Have a briggs and stratton 6.5 lawnmower that idles irratic.It revs up then down the whole time it's on.Doesnt die out just inconsistent.I took the top off and the I can see the am movng back and forth as you would do if you were reving it.Weird.I can't figure out why or what is making it do that.Any suggestions? I would really appreciate.Thanks

Have somting in carb you could try a carb cleaner in you fuel.or take the carb off and clean it.