Before trying the old carby Norm ,it's worth adjusting the fuel mixture first ,you may have tried that already.
I removed the screw just to see what position it was in and after removing the screw it went in 4 and a half turns to where it was running so just as a position to check if your one is adjusted differently, then adjust to see what position runs best.
Another way to check if your primer is supplying the right amount of fuel is to have the choke off then open the throttle while pumping the primer if it runs better when pumping the primer there is a lack of fuel when not pumping on the primer.
