out a lot less voltage than most of the battery powered CDI ignitions. never measured it but i think it's probably around 10kV instead of the normal 15-20kV