Metal-clad spark plug caps.

[forchetto]

Those amongst us that were bikers in the 70's when men were men and bikes leaked oil all over the place, should remember the troubles caused by the metal-clad plug caps used on Hondas and the like. After a few thousand miles any moisture or light rain used to cause cylinders cutting out due to internal shorting.
Wise dealers changed them even before delivery for all-plastic ones to avoid trouble.

I thought I'd seen the last of the b******s, but they're back on Chinese bikes folks...
There's no need to change them, the metal cladding is fitted with tiny bent-over tabs and can be removed, leaving a normal cap behind.
They were fitted to screen interference from the plug and stop radio and TV interference, but nowadays with FM, UHF and digital transmissions this does not happen.
If you get misfires in the wet, remove the metal cladding or change to an all-plastic cap.