If the engine has had a lot of running, it 's possible it's starting to wear out. That means that the piston/cylinder fit is starting to become too loose. That will cause reduced RPM at full throttle, and an idle that is not as good. A worn engine will have little range for the needle from too lean to very rich. That's because you need a certain amount of fuel to make up for the loose fit...the oil in the fuel helps with the compression seal. A new piston/liner assembly should take care of things.