I have a good little Kohler but recently it has started running a little strange. The machine has almost 1500 hours and was bought new in 2000. The concern is that when I run at anything less than full throttle it lopes. Another strange thing is that when the fuel tank gets to be about 1/3rd full it also lopes. By loping I mean up and down with the RPMS it still runs bet smells rich along with the up and down revving. I thought it may have been the fuel filter but I changed that and also flushed the tank and cleaned the screen on the fuel filter but still have the same problem. When the tank is full and I am running full throttle it runs like a champ. It would lead one to think that mayby there is a fuel pump issue but could it be something else? I know EFI's are sensitive to many things but I don't want to guess and be wrong. Any help is appreciated.