Hi everyone

So I bought a 1980 rx7 it ran good no issues really I wanted to change up some parts so I installed a new manifold gasket, new radiator, a new oil cooler, new headers, new spark plugs and wires and coil blasters I had it sitting for two months and did not start it just installed these parts today now when I start the car it runs high rpms and comes down and cuts off in a matter of about 15 secs by the way don't have a exhaust pipe just headers right now. Does anyone know what can the problem be?