rough running

[azolakov]

My 1969 C/10 runs smoothly until it warms up for a couple of miles. Then it's still okay until I stop, like at a light, when I begin to hear it popping from the exhaust. Then when I start off again, it runs very rough with the popping sounds and poor acceleration until it gets up to speed again, like thirty mph or so. This continues until I stop and turn off the engine for a few minutes. Then, like magic, the problem disappears, and the truck runs smoothly again, idles contently at stop signs and lights, and gets me where I'm going without threatening to quit any more. I have a 250 engine with a Rochester carb. What can cause this behavior?