Thread: 4.2 Idle Screws

  1. #1


    In trying to perform the bypass on my computer, I have come across numerous articles on the idle screws (curb idle, fast idle, etc.). Can someone explain and help me locate which is which. I am forever in the search of easy performance improvements and have come to realize that this is the place to learn![addsig]
    1989 Sahara, 4.2, ignition upgrade, Pioneer CD, 8\" sub, 1\" shackle lift, Bombproof motor mounts. Lots to go!

  2. 1048396

    The fast idle is on the back left of your carberator (near the throttle linkage), it seats on a stepped cam that is controlled by the choke. Curb idle is bottom left (under the fast idle) and sets the idle against a fixed stop.

    There is also an idle screw on the idle solenoid (to the front of the throttle linkage) controlled both by vacumn and electricity, on my 90. Mine generally seems to engage and disengage without rhyme or reason. But I do know it helps the motor keep from dieseling on shutdown (running after the key is off) if it happens to be engaged. Maybe one of the other guys can tell us both what exactly it´s supposed to do (besides being a pain).[addsig]

  3. #3


    Thanks for the info. I'm getting ready to do the Nutter by-pass, and part of it is to screw in the idle jets so the needles are connected to the stepper motor (for a full rich condition). Have you performed that upgrade - any results? I also can't coinceive of what type of individual could create the spider's nest of vacuum lines on that 4.2. Amazing, isn't it? My pulse air system is rusted out so I just capped off the lines going to the check valve. Hopefully that won't cause any problems. [addsig]
    1989 Sahara, 4.2, ignition upgrade, Pioneer CD, 8\" sub, 1\" shackle lift, Bombproof motor mounts. Lots to go!

  4. #4


    I don´t know what your situation is with smog tests where you´re at. But I´ve been hesident to do the nutter bypass, until I understand the effect on the rest of the MCU functions. I might try it someday, but I´m gonna do it so, it can be undone.

    They had an alternate method, to fully seat the metering rods on the stepper motor. Remove the stepper, pull the rods out by hand and reinstall carefully so they stay fully seated, if I remeber right. And then don´t plug the stepper motor plug up again.

    If I didin´t have the smog people to deal with, I´d pull the whole works out by the nutters, and put in a Chevy HEI or any magnetically triggered distributor, that would fit, and hook up an old Dodge ignition module. I hooked up a dodge ignition module to a BroncoII, bypassed the computer, the computer was happy as heck and showed no trouble codes.

    The vacumn lines, and the smog stuff, on the carburated YJ´s seem to be mostly an afterthought (afterbirth). But it really isn´t that complicated, heck I figured out most of it.


  5. #5


    I'm here in FL, so emissions aren't a concern - to date. I've also completed the ignition upgrade using a more powerful coil and a larger dist cap and rotor (from a ford 300cuin straight six. Right now idle is smoother with this mod and it wasn't to difficult.

    I am new to this site and really appreciate all the helpful individuals sharing their thoughts and experiences. Hopefully one day I'll be able to offer advise.[addsig]
    1989 Sahara, 4.2, ignition upgrade, Pioneer CD, 8\" sub, 1\" shackle lift, Bombproof motor mounts. Lots to go!

  6. #6


    Most everybody that´s interested enought to read the posts is on a learning curve. About the time I get one system or model year, under control, the people at Jeep change the game.

    One thing you could do for me, is check the coil with the ignition on, with a voltage meter, + coil pole to ground, should read about 7 volts (plus or minus a volt), while it´s starting, it should read battery voltage. I would like to know if the Nutter by-pass affected this?[addsig]

  7. #7


    One other guy that tried the bypass ended up with a lean burn condition. But I believe he changed carb also.

    Might be a good idea to check the number one plug every so often, if it´s to light colored, might be a problem eventually. One good high speed run with a lean motor can damage things.[addsig]

  8. #8


    I'll check the voltage of the coil once I complete the by-pass. I have one project to complete prior to actually getting into the wire cutting. I also have to find the ignition module to be able to splice the infamous purple wire. Basically, I have a bit more investigating to do before my first cut.

    Since I have now removed my pulse air tubes from the loop, it is time for a new cat and muffler system - the one I have has more holes than swiss cheese. I wanted to see if removing the pulse air from the loop had any detrimental effects first so I can get a cat without the extra flange on it. New Exhaust should be this week then Nutter. [addsig]
    1989 Sahara, 4.2, ignition upgrade, Pioneer CD, 8\" sub, 1\" shackle lift, Bombproof motor mounts. Lots to go!

  9. #9


    I changed my plugs when I upgraded the ignition - they indicated a lean condition. I've read that this could be attributed to vacuum and/or air leak in the system. What other ways can you change a lean system? Does it have anything to do with timing? I'm a bit out of my league here. [addsig]
    1989 Sahara, 4.2, ignition upgrade, Pioneer CD, 8\" sub, 1\" shackle lift, Bombproof motor mounts. Lots to go!

  10. #10


    I´ve had my share of vacumn leaks, really messes with the idle. Good luck, let us know how it turns out. [addsig]

  11. #11


    I´ll have to go back and read the Nutter bypass again, he said timing should be set to 8 degrees BTC, after the mod. But I don´t remember him giving an RPM for setting the timing, I better double check. If I remember correctly the timing on a stock motor is set at 600 RPM, at idle like alot of motors. But I always double check the old book, just to make sure. RPM and remembering to plug the vavumn advance, has alot of affect on timing.

    Motor burns pretty lean anyway, supposed to be 14 to 1 fuel/air ratio. When it´s working right,

    My plugs are usually a little sooty, middle gray to charcoal color, but I do alot of stop and go driving.[addsig]

  12. #12


    Depends alot on where it´s burning lean, at low RPM´s or at highway speeds. There are two usually capped screws on the lower front of the carb, that adjust the low speed jets. Standard setting is usually about 2-2 1/4 turns out after screwing them in all the way to the seat gently. I ended up with about 3 1/4 turns out from the seat, doing it by the book, with the stepper still hooked up. Don´t know how the book method will work out after the bypass, might pay to hook the whole works up to an exhaust analizer after your through and writing the results inside the hood for future reference. I always write down the initial settings, the first time I screw them in (screw them up), just for a reference point and write the results on a piece of tape in the engine bay.[addsig]

  13. #13


    So screwing them out will richen the burn? Wow, I didn't realize all of the intergral working of the 4.2 prior to purchase. I've had 2 wranglers prior to this, a 93 4.0 and an 89 2.5 so all this carb stuff coupled with the amount of vacuum lines starts a big learning curve for me. Fortunately I got a good deal (for now) on this ride so all the extra is worth it. I'll just have to prioritize my projects and do them right. [addsig]
    1989 Sahara, 4.2, ignition upgrade, Pioneer CD, 8\" sub, 1\" shackle lift, Bombproof motor mounts. Lots to go!

  14. #14


    Clockwise lean, counterclockwise rich, only works off idle (lower RPM´s) other jets kick in at higher RPM´s but there is some overlap.

    Another guy mentioned, swapping to a dodge carburetor from a early 318 (Carter BBD). Havn´t heard how it worked out yet. I´ve got a couple on the shelf in the garage.[addsig]

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