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Easy Fix for Jeep 258 Carter BBD Idle Problem

by Terry L. Howe

Carter BBD

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.

Carter BBD Apart

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.

An alternative to removing the choke plate and all is to just remove the top of the carburetor. The advantage of this approach is you can make sure there isn't a lot of junk in the bowls, you can check your float adjustment, etc. It also may help if you have power brakes because the master cylinder may be in the way of getting the choke rod out.

Contributor

Thanks to Randy Peterson for posting this solution a few years back. I've used it on various Jeeps many times since.

Last modified Wednesday, 01-Dec-2010 08:33:40 MST


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alan (5/29/2013)
Good article. Did the nutter on my cj7 85` 258 4spd has any one ever ever made a device to make the stepper adjustable.

Redrockerfan (2/20/2013)
Thanks for the write up on this issue. I was starting to think i just bought a junk yard bound jeep lol. Did everything you said and the jeep runs like a champ.

Boulder CJer (10/23/2012)
I have an idle problem with my 86 CJ7 that sounds very much like this issue, however my well regarded jeep mechanic is telling me it is related to a failed decel valve (part J3237284). I also understand this part is no longer made. Any idea if this feedback is legitimate and if so where I might be able to dig up this part.

JIM (4/1/2012)
DID THE RANDY PETERSON EASY FIX THING WITH NO CHANGE. MOTOR STARTS & RUNS FINE FOR A FEW SECONDS, THEN STARTS TO BLOW BLACK SMOKE.
REMOVED THE TOP PLATE OF THE FLOAT CHAMBER & FOUND NO FLOAT?
STARTED THE ENGINE & IT RAN FINE UNTILE THE FLOAT CHAMBER BEGAIN TO FILL UP WITH GAS, THEN IT STARTED TO BLOW BLACK SMOKE & STALLED OUT.


Mexican Side (3/22/2012)
I just get realize that I have a 258. I thought that it was a 242 because my jeep is an 1990 XJ fuel injection. If you could give some tips it would be great.

Andy (3/7/2012)
Hello Terry,
Your article was very interesting. I've owned an 86 cj7 for 13 years. I got this issue, once the engine gets hot, the engine stutters when I step on the gas. It will eventually pickup and run, but it continues to act the same. Any help would be grateful.

Cj5 (11/15/2011)
Any advice on an engine that I cannot get to stay running below 2,000 rpm? I bought the CJ5 not running and the previous owner had rebuilt the carb, I went ahead and put another rebuild kit in it but can only start it off of starting fluid and stay running at real high rpm. Any suggestions or spec sheets on how the original carb was set up with jets etc.?

Pat (5/6/2011)
Follow the instructions and everything will fall into place even if you have no idea what your looking at. A schematic of the carb would help most people but this is so simple most could do it with their eyes closed. Thanks for a fine article. I did it but still have the rough idle and stalling when put in gear but it is better. Intake manifold bolt do come loose for sure check that too. I love my jeep too much to get rid of it because of this. I am looking at this as a test maybe my jeep wants to see if I really love her. Bi--h

SImon Benn (5/4/2011)
Fellow englishman now living in COlorado Springs and teh frst thing i did was get myself a Jeep. It came with an idle problem too, so I will try this and I am by no means a Tech, not even sure what a choke rod is???? But I will try and get this done, thanks for this!!! Video would be awesome too!

70 Challenger (4/26/2011)
Terry, Same goes for most any 2bbl carb. Problem is most grease monkeys don't know diddley about gauges and measuring much less following instructions. The last comment of your solution is best by any means,that is if your inclined. There is a lot of people who need simpler means (other than comic book type) to tackle carb solutions. Show and Tell is is fine in automotive manuals but still audio-visual is what most want or need to simplify matters. Thanks for your support and look up my blogs and help just the same. 70 Challenger

Carlos (4/13/2011)
Anybody knows where I can get the computer(new) for my Carb-Carter BBD Type C2 # 8394S 2 barrels ? I appreciate any information about. nostromo9805@yahoo.com

Andrew (4/8/2011)
Do you have any pictures? I do better when i have seen it done.

Keith (4/5/2011)
I'll give this a shot. I've never worked on a carter like this before, but how differant can it be from any other carb? right?

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