Hi everyone, I just purchased a second hand 250cc farm quad ( atomik) when I bought it yesterday it sounded sweet and no rattles. I changed the oil over today ( the old stuff was pretty thick and black). and as soon as I started it it was making all sorts of rattles and knocks that appear to be comming from the top end. Any ideas? what could of happened just by changing over the oil? Any diagnostic check I can do?

I checked the plugs and they were pretty black and sooty but cleaned them up and seem to be fine. The bike seems to splutter a bit now too...

BTW although unrelated the oil in the diff was like molasas so i changed that over with bike gear oil, it that ok?

Cheers