Looks like a "Weedeater" brand engine. In my experience they are a very cheaply made engine that is low on power. One thing you can do to help is pry apart that muffler, and remove any and all baffling or noise suppressant materials inside. Then provide one 5/8" or larger tube outlet, or two tube outlets of about 1/2" diameter. Then try a 16 x 8 prop as mentioned. That engine will never do well with an 18" prop.

If you want to go a step farther, look into the carb throat and see what number is cast into the throat. Then we can tell you if a larger carb would help as well.

Of course, all this is assuming the engine is in good shape to begin with. I have seen engines that actually ran quite well gain 800 rpms (which is a lot of power!) when the worn piston ring was replaced....

AV8TOR