A little trouble shooting...

[Chevelle454]

This is a little off topic, but I need to get this car fixed quickly. My '84 Caprice has a 305 V8 (carb.).

I recently changed the valve cover gaskets, and when I put it all back together it runs rough. I changed plug wires and the O2 sensor, but it still is not how it was. When I first start it up, it runs perfect, like it supposed to. After about 10-15 minutes of driving, it will have no power until about half-throttle. It will cut out and backfire out the carb a little bit sometimes. I have to push the gas way down before it will go anywhere.

All vacuum lines are leak-free and connected. Emissions lines and harnesses are all intact. The only thing I can possibly think of is the ignition timing. But it wouldn't run good for the first few minutes if the ignition timing is off, would it? Or could it be the electric choke?

I need help, quick. I am getting 10mpg instead of the 20-25 like I usually get...

Thanks...
Scott