I have a 1980 F150 with a 351m and a 2 barrel carburetor on it. When I try to start it won’t start unless I hold the gas down and as soon as I start to let my foot off the pedal too much it sputters and stalls out. I’ve tried adjusting the throttle screw and the two air/fuel mixture screws on the bottom of the carb and nothing. Any ideas? Or somewhere where I may have gone wrong?