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?)
Well, after 333,000 good miles my 1987 E16S Sentra has developed a
> very unstable idle. It started out as occasional stalling during warm-
> up. It got worse and began a rolling idle (rpm up and down) when cold
> and during warm up with more frequent stalling during warm up. Now
> there is always a rolling idle whether cold, warming up or hot. Once
> warmed up it will sometimes idle at very high rpm and sometimes it
> will not idle at all but flat out stall as soon as the clutch pedal is
> depressed. The carb primary and secondary are fine as once you get
> past idle it runs great. Also, it is an idle speed problem and not an
> idle mixture problem as there is no black smoke and the plugs look > fine. >
> There is an idle speed control actuator which operates on vacuum
> providee by a vacuum control modulator (VCM). Due to the rolling idle
> I suspected the VCM, so I tested it per the factory service manual and
> it checks out OK. The voltage (12v) and the vacuum (2" Hg to 3" Hg)
> are within specs. There are two small air filters that filter outside
> air to the VCM and the idle speed control actuator and they are not
> clogged. So I'm now thinking that the idle speed control actuator
> itself is going bad.
>
> Has anybody had any experience repairing/replacing this part? Can it
> be replaced without removing the carb from the manifold? Anybody know
> of where one might be able to buy one short of a junk yard (likely to
> be in worse shape than mine). I'll appreciate any advice. Thanks in > advance. Al >