Why do my ejewei cfmoto scooter slows down after running about 30 min.

Is this a sound the car doesn't normally make, or are you asking because you want to make the car louder when you press the accelerator? Does the engine seem to idle and run fine?

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Sounds like the throttle is connected backwards. The parking brake maybe hanging up. if the scooter is run for 10 mins forward, then carefully check with your hand that the parking brake is warm and not hot. Hot indicates brake slowing you down and making the motor work too hard. For other answers to Scooter Problems goto my free app.
Scooter Doctor App

I’m happy to assist further over the phone at https://www.6ya.com/expert/bill_bcbb89783b08c489

you have bad valves seats probably broken loose do to overheating replace short block or by new motor short block cost to much 300.00 new motor 225.00 to 245.00

check wheel bearing...transmission output shaft bearing. noise is louder because when bearing slow down, they get louder from worn down ball bearing inside the cage, space is developing between the ball bearing..caused by lack of lubricate, or just worn out bearings

Hi bwp1760! Have a look at this site: https://www.motorcyclemanuals.info/motocycles-atvs/cfmoto/
There are many of manuals for CFMoto motorcycles for free download.

Sounds like you have weak batteries or a bad brake. You can measure the brake resistance at the connector between the front and rear halves of the scooter. The brake is good if the resistance measures between 30 to 65 ohms.
You should load test the batteries to see if they are good. It takes battery power to run the motor and release the brakes. Weak batteries will cause the brakes to drag and the scooter to run slow.
A new brake costs about $70 and new batteries start at $40 each.
If you do not have a beep fault code, nothing has burnt out yet. I would have the scooter checked out soon by a qualified technician. Hope this helps.

Ohm chech the brake. It should be around 35 ohms.