This thread might shed some light on it but read the full thread and or even bookmark it for now.
http://www.svrider.com/forum/showthread.php?t=95514
It's out of my my knowledge base bike as my 1999 Curvy is Old-school idle adjuster knob.
However first check that the throttle cables are OK at the carb end and also look for Air-leaks into the carbs from the air-filter.
HTH
