rough idle when tank is full?

[DRobb's_85]

My 85 k10 has a strange problem. Every time I fill the tank up, it will run rough on idle until I drive it down to 3/4 tank, where it then smooths out. It runs great from then on, but that first 1/4 tank of fuel causes it to run rough at idle speeds only, for some reason? It has a new 350/edelbrock carb in it with 650 miles on them, the fuel pump is new, and I am running a fuel filter. This just stumps me. Any idea of what it could be? For now I guess I'll just fill it 3/4 full until I can get this figured out.

[f.monroe]

Maybe the tank isnt venting correctly and it is pulling a vacuum on the tank . Fill it up , drive it a little until it starts running odd , then pull over and open the gas cap and see if there is a difference . Just a thought .
Frank

[DRobb's_85]

Ok I filled it up today and it looks like it is a tank venting problem. It started running rough within a minute of leaving the gas station so I pulled over and started loosening the gas cap and there was a ton of pressure being released. I ran it for a minute with the gas cap off and it started smoothing out. The gas cap is a venting cap with the little button on it. It cant be the cap as it is fairly new and it ran fine for a few hundred miles so what could it be? Thanks.

[rfmaster]

DRobb's
Check your charcoal canister - and all vacuum tubing connected to it. Make sure that vacuum hoses are not pinched off.
Remove charcoal canister (usually mounted on driver side next to radiator) and replace small filter - bottom of the can. On top of canister you should see vacuum hose taps - make sure that they are connected to correct location (one should go to gas tank, another to carburetor vent - check edelbrock documentation).

//RF

[James McClure]

Be real careful about those charcoal cannisters. Sometimes the diaphram on top goess bad and it will literly suck gas right out of the tank and seeing as the tank vents thru the cannister and your tank is building pressure it's a good bet the cannister is toast IF, IF the gas cap is truely good, BUT, it sounds like the cap is not venting properly because it runs fine with the cap off. I think you may have answered your own question. jim

[DRobb's_85]

Its not the cap..I replaced it today and the problem is still here. I tried running it without it and it still runs rough. I guess I just imagined it running better without it the first time. So now the charcoal cannister, I gave it it a once over and everything is hooked up and tight but I will look into it some more tomorrow.