Take the float bowl off off the old carb. Unscrew main jet. Look at number printed on main jet. If someone didn't drill it out bigger that should tell you what jet size you have. Check the needle position as well.
"Yep sounds like it is running too rich which is not unheard of on the stock Urban. I would try a smaller main jet. Assuming you have an 85 main jet in there, pick up an 82, an 80 and a 78 from treatland.tv. Try the 82 first and report back.
And it's not uncommon for a previous owner to drill the main jet bigger in the mistaken belief that that would make it go faster. I've had that happen. Had a 78 jet in an Express SR only to determine later that someone had drilled it out to a 92 or so."(edited)