Not knowing what you have for a engine I would say just adjust the idle screw. Also are you sure when you mow long damp grass that its not normal for the engine to bog down??
Cut the grass more often, set the cut height higher when the grass is long and don't mow wet grass.
There are limits to what a small engine can do on a rotary mower.
If you can not manage the above go buy a drum mower
