Sparking on 1 cylinder only

[Hawthorn 7]

Hello all, I bought a 1979 Z1000 a few weeks ago. When I collected the bike all was fine, today being nice I thought I’d use it for work. Fired straight away but only firing on 1 cylinder. Quick check round and there is no spark on cylinder 1,3 and 4 and a poor spark on cylinder 2. The previous owner has fitted a aftermarket electronic ignition.
I will take to a local motorcycle mechanic but thought I’d get some feedback.

[zed1015]

Most likely fouled plugs or damp/shorting leads etc.
Weak spark on 2 indicates that the dual pole coil should be ok to spark 3 so i'd try all the plugs in 2 cap first to see they spark there.
If not it's the plugs , if they do then look at the leads and coil connections.

[Garry.L]

Stored in a Shed or detached garage?..

It's been VERY cold in recent weeks meaning your bike would have got very cold. In the last few days it's warmed up a lot, so nice warm(er) air hitting your very cold bike causes it to condense making everything you can't see nice and damp.