My ATV will start and idle fine, but when I increase the throttle it dies. I changed the fuel filter unit on the bottom of the tank and have the same results. Any suggestions??

 

Probably time for a carb rebuild. Please note I said rebuild not a take apart and clean! Due to its age you probably have some dried out gaskets and a thorough and complete cleaning is going to be needed. These were popular quads in their day so getting a good quality rebuild kit should be fairly easy to find. A good flush of the fuel tank might be in order if it has been sitting for awhile. While you have the carb off, check the mounting flange to be sure there are no nicks and all is smooth for a good seal, and do replace the air filter. I would bet she will run well after this.