Many of you will be attempting this modification on a stock computer controlled 258. I believe the Jeeps in question are the model years mid-1982 through 1990. These will require bypassing the computer from controlling the ignition timing. A simple unplugging of the Duraspark ignition module will take care of this.
The jury is still out on whether the stepper motor needs to be fixed. Based upon the oxygen content in the exhaust and other inputs, the computer varies the air/fuel ratio (via the stepper motor) to maintain an optimum 14.7:1 ratio. The O2 sensor provides the majority of the input to the stepper motor, but not the only input. According to all of the information the ignition plays no role in determining air/fuel mixture. I would try the HEI modification without messing with the stepper motor and if everything seems to work great then leave it alone.
However, most people do fix the stepper motor because it offers some benefits of it's own. Refer to the write-up from John Nutter that explains bypassing the Jeep’s computer from controlling the air/fuel mixture and ignition. John Nutter Ignition Bypass. Keep in mind that the only thing you would need to do is ‘fix’ the stepper motor’s idle position. You don't need to do any of the wiring modifications John makes because the HEI install does this for you, in a much cleaner and easier fashion! A tip on the "Nutter Bypass" - carefully take notice of how many turns you make when screwing in each idle jet. You will then be able to easily return to the original settings and adjust from that point when everything is done. The rest of the ignition modifications discussed on his page will be unnecessary because the HEI now becomes your Jeeps complete ignition system. In short, you will end up with the stepper motor and ignition module unplugged.
