On the 1200's there is a vent valve located on the firwall, if this valve does not vent properly like it is sopposed to then the fuel tank cannot vent . It will run for for just a bit ,then die . To test this theory drive it till it dies, open the hood then undo the inlet side of the fuel filter , if it's not venting you will hear the suction when you pull off the fuel line .That means the valve is not operating properly.