I have a 01 426 that has a slight stumble when the throttle is barely cracked. I have tried jetting with and without the bk mod and have not been able to cure it. It was a very slight stumble and I could live with it. I installed my new TA pipe and it made this problem more pronounced, eventhough the bike runs better everywhere else in the powerband. I think with the louder tone of the TA, you can hear the stumble better. I can whack the throttle open from an idle and the bike will almost never die. It pulls hard and runs great. The problem is when the throttle is barely open. (maybe a 1/4" of movement) I notice this when creeping very slow in 1st gear or when I am breaking into a turn and slighlty blipping the throttle. You can hear the stumble and it sounds as if it is going to die at times. But as I exit a turn and roll on the throttle, it runs great. I guess my question is what should I look to? Is this a pilot jet problem because the problem is only during very slight throttle openings? Has anyone else dealt with this?? I do give the TA pipe a thumbs up. It makes the bike rip, and has an awesome sound.