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I am working on a 2005 Daelim 250 Scooter which has Kymco engine and parts and electronics. The scooter sat up for five years. I drained the gas tank and put in new fuel. I removed the carb and cleaned it thoroughly. Still, the scooter has poor acceleration. I have since replaced the drive belt and installed new rollers. The Throttle Positioning Sensor reads correctly. The air cleaner is clean. The exhaust has no clogs, but still, poor acceleration. The scooter does not get over 45 mph. When the scooter is on the center stand, you can hear that the engine does not want to rev correctly. Does anyone have ANY ideas what the issue could be?

The engine starts right up and idles perfectly but just has poor acceleration. I'm at my wits end.