Did this just start or has it always been like this?
Just started... Something needing the choke to run is a fuel/air mixture problem. Pulling the choke increases the fuel and decreases the air in the ratio. It may be a clogged jet, damaged gasket/diaphram, air leak, etc. clean it making sure the jets and venturies are clear. Check the pop off (needs a special tester). Spray carb cleaner around the intake gaskets. If all else fails, buy a carb rebuild kit, replace the intake gaskets and check the reeds.
Ran bad since you bought it... any of the above my be the problem, however, you may simply need to tune it. It may be way lean on the bottom end, or something of the sort, but that's unlikely if it's running well on the top end. The bottom end effects the top end, tho they could have tune the bottom end lean and the top rich... again, unlikely.
Also, if it runs fine when it starts and then later it wont run right, it could be lean and over heating.
There's lots of causes. It's hard to say. There's a few things to check.
