We have 2 weed whackers, 1 hedge trimmer and another piece of equipment that will start right up. But will not increase in rpm when you pull the trigger. It has us stumped. 1. They didn't start doing this at the same time. 2. We changed fuel filters, checked for a clogged muffler, took carbs apart and cleaned, put a new carb on one, new plugs, sealed intake gaskets, checked for a carbon build up in motor. Age ranges from 3 months old to several years old. We maintain our equipment VERY WELL and use synthetic 2 stroke oil.