problems with a stiga 50s rental?

just bought a new stiga 50 s rental.
Very disappointed after using once.
Splutters when cutting anything long even when height set to maximum.
Starts OK but very little difference in power of engine. It sounds just like a mower on idle and when the throttle is moved there is little change.
The manual suggests that the govenor and power levers should be synchronised but this makes little difference. There appears to be a lot of slack on the cable at the handle end where it seems to be rigid and bent for ? some reason.
A local lawnmower man suggest the stiga 4.5 hp engine is poorly developed and B&S would be a better engine and could be "tuned".