Mountfield HP470 Lawnmower > Rev/Throttle issues?

Have Mountfield HP470 lawnmower - with new spark plug & diaphram, plus cleaned/serviced carb & fuel tank. However, it's difficult to start & seems to not rev high-enough, which means it can readily stall. Any suggestions? And any way to make it rev higher - can't seem to adjust the throttle cable? Rgds, Mark