KLX 250 Carb issue

[ShadyRascal]

My buddy has an 06 KLX 250S with a carb problem. When you rev it up it holds rpm and drops back down very slowly. I beleive there is a problem in the vacuum system to the carb.

The carb has dyno jets set at the suggested specs, hole in the airbox lid, and stock pipe. The jet set is installed properly.

The vacuum line begins at the front of the cylinder head, comes back to a valve of some sort, and from the valve which is located at top of the head, a small vacuum line to the carb and a large one to the airbox. I think the problem is in there somewhere as this is a CV carb and the diaphragm isn't dropping back down as it should. (Yes the spring is in there too!)

When you pull the vacuum line off the airbox and plug it, it appears to slightly improve the situation. At idle you feel (that airbox line) pulsing/blowing a slight amount of air, and when revved it pulls vacuum.

Any ideas here? I've done these CV carbs on my atv's and bikes many times and haven't had one stump me like this. Gotta be a vacuum issue but I don't know anything about the circuit on this bike.

Thanks for any opinions,
SR