So I have been running into this issue lately and I have not been able to figure out what's been going on. I figured it would be best to upload a video instead of me trying to explain it. But for context as you can see in the video, The bike was running/idling just fine and revving fine as well. Put the choke down and revved it again and then all of a sudden RPMs dropped and when I tried to rev it towards the end it almost sounds delayed and struggled to rev. It's gotten to the point where the RPMs drop so much that the engine just eventually dies out. (Apologies for the bad video and audio quality, it was recorded with a fake $30 GoPro)