Yes I was out on a ride. I went to overtake a car and there was a slight hesitation before the bike picked up and from there it wouldn't rev. It is ok at lower revs but as soon as you try to push it on a bit it starts loosing power almost as if it has carb icing or is out of fuel. Haven't tried the spark plug test, but we put new plugs in when we stripped the carbs. When it is on the stand in the shop it runs up through the revs with no hesitation.
Sorry for the long delay, been a hectic few days.
A compression test was done, and as far as I am aware everything was okay.
The bike will run pretty well without the air box fitted but as soon as you put it back on it won't run properly it just won't seem to rev past 6500-7000 rpm.
No mods have been done they are the same carbs that have been in the bike for the last eight years.
All that has been done is the diaphragms, sliders and needles have been replaced.
Yes the problem started whilst out on a ride.
I will check to see if what you say about the spark plugs is correct.