Gilera 633721 Workshop Manual page 258

Injection
YES go to 17 NO go to 1 9
17 - Check the secondary resistance.
Measure the resistance between one of the primary terminals and the spark plug cable output
Electric characteristic
Primary HV cable output =
3.1 K ± 9%
YES go to 18 NO go to 19
18 - The coil is conforming.
19 - Replace the coil
Inspecting the spark plug shielded cap
Measure the shielded cap resistance.
Electric characteristic
Resistance:
5 KΩ
INJEC - 258
Nexus 500 euro 3