Most of my carburetion experience was prior to all of the high-tech goodies (computerized crap) that was added on to cars past about the mid-70s or so. You will probably find that a bad idle will NOT necessarilly show up just by inspecting the spark plugs. Usually the engine idling time is very minor in comparisson with its road-speed operating time. That tends to burn-off any small amounts of carbon that may be acccumulating while the engine is idling too rich.
An idle that is variable may signify you could have some loose bolts that mount the carb to the intake mani. My girlfriend had a problem once, where she had a franchised car dealer shop (aaargh!) do "repair work" and they screwed it up bad. They charged her $432 for what was a tune-up, plus they replaced a slightly leaking head gasket too and messed-up the whole deal. The engine would then NOT idle properly. I'd bought a $25.50 carb kit for her 86 Escort and went to replace some carb parts & set the float height. etc. etc. As soon as I took that carb off that intake manifold I found her trouble (in addition to her having a F-O-R-D). The lousy beggars who replaced that head gasket (which could probably have been tightened in 1 hour "free") had broken the OLD gasket that the carburetor SITS on - that's on top of the intake manifold! Those jerks had RE-USED to same busted old carb. gasket which had a 1/8 inch chunk missing altogether.
Another case: 2 old biddies drive into a service station where I was working. They ask me to tell then why their old plymouth was running so bad -and why it didn't wanna start. I found the 2 nuts on the studs that hold the carb on the intake manifold so LOOSE you could see that 1-barrel Carter just a-dancing on top of the engine. It was so loose I couln't see who that old car even started. So I fixed that free for them (like a chump-idiot) and I recommended I could replace the 6 plags that were HORRIBLE just for the price of the plugs. Those dumb broads said "No" and just drove off! (Hey - is the phrase "two dumb broads" redundant?)