Still Got Problems

[NewbRider15]

I have a 1986 Kawasaki ZL400 Eliminator (33bhp restricted).
I have recently put in new spark plugs and now the engin starts up and ticks over fine. The problem is, is that on longer journeys, or just randomly, one of the cylinders will stop firing and I will be running on three cylinders.
Occasionally it will drop to two for a short period. Then it will stay that way.
Stopping it and starting it again doesnt do anything. It will run on all four for a while and then lose one. I can tell by the noise and the serious drop in power.
After leaving it overnight it will run fine again in the morning but as soon as its traveled a while it will do it again.
I dont get what is causing this.
Can anyone help?
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Now on - 1986 Kawasaki ZL400 Eliminator (33bhp restricted)
Own - 2009 Huatian HT50QT-25 Scooter

[G]

An easy first diagnostic is to keep a spark plug with you.
When it goes down to 2, whip each of the HT leads off and check each one in turn to see if the plug is firing.

Probably a coil out if that's the case. Could even just be some dodgy HT leads or something (heat getting to them somehow, maybe).

[NewbRider15]

Cool thanks, I'll give them a check and reply when i've done that.
____________________
Now on - 1986 Kawasaki ZL400 Eliminator (33bhp restricted)
Own - 2009 Huatian HT50QT-25 Scooter

[NewbRider15]

We took the bike to a garage and they tried all sorts but it turns out its not a happy bike when its restricted. so... yeah. Figuring out what to do next.
____________________
Now on - 1986 Kawasaki ZL400 Eliminator (33bhp restricted)
Own - 2009 Huatian HT50QT-25 Scooter