Rough, unsteady idle problems are all too common with the Carter BBD on the 258 CID engine. This can be caused by several different things, but the most common is a too-rich or too-lean fuel mixture. You can get a clue to which it might be by removing the lid of the air cleaner and looking down at the metering pins . If they are all the way forward, the mixture is too lean. If they are all the way back, it's too rich. This assumes of course that all the other systems are working. If it's too lean, check for air leaks around the carburetor base and the intake manifold gasket. Check all vacuum hoses for cracks, breaks, and leaks. A bunch of little leaks add up to one big one. If the hoses appear cracked and aged on an older engine, it would be worth your while to go to the auto parts store and buy new hose and just replace them all. It's inexpensive and not hard to do. Check the Evaporation Canister for correct operation. It should not be purging at idle. For an easy solution to a common Carter carburetor problem, check out Terry L. Howe's excellent page at: