My 1979 F100 Custom Runs fine but when approaching a stop sign or stop light it shuts off, this happens every now and then, it also has a problem starting and sometimes requires the gas pedal to be pressed down to the floor, it has a new carburetor on it, what could be causing this?

 

If it cuts out when you go from moving to stopping relatively quickly, it's a sign that the fuel level in the carburetor fuel bowl is too low. If you have to start it with the pedal to the floor, that means it is flooding before you start it. Over half the time this is due to operator error - by pumping the pedal too many times before starting it.

To start the vehicle cold, it only takes one pump. This is not to "prime" the carburetor as so many people seem to think - it is simply to free the fast idle linkage so that the choke can close. The pedal does not need to be pumped at all to start a warm engine. All that is required is to slightly hold the pedal in while the key is turned to open up the throat of the carburetor slightly. If doing this doesn't start your truck, there is a problem.

If you are starting the truck as I have described and still have the flooding issue, a blown power valve or needle and seat problem are likely issues. Is this a remanufactured carburetor from the parts store? Problems like this are very common with bolt-on carburetors because it's up to you as the end-user to make sure the carburetor is calibrated properly for your particular application. When you buy a rebuilt carburetor from Autozone (ick), you have no way of knowing from which vehicle and associated emissions calibration the carburetor originally came.

As a side note, quick cutting out is often associated with a failing ignition module or distributor Hall Effect sensor (often called a pickup module). This was not my first suggestion because you mentioned the problem occurs when there is a change in speed.