Small Engines (Lawn Mowers, etc.): lawnmower sucks, float carburetor, turn the screw

Question
my lawnmower loses power when under load. it has a float carburetor with one adjustment screw but i havent messed with it yet. is there anything i can do to figure this thing out?

Answer
Hiya Todd - Not knowing the make/model you have, I'm gonna take a stab at this.

The one adjustment you have is a high speed mixture screw.  Idle mixture is capped.  Sounds like your engine is running a bit rich, turn the screw clockwise 1/8 turn at a time and try it again.  It should run better immediately.  Keep track of your turns in case you go to far and it quits (turn it back 1/8).
Lemme know how it goes !