For the brave-of-heart and the compulsive tweakers, here is an ignition modification that will improve the performance of the 258 CID six-cylinder engine noticeably.

Be aware: Modifications that disable the computer may very well render the engine incapable of passing an emissions test, and should be performed only on vehicles intended for off-road use.

And here are diagrams of how it works:

I'm told that the wiring on a YJ is the same, except for one difference in wire color --- The wire from MCU harness terminal #47, going through the firewall to the connector at the ignition module, is orange on my CJ, but is light-green on a YJ.

Note: With no distributor signal input, the computer does not know the engine is running, and remains asleep. In this state, the input sensors and controlled devices have no function. The idle speed controls, BBD stepper motor, and the Pulse Air system do not work.

Your buddy borrowed your timing light and never returned it? Here's a trick I learned from my brother. The timing can be set to surprising accuracy with a vacuum gauge. Hook the gauge to manifold vacuum and run the engine at curb idle. Rotate the distributor clockwise to retard the timing. Now slowly rotate it counter-clockwise to the point where you just reach the maximum vacuum reading. Back off (clockwise) until the vacuum drops by 1". Lock the distributor down. Timing can be set to within 1° using this method.