Kuma recommends that an insulated chimney liner be installed into a masonry chimney for good draft performance. This liner should meet the high temperature requirements (2100°) per the UL 1777 standard for chimney liners.
If you cannot fit an insulated liner or do not wish to spend the money on an insulated liner, you need to make sure that there are no code/clearance violations, cracks, or damaged mortar joints. Please read this excellent article by Jim Hessenauer for more detailed information on this topic: https://www.cheapchimney.com/blogs/diy/do-i-need-to-insulate-my-chimney-liner