The cap is vented thru the carbon cannister in the rear of the scoot. The vent is switched on and off by a valve, left side under the footboard, out board of the frame rail (whew!). No vent if this valve sticks. I removed this valve when I had the Yager and coupled the line directly to the cannister. I also capped the vacuum source on the intake manifold. Got rid of the problem.
Karl