Another thought...you might want to pull one plug wire at a time from each cylinder. If the engine sounds different (roughness) with each plug disabled...all is good with that cylinder. Then re-plug the wire and go do it again to the next plug. If the engine sound (roughness) does not change, then that cylinder is suspect. Doesn't tell you WHAT the problem is. But helps you to isolate WHERE the problem may be.
Another check for a vacuum leak is (with the engine running) to take a propane canister torch, open the valve, and point the nozzle around the base of the carburetor, intake manifold, and other places where you might have an intake leak. If in any one place you hear the idle pick up, that is a place you have an intake leak.