Thank you all for your input.
I have fixed the problem.
I took the carby off again & removed all the jets.I ran a thin fuse wire thru all of them & one of them was partially blocked which didn’t dislodge with washing in fresh petrol or blowing compressed air through.
The bike works perfectly now ...

Thanks for your suggestion. I mixed fuel at 1.5%
, tried it & it’s bogging down still under load. I have taken the stator off & checked the resistance . It reads 302 ohm & the manufacturer says it should be 320 ohm.allowing for some tolerance that difference shouldn’t make a difference, should it ...

I purchased a new beta 80 for my son to learn how to ride trials in May 2015 . He has ridden it 8-10 times without any troubles & a few weeks ago he went to ride in his first trial for the year & in the first section went around a tree & went to accelerate & the power was bogging down & konked out ...