Hello all, my uncle picked up an atv from someone. We got it up and running but when taking off and while driving, you will hear some clunking noise, and some lag in the throttle, anyone have an idea of what it could be?

We have no idea what kind of atv it is, there is no markings or numbers anywhere on it. I have attached some pictures if you could help me in identifying it. I think its this one Import wholesale China ATVs from Chinese Quad suppliers but when i search that model number I cant find that quad anywhere but that website.