1987 Jeep Wrangler YJ: The Real Rough Idling Cure

Jeep YJ

I May Not Be A Mechanic...

... but I learned basic mechanickin' as a teenager. Out of economic necessity in the late 50s, kids had to learn to maintain their own cars. Engines were simpler then; no computers, very little plastic, lots of space in the engine compartment, and no metric wrenches required. All you needed was access to a standard SAE socket set, a few open-end and box-end wrenches, and maybe a few bucks for used parts now and then. Of course, gas was only fifteen cents a gallon, too, same as stamps.

Rough Idle? Stalling?

Does this sound familiar?: One day, my YJ just died when I stopped for a light. I got it going again and by pumping the accelerator, I managed to limp home -- had to run a couple of stop signs, though, because it just flat would not idle at all. I Googled a few articles on rough idling Jeeps around the Web. None of the web *fixes* for rough idling of Jeeps with the much disparaged Carter BBD that I tried seemed to have a long-term positive effect on my Heep's performance.

Many things can cause rough idle; improper timing, poor idle mixture adjustment, faulty ignition wires, and most common of all -- vacuum hose deterioration or disconnection. The maze of hoses under the hood of a carburetor breathing Jeep is very susceptible to this last one. But that's not what this article is about.

My sister came to visit and we took a day trip to Oregon Caves. The last few miles were 6% uphill climbing with hairpin turns. By the time I got to the top, the Heep was coughing and sputtering and would not idle at all. It stalled as I parked. Later, the Heep started up again, but all the way down the hill after our tour of the caves, it backfired and shuddered. I had to stop and crank the idle up to keep the throttle plate a little open to make the stoplights on the way home.

Inspection of the spark plugs revealed a lean-fuel condition; the plug insulators were uniformly a very dusty white color with tinges of tan.

A few days later I took the Heep, still coughing and stalling, to a local garage. The mechanic there blew out the idle ports to get it running again. I got him to show me what he did. $75 lesson learned, or so I thought... (turns out blowing like that can collapse your floats, if they're the older brass/bronze ones!).

I had to do the same thing three or four times over the next two weeks (blow out the jets, not take it to the shop!) I came to believe that all I was doing was blowing any obstructing material back into the float bowl to plague me again after hitting another few potholes in the road.

I have since learned that on some carburetors, using compressed air to clean ports on *assembled* carburetors can collapse the floats -- be advised.

Finally, I had had enough. I am over sixty five years old and had not worked on an engine in many years -- but I was determined to solve this myself. I got out my tool box. I took the carburetor off and disassembled it, finding some kind of crystals growing in the float bowl. I don't know for sure what they were, but I dumped them out on a rag, and a couple of hours later they were gone. The only thing I can think of associated with petroleum that might act like that is naptha. Naptha or napthalene is a common gasoline impurity, and is more common in cheaper gas. I purged my entire fuel system and installed a filter in the fuel pump inlet line when I applied *the cure*. The filter may not stop crystals growing in the carburetor, but it will keep tank scale and other dirt out of the carb. My YJ with its Carter BBD now runs like new. No lie.


Curing Rough Idle

4.2L straight-6 engine

This applies only to the 4.2L (258cid) I-6 engine with the Carter BBD 2-barrel carb. The cure works if all other features of your Carter BBD are functional.

The cure consists of removing the carb from the engine and cleaning it according to the following procedure. Tools you will need include:

  1. Air cleaner removed

    For fire prevention and personal safety, perform only on a stopped, cold engine, in a vehicle resting on level ground with the parking brake on. NO SMOKING! Remove the air cleaner inlet duct and the air cleaner cover. Push up the air cleaner and prop it out of the way on its tall mounting stud.
  2. Dismount the low idle solenoid from the carburetor by removing two philips screws holding it to the carb top.
  3. Stuff a rag under the joint between the flexible fuel line and the carburetor fuel inlet line to catch any leaking fuel, and detach the hose from the inlet line. On my engine, this was the outlet of the main fuel filter.
  4. Low idle solenoid removed

    Disconnect all hoses and electrical connectors from their attachment points on the carburetor. Un-clip the top of the throttle link rod from the pin on the throttle crank arm.
  5. Remove four (4) nuts holding the carburetor to the intake manifold, lift the carburetor off the studs. Stuff a rag into the intake manifold opening. Without trying to dump out its fuel, keep the carburetor upright and remove it to a well-lighted bench in a ventilated area for disassembly.
  6. Clips, fuel inlet, and idle screws removed

    Being careful not to rock the carburetor around too much, remove three circlips from the link rods for the throttle pump arm, thermal choke arm, and the idle cam.
  7. Unscrew the brass fuel inlet fitting (9/16"). Notice the orientation of the fuel cutoff needle valve for replacement later. The pointy end goes towards the inlet line, as shown in the photo.
  8. Remove the idle mixture adjustment screws and their springs. These are located in recessed holes at the front base of the carb.
  9. Remove the top of the carb by removing the remaining four (4) philips-head screws.
  10. Carb cover removed

    Carefully lift the top off and notice that there is a small steel ball (check valve) below the acceleration piston. Also notice the spring in the center well. Take these out and set aside before proceeding, to prevent losing them.
  11. Empty float bowl

    Remove the float assembly to gain access to the float bowl. This is where I found that my carb was cluttered with some attached and some loose granules or crystals. They looked like orange-ish rock salt.
  12. Use a rag to swab the fuel and any sediment or contaminants out of the float bowl.
  13. Now clean everything with carburetor cleaner. Get all foreign material out. You should also unscrew the idle and main jets and spray carburetor cleaner through every port you can find to get the thing as clean as possible. Use low pressure compressed air too, if you have it, to blow out all the ports. Lacking a compressor, a tire pump will work.
  14. Now put it back together in reverse order of disassembly. Getting the top cover on involves getting four things in their right holes -- the top should seat all the way around with only a little pressure to compress the acceleration piston the last 1/8th of an inch. Jiggle the top on until this is so.
  15. You should not have any parts left over!

Idle mixture adjustment

After remounting the carburetor to the manifold, and attaching all the hoses and electrical connections, screw in the idle mixture adjustment screws gently just until they fully seat. Then back them out 4 or 5 full turns before trying to start the engine. You will probably have to crank for 10 or 15 seconds to get it started, but then it should fire. Screw the idle mixture screws in one at a time until the idle speed drops, then back out until it smooths again. Repeat with the other mixture screw.


If you can't get the engine started, or the idle is still rough or it won't idle at all, check these things in order:

  1. Verify that all the vacuum hoses are replaced in their correct attachment points.
  2. Spray a little carburetor cleaner or starting fluid into the carb throat, then try starting. If it runs for a few seconds and dies, you just aren't getting fuel from the pump yet.
  3. If the idle is still rough, remove the air cleaner cover and verify that no gas is visible coming from the main jets (in the venturi throats) at idle -- if this is so, you probably got the fuel cutoff needle valve in backwards.
  4. If the idle is still rough, check the distributor cap for carbon tracks or cracks, clean and gauge the spark plugs, and confirm the timing is correct.
  5. Perform the idle mixture adjustment as detailed above.
  6. Take the vehicle for a drive, and notice any roughness, hesitation when shifting and accelerating. If it hesitates when accelerating, either you forgot to replace the distributor vacuum line or got it in the wrong place, or you forgot to replace the little steel ball (check valve) under the acceleration pump. Most commonly one of the hoses on the Decel valve behind the carburetor that connects to one of the pulse/air hoses at a tee has come off the Decel valve. I had to install a clamp on that one.

If all else fails, perform a Weber 34 DGEC Conversion!