I just built a similar scooter from a box of parts someone gave me. I had the same problem. There should be 2 screw needles on the carburetor right next to each other. One is the low speed adjustment and one is the high speed adjustment. Your problem is the high speed adjustment. The needle closest to the motor is the LOW speed adjustment. So you need to adjust the one that is closer to the air filter (further from the motor). The way to do this is to put the scooter up on a block so the rear wheel can turn without it touching the ground. Start the scooter and let it idle. Once you have a screwdriver on the high speed screw, rev up the motor as far as it will go. Full open throttle. Now turn the screw a half a turn in either direction and notice how the motor sounds. If it gets way worse, turn it in the opposite direction. Eventually you will find the smoothest position of the needle. Meaning the motor is running smooth and not popping or running badly. You'll know when you get it right. IF you do all this and it still doesn't get any better then you need to get a rebuild kit for the carburetor. It might not need to be completely rebuilt, but since you have to open it to clean it you might as well put the new parts in. If you have a Walbro brand carburetor, DO NOT use an air compressor to blow stuff out of it. Use carb cleaner and an old toothbrush or whatever you can to get it clean. Just no hi pressure air. There is a one way valve in the carb that gets damaged every time if you use air. Once this happens, you will need a new carb.
If this is all beyond your mechanical abilities, take it to your local small engine shop. It shouldn't cost you more than $50 to have the carb rebuilt. Good Luck!!