chevy4ever

Hey all, i have a 2000 suburban that when i turn it off and try to restart it after about 30 minutes it is real hard to start. If it sits for several hours it starts fine. Same as when i start it for the first time in the morning all is fine, just after it warms up then sits for anywhere from 30 minutes to 2 hours it is really hard to re start. Also there is black {rich} soot after the exhaust pipe on the vehicle. obviously it is burning rich. no codes are set, runs great once it starts.
any ideas on what to check would be great.
thanks.