My 99 24v has always been touchy in terms of gassing up, I can't run it at full volume unlike my others, 94 and 93... been that way since we bought it new, I'm pretty sure that along the way the design of this system, starting with earlier models, has been a source of refueling issues.. as far as venting, tanks don't vent directly to the atmosphere anymore, I think there's some sort of vapor recovery built into the filler pipe, supposed to prevent vapors from escaping as you fuel up.. if you're experiencing a major change in operation, possibly something has broken internally or failed??? I'll have to check my manual for any info, maybe another on here can help out in the meantime.