Why does my car only run when I push gas petal to the floor yet car turns off when I let off the gas?
Sounds like the idle might be messed up. Take to mechanic.
Perhaps you need to get your carburetor overhauled.
Let the pedal go slowly i bet your car is not automatic?
Need to know the make, model, and year of car to really tell
When you reinstalled the head you got the valve timing off. Go back and make sure all the timing marks are exactly correct.