2003 Mitsubishi Challenger Morning start up

First start up in morning or when vehicle has been sitting unused for more than a day, cranking over ok but takes several splutters before ignition fires fully, small backfires during startup. I have added injection cleaner a few weeks ago assuming that was problem and also Mitsubishi service centre has replaced platinum spark plugs. Petrol good. Could problem be petrol starvation. Although after starting engine runs excellently and restarting does not show problems.
Please advise