1995 GMC Jimmy sometimes slow to start

I have owned a 1995 gmc jimmy 2dr 4wd with the 4.3 motor and 96000 miles on it for a year. Since I have owned it there are times when you have to crank the engine for 6 or 7 seconds before it fires. Then there is a puff of black or grey smoke and it runs rough for a second and then runs fine. There are times when the starts are normal. If you stop at a store and restart after a few minutes it is fine. Colder mornings are better. I realize these vehicles are prone to fuel delivery issues but I would like to narrow down my search. Aside from this the vehicle runs perfectly. I have changed the fuel filter checked the spark plugs and installed new ground and positive cables to a new battery.