With out the BK mod, the accellerator pump squirt is way too much. In other words the pump spray lenght is causing a rich condition to the point where it may not foul the plug, but causes the motor to go "lazy". So the short answer is: Too rich on the pump. That is one of the ways they keep the bike from detonating with pump fuel with 12.5:1 compression. The other way is 3-D ingnition timing. Now some may argue, but this is what I understand.