I have a lawn mower and it runs ok, but the engine revs up and down.?

it sounds like your having a fuel to air ratio mixture problem... and ALL carbs can be adjusted just need to look closer.............. after you find the adjusting screw turn it all the way to the right to close the air off then turn it to the left 1 and 1/2 turns to the left then start the motor then you can adjust it right or left to stabilize the motor as needed.note that you remove all gas from it first and DO not run any premium fuel in it..............use just normal 87 octane in it this is the way it was designed..and dont run any stabilizer in it till youre ready to winterize it....but put new fuel in it and go for it.....good luck