Hi Lozza,
Nothing really wrong with the Keihin, although I suspect its worn.
I have Mikunis on all my other bikes and understand them well and have heaps of jets etc, so its more simple for me to have a Mikuni...
And if they can make a Bultaco run and start reliably - then I want a Mikuni!!
Its a 38mm size Mikuni.
Cheers,
Mark.