The Royal Enfield Bullet motorcycles manufactured before 1994 were equipped with what is commonly referred to as the "iron barrel" engines. These engines had a heavy crankshaft, which contributed to the distinctive thumping sound and feel that is often associated with the Bullet.
The heavy crankshaft design was known for its durability and robustness. It provided a unique riding experience, characterized by strong low-end torque and a slower-revving nature. Many enthusiasts appreciate the thumping sensation and the classic feel that the heavy crankshaft offers.
This change was aimed at improving the engine's refinement, reducing vibrations, and increasing overall performance.
So, if you have a 1991 Bullet, it is likely to have the heavy crankshaft design. This is one aspect that contributes to the classic Bullet experience. If you prefer a more refined and smoother ride, you might consider newer Bullet models with the UCE engines.