Hi guys,
We have a '99, and once in a while, with no real indication as to why, it will begin dying any time the clutch is pulled in, for downshifts, stopping, etc.  While it is doing this, the idle also drops to a lower rpm than usual.  It will do this for a while, then out of the blue idle will increase back to normal, and the dying stops.  The only other recent issues were fuel flowing out the carb vent hose, but this was fixed with a new fuel bowl needle/valve assembly.  To the best of my knowledge, the carb jetting is stock.  I recently put a Screamin' Eagle muffler on, but this problem existed before this mod, and has not changed since the new muff.  It's really frustrating, as it only happens once in a while, lasts for several minutes, then goes away.  The bike will usually re-start, idle ok, run ok during acceleration, then die again when clutch is pulled in.  Then, a few minutes later, it goes back to normal.

Thanks, but it's a brand new petcock, so that's not the problem.  I had bought it a while back thinking it might be the solution to a different problem, and then thought some issues with the vacuum diaphragm might be causing this problem, but apparently that wasn't it as the problem continued after installing the new one.  I revisited the idle mixture screw setting, and adjusted the idle speed accordingly, and so far so good.  Not sure why it would only do it sometimes, but we took it out for quite a while after the latest adjustments and so far no problems.  Time will tell.

check the coil, & connection between the wire & the cap.

and what verslagen said

Verslagen,
We had the new petcock already, so I installed it to remove that as a possibility.  The vacuum line itself shows no adverse effect with the "spray test", so no apparent leak there.  I had wondered if maybe the vacuum diaphragm inside the petcock was the weak link, but obviously it's not that either.  So far, readjusting the idle mixture and idle speed have taken care of the problem.  I'm gonna keep the spray with us on rides, and double check for vacuum leaks while it's actually happening, if it happens again, just in case I'm missing someething.  So far, so good though.