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?

[68gmsee]

Are you saying that it runs rough until it warms up completely? That's what it sounds like you are saying in your post.

If that's the case it could be your choke if you have an automatic choke.
Also, I'd definitely check for moisture in the distributor, bad wires (cracked), and just do a complete tuneup to eliminate any ignition/electrical problems.

I used to have an older vehicle that did that in the mornings and found the plug wires were bad. You could see them arching at night by spraying with a fine mist of water. This car would also stop sputtering once it warmed up and all the stuff dried out.

[azolakov]

No, it runs rough after it warms up and continues until I turn off the engine for five or ten minutes. Then it's fine the rest of the tyrip.

[azolakov]

No it runs rough after it warms up. Then if I turn off the engine for five or ten minutes, it runs fine after that. It has new plug wires and a manual choke. I did a tuneup which didn't help--new points and condenser, cap and rotor.

[68gmsee]

Man, this is a bugger bear... Here's the way I understand it. (correct me if I'm wrong)

It runs smooth when you first start it. Then runs rough after it warms up.
It stays running rough after it warms up until you turn it off for 10 minutes or so. Then it runs smooth after that.

Here's some more questions.

Has it always run like this or did it start after you made some changes?
Have you converted to power brakes?
Also, is there anyway you can check your exhaust when it's running rough for signs of white or blackish smoke?

It could be a fuel or vacuum issue or even a leaking head gasket but not sure.

Next time it starts running rough, choke it slightly to see if it smooths out some. Fuel pumps are cheap so you can eliminate that quickly enough by replacing it.