The BK mod acutally stand for Brian Kinney. He came up with the modification. The mod itself is a limiting screw on the accelorater pump of the carb to limit the pump squirt. In short, the stock pump shot is way too long, basically creating a rich condition that will temporarily cause the motor to either bog or just get lazy. By limiting the pump shot and enrichening the pilot jet about 1 size, depending on the rest of mods that may have been done. Also when doing this mod, you should adjust the pump timing of the squirt. This seems to make an even bigger difference in the "bog" that can occur when the throttle is wacked wide open all at once. Its said that the pump shot should occur just under the slide as it goes up. I adjusted my with the bike running so where it is actually I dont know. I know this, I have no bog anywhere, as a matter of fact, I had to increase the play in the throttle, because even the slightest crack of the throttle on my bike causes severe acceleration. As you can imagine, sometimes this is good, sometimes not, like crawling through a wooped out rutted corner with captain goslow in front of you and one of those little woops can give you an inadvertent throttle blip, which on my bike means contact with captain goslow. Motoman393 has a great set of pics on this mod on his site. It'll only cost ya about a buck to do, if you have decent set of tools and the proper size tap.