Lawnmower smokes like a chimney!!?

I just bought a used lawn mower. It is non-returnable. It is a 4.2 HP. I prime it & start it and it smokes really bad. Also, it is not running at it's full potential. It will run for about 10 minutes idle and about 2 minutes if you try to mow, all the while smoking like a chimney. I put fresh gas & oil in it before I started it. There is no choke or throttle on it either. Any suggestions would be wonderful!!