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on March 13, 2008, 04:03:00 PM

The "Nutter" Bypass 

All CJs and YJs with the BBD equipped 258 (4.2L) engine are starting to get old. One of the seemingly inevitable consequences of aging in these vehicles is the ECM (Electronic Control Module) computer or other portions of the stepper motor controlled carburetor or the complex pulse air emissions system failing. The performance of the vehicle can be dramatically altered by any sort of failure or even a mild degradation in the ECM or emissions system. These systems control both the ignition timing and the air to fuel ratio on these vehicles, and if either is off then a significant loss in power or overall driveability may result. 

Typical symptoms of a malfunctioning ECM or emissions system may include: surging or missing at idle, knocking or pinging, loss of overall power and/or an increasing inability to maintain freeway speeds. In extreme cases the ignition may actually cut out completely while running or the engine may not start. There are other reasons that this modification may need to be done as well. Any change to a different type of carburetor, or to a TBI fuel injection system such as those offered by Holley or Howell may confuse the ECM resulting in the the spark timing being off. The addition of high performance parts or engine modification such as head modifications, headers, or intake modifications may also require that this sort of modification be done because the modified engine may operate outside of the parameters for which the ECM was designed. 

This modification will do two things. It will rewire the ignition wiring to be basically the same as that found in an '78 to '82 CJ (with the 258 engine, but without the computer controlled version of the Carter BBD carburetor) and it will make the stock carburetor with its computer controlled stepper motor work with the rewired ignition. These instruction are based on my own personal experience and hundreds of Jeeps have been modified in this way since the first version of this modification was published. 

The following steps all apply to anyone keeping the BBD carburetor. Steps one and two may be skipped by those changing to another type of carburetor or to fuel injection. 

Warm up the Jeep long enough for it to be operating in closed loop mode if the ECM is still functional.

Screw in the idle jets so that the needles that are controlled by the stepper motor move. The purpose of this is to fool the ECM into putting the stepper motor driven needles into the full rich position. When this happens, you will see that the needles will be all pushed the way to the inside of the carb and the tone of the air going through the carb will get much lower and the engine will slow down and start to miss. At this point you should shut off the engine and disconnect the plug for the stepper motors on the back of the carb. The plug stays disconnected from now on.

Before the modification

An alternative to steps 1 & 2 is to remove the stepper motor from the back of the carburetor and fully extend the needles. Replace the stepper motor while being careful to not push the needles into the body of the stepper motor.

Changing the wiring: On '83 and later CJ's and YJs with the 4.2 the purple and orange wires from the distributor go to the computer under the dash instead of directly to the ignition module like earlier CJ's which were equipped with the Dura Spark ignition module. In summary: Orange wire at the distributor connects will connect to the orange wire at the ignition module and the purple wire from distributor connects to purple wire at ignition module. 

To do this you will have to pull the wires out of the split loom covering and follow them carefully. The orange and purple wires from the distributor both go into the firewall between the battery and valve cover, and there there is also an orange wire from the ignition module that goes into the firewall at the same place. Cut the two orange wires and splice them together. Cut the purple wire here as well .

After the modification 

There is no purple wire coming out from the plug near the ignition module, so you will have to add a section of wire. Find where the purple wire is that goes into the plug from the ignition module is, splice your new wire into that purple wire and then run the new wire up from the splice to where you cut the other purple wire and connect the new wire to the cut purple wire (near the firewall). I used 16 gauge wire for the section I added and I have always used insulated spade terminals at all the connections. A perfectionist would have soldered and used heat shrink tubing to insulate the connections.

Disconnect the vacuum line going to the distributor and plug the port that the it originally went to. Now attach the disconnected vacuum line from the distributor to a tee that you will have to install in the the ported vacuum line. The ported vacuum line is attached to a fitting on the BBD carburetor located on the side towards the valve cover, about 1/2 way up the carb.

Set the timing. Disconnect the vacuum line at the distributor and plug it. Set the timing to 8 degrees BTDC. When I first started the test vehicle the timing was retarded (about 6 degrees ATDC). Don't let the motor run for long like this or it may be damaged. Know how to adjust your timing, have your timing marks clean and the distributor clamp loosened ahead of time so you can advance the timing to 8 degrees BTDC (Before Top Dead Center) quickly.

My results: My 258 now revs smoothly to 4000 RPM, that's right 4000 RPM! Before this it would only get to 3000 RPM and then start to bog and miss. It also has a bit more power below 3000 RPM and the low end torque is definitely stronger at 600 RPM than it used to be. I idled up a fairly steep driveway in 1st gear high range and it went down to 400 RPM and kept pulling, before the conversion the engine always died when I did this in this driveway. 

Since the original article appeared, I have received hundreds of e-mail messages about this modification (literally - at least 2 messages a week, sometimes 2 a day for up to a month after a new newsgroup or mailing list discovered the article which was originally published in 1996). Out of these hundreds of messages only two people have told me that they did not see much of an improvement, while most of the rest seem to have been as amazed as I was by the improvement. I'm very pleased that all those CJs and YJs are running better and I hope that this reworked version of the modification will help even more people. 




Posted by South Mountain Man

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