K&B's have a history of mismatched carbs.

The 40RC I had came with a .61 carb with a smaller barrel hole. The throttle response was poor and the midrange mixture was always too lean. Apparently some of the .61's came with carbs designed for smaller engines.

K&B has some good carbs and some bad ones and some were mismatched. But they have had enough bad ones that it seems to have become common practice to put a Perry carb on if the engine does not seem right.

Perry carbs are good carbs. They can be adjusted easier than some of the K&Bs. Also, if the Perry Pump is used, a a Perry carb may be required.

Another thing if a carb is damaged on an engine, it is usually significantly cheaper to get a perry carb. On my 25FSR, the OS carb is $57, the perry is $35. You can guess which carb I will replace it with.