The K&B 61 is a good engine is a good engine with or without a pump. If you you have a pump size carb you need to run a pump as there is insufficient vacuum for full power without it, even with muffler pressure. With the pump you do not use muffler pressure. William Robinson, another moderator, can be considered an authority on these engines, especially the older ones. For the newer K&B's refer to the mfg: http://www.mecoa.com/kb/index.htm and the new mfg of the Perry carburators is: http://www.perrypumps.com/prod01.htm

Good luck and enjoy the engines.