whats the difficulty with running my powerdevil petrol mower?

i can get the engine to start,but it 'rolls' up and down the revs,regardless of bar being held,if i change throttle from start to fast or slow rev,it either ticks down to a cut out or doesnt move at all!! i have cleaned the carb out,float is fine,and it is fine to run upto high rev-im just pulling my hair out to mend it! is this big problem or am i overlooking something?