So recently got my hands on a used 2021 Coleman AT125EX, slapped a new carb on it, new spark plug etc. should be no reason it shouldn’t run bad, but of coarse it does. Quad fires right up and idle great put it in drive and it barely takes off doesn’t rev up high and go no where it just doesn’t go, doesn’t stall nothing just has no power really, but once you get it going it rips, I don’t see it being a carb issue but I’ve done and tried everything, any input? Thank you!

 

Looks like a stretched Honda Cub clone engine. Check the throttle lever limiter on the handlebars is not wound right in. I would have thought it has to be the carb, try using the old plug first, but I would be cleaning the old carb out and re-fitting it, making sure the O rings for the manifold are in place.