Sorry mabye I do not listen well. But in my own defense the jeep I own is practically new even though it is a 1985 cj-7. What I mean by that is that no one has moved vacum lines around under the hood. Everything is the way it came from the factory. Except for new (rebuilt carb) air tubes spark plugs and the like. So I do not know how the line could have gotten changed.
The way I understand the evaporation canister, is that when the engine is started and warmed up, ported vacum at the purge signal cap opens a valve and the trapped gas vapor is pulled into the carb by way of the purge line and burned. prior to this, cansiter purging should not take place at idle or when the engine is cold. The cansister causes trouble when the purge diaphragm under the cap is leaking, which forces the engine lean, resulting in a rough or unsteady idle. That is my problem and I need to know if it is possible to bypass the canister or elimante it all together. please forgive my ignorance.
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Sounds like the vacuum switch on top the canister might be bad.

I don't have the vac. diagram for your jeep, but there should be 3 lines on that. Purge, Purge Signal, And tank vent. The Purge line is the line that sucks on the vacuum canister. The Purge signal line should be the vacuum line from ported vacuum. The tank line should be coming from the fuel tank venting the tanks extra pressure into the charcoal canister. There should be a constant vacuum source on the purge line and vacuum on the purge signal line only when the jeep is warm and running at part throttle.

Check to make sure that the purge signal line is working right, if you do truly have vacuum only warm and off idle, then you need to replace the vacuum switching device on the top of the canister, or get a new canister (whichever is available). If you do have vacuum at the purge signal line at all times, look more into lines themselves and the carburetor.

Greg start the jeep and then pull the lines off of the carb to see if they have vac at idle. When you find the off-idle one it will not have vac in it. See if it can be followed to the cannister. I am pretty certain that hose is where the cannister vacuates to.
The cannister does keep some raw fumes out of the atmosphere but I DOUBT a sniffer can tell if you have plugged the vac lines to it..
Big Jim <img src="/forums/images/graemlins/pfft.gif" alt="" />

I've got a can of peanuts here if you care to empty them and use it for testing if you desire.... Got some empty coffee cans if you want to give them a try....

BJ ported vacuum first goes to a coolant temp switch so that it is not trying to pull EGR/evap in when it's cold.

The best way is to work backwards or just test it once it is warm.