I just bought a 69 mach1 and it ran pretty good. Well all it did was sit in my driveway and now when I start it, it wont idleor stay running even on the gas. Ill keep it at like 3000rpm and then it just dies like someone turned it off.
It only sat for a week but the gas was very low and i thought it wasnt getting enough fuel cause its also on a slight slope so I put 5 gallons in it and still does the same thing. It has a moroso fuel pump.
when you get your gas tank way low, it starts sucking up all the rust, and the other crud thats accumulated over the years at the bottom of your tank.
change or clean your fuel filter, some may have found its way to the carb too.
There is also a filter in the tank. I would look at changeing that one before you change any filter you put on. I would imagine that one would clog first.