[urq] Spark Arches Around Distributor Causing Missfire?

[Maurits Jonkergouw]

Distribitor cap is now 4 years old and does not show visuable cracks. The HT lead in question (also 4 years old) is of the Luminition HV-3 (8 mm) silicon type and 'looks' ok. What are the usual causes for leaking spark arches?

Thanks.

Regards,
Maurits

1986 quattro GV
1982 Coupe GL DD