I have a 79 Ford F100 with a 300 Straight six and it will not Idle or even stay reunning when I let off. It seemed to be running rich so I installed a new carb. After installing a new carb it ran fine for about 5 miles then it started acting up again. It runs great when I have my foot on the gas. I think it may be the catalytic converter. I know it is in tough shape, but could this be causing the problem? I would really appreciate any help.

I believe your problem is ignition system related. Hopefully, the suggestions below will help.

1. Test/replace the Ignition Control Module (focus on this first):

http://www.partsamerica.com/ProductList.aspx?PartType=194&PTSet=A&SearchFor=Ignition%20Module%2FControl%20Unit

2. Test/replace coil:

http://www.partsamerica.com/ProductList.aspx?PartType=190&PTSet=A&SearchFor=Coil

3. Replace distributor cap, rotor, spark plugs (Motorcraft), and plug wires.

4. Set idle speed and timing.