Easy Fix for AMC 258 Carter BBD Idle
Problem
by Terry Howe
The Jeep 258 (4.2L) I6 isn't a power house, but it has gobs of low RPM
torque that makes it a great engine off-road. The most common problem I see
with the engine is it's inability to idle. Between the 258 in my '81 CJ-7
and a couple friends that have 258 equipped Wranglers, I have fixed this
problem half a dozen times and the cause of the problem has always been the
same. The problem has always been caused by clogged idle tubes that cause
fuel to drip out of the venturis and make the Jeep run rich at idle. Fixing
the problem is relatively easy and once you get past this problem, you will
get much more enjoyment from the 258 and the Carter BBD carburetor.
The Carter BBD is a two barrel carburetor that was available on late 70s to
late 80s Jeeps with the 4.2L engine. There are two variations of the
carburetor, one is computer controlled and has a stepper motor on the back
side. The computer controlled version was used after 1981. Other than that,
the two versions are the same although it seems the stepper motor version is
a bit more troublesome.
The Symptoms
The symptoms of the problem include stumbling and sputtering at idle. In
advanced cases, the Jeep will stall at every stop sign and will only run at
high RPMs. Gas mileage will suffer since fuel will just be sloshing out at
idle. Sometimes the idle will be turned up to a high RPM to avoid the
problem. Typically, the Jeep will run fine at higher RPMs (unless there are
also other problems.)
For a sure diagnosis, park the Jeep with the engine off and remove the air
cleaner cover. There should be a plate over the throat of the carb, the
choke plate. If you open the choke plate you should be able to see down the
throat of the carb and you should see two screws with holes in the middle of
them. Next to them are two passages with a nozzle in the middle of each.
This thing is known as the venturi, when air passes by, fuel is supposed to
be drawn out through the nozzles. If the idle tubes are clogged fuel will
drip from those nozzles during idle.
In order to see if fuel drips from the nozzles at idle, you must start your
Jeep with the air cleaner cover off and look down the throat of the carb.
The Jeep Technical Service Manual recommends that, when you do this, you
cover the air cleaner with a piece of plexiglass since the engine can
backfire through the carb and a flame can shoot out. Since I am reckless and
like living dangerously, I never do this.
After you have chosen the wise or foolish path, start your Jeep and open the
choke plate. If your idle tubes are badly clogged, you will see fuel
dripping from the nozzles at idle (if your Jeep will idle at all.) If you
don't see fuel dripping, but your idle is still poor, open the throttle a
bit with your hand or have a friend hit the accelerator. You should see two
even streams of fuel and no dripping from the nozzles. Any dripping means
clogged idle tubes.
While you are doing this, make sure you don't put your hand or anything else
into rotating parts like the fan. Keep your tie away from that thing (some
people just want to look good all the time.) Same goes if you are a "long
hair, freaky" Tesla type. All joking aside, I've heard some bad stories.
The Solution
Neither removal of the carburetor nor a complete rebuild is necessary to fix
the problem. To fix the problem, start with you Jeep off and parked
securely:
1.. Remove the air cleaner cover and air cleaner. You may need to remove a
few hoses and wires to get the air cleaner out of the way. Make sure you tag
them all so you can put them back in the right spot. It's often easiest not
to remove the heater hose that goes to the exhaust manifold, if you have
one.
2.. Remove hoop that holds air cleaner.
3.. Remove the two screws holding the choke plate with a 3/16" socket or
small flat blade screw driver depending on what screws you have holding it.
Be careful not to drop the screws down the manifold unless you enjoy
fishing. Remove the choke plate.
4.. If your carb has one, remove the plate on the side that covers the
choke linkage so you can access the screw holding the choke rod. It may be
necessary to drill out a rivet to get it free.
5.. Remove the little snap ring and screw (1/4" socket) that holds the rod
that holds the choke plate and remove the rod. A screw driver will normally
push off the snap ring.
6.. Remove the two screws with holes in the middle of them and carefully
remove the venturi cluster with the two little gaskets. There should be two
idle pickup tubes pushed into the venturi sticking out of the bottom. If
they have fallen out, that could cause your idle problem.
7.. Blast the venturi and inside of carb with lots of carb cleaner. Be
sure not to dislodge the check ball in the center.
8.. With a long thin pin or piece of piano wire, make sure the idle tubes
and nozzles are clear. After you have run them through, spray with more carb
cleaner.
9.. Put the cleaned out venturi cluster back with the two gaskets and two
screws.
10.. Replace the choke rod with snap ring and screw and replace the choke
plate. Make sure all the screws are tight that hold together the carb body
at this time. Often the screws will loosen up and dirt will be sucked into
the carb.
11.. Replace the plate covering the choke linkage with a sheet metal
screw, or leave it off.
12.. Replace the air cleaner.
Smooth idle should have returned after this simple process. If you still
have idle problems and don't have any dripping, make sure all the vacuum
hoses are hooked up and in good shape. Also check for leaks around the
intake manifold or a loose carburetor.
To avoid repeated clogged idle tubes, drill out the idle tubes to 0.032".
This will greatly reduce the frequency of clogging and it is a procedure
that was actually recommended by Jeep for a while.
Idle Problems
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.
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