Bike Starts revs to 5000rpm then drops slowly and stalls

[Newbiescott]

hi team

yeah so my 93 Gs500e  has.come a long way. ....but now when I start it even when cold or hot it revs to 5000 rpm and then slowly comes down after 20 seconds then idles briefly and stalls.  Then it sounds like it back fires out of the carb.

Any ideas where to begin  

thanks in advance

Scott