UPDATE TIME AGAIN!!

Finally got time over weekend (damn kids) to have a good look at the FZR.

Drained fuel system, cleaned carbs, filters, jets, replaced fuel lines, checked flow to carbs, checked plugs, adjusted idle screw etc. Had also disconnected all electrics in hope that things would reset (if you know what i mean).

End result is that bike starts easily, idles smoothly, revs fine through the range - until you go for a run.

So have kinda decided that fault must be electrical - gonna invest in a new voltage reg (last one was used from fleabay).

Haven't priced a genuine one yet, but plenty of non-genuine on t'internet for £60 region.

Will keep you updated.

could it be possible (that im an idiot) that the cat converter is clogged?

just a dumb thought. I dont know if cycles even have them.

As far as i'm aware (though i have been wrong before) there is no cat.

Thanks for trying to help tho!

As far as i'm aware (though i have been wrong before) there is no cat.

Thanks for trying to help tho!

sticking valve(s)? carboned up? try a compression check as from what your saying it's mainly under load that it's failing.

When you have been checking all these wires did you check the primary coils and the cdi trigger, could be that the coils are breaking down and the heat absorbed from sitting while you filled with fuel screwed with the resistances.

It would give a weak or erratic spark.