G'day Keith,
If you have auto adv,/ret., have a look at the springs, make sure everything looks OK.
Also, could be starving for fuel, have both petcocks on, make sure petrol tank cap
breather is clear, then if still no good, I would dismantle your carb, & give a good
blow out & clean.
Just try pressing the primer button on the carb down while its running, & see if it's
reving better!
Also, how do your plugs look?
regards, Cus