STEP 5
Ignition System
Steps 3, 4, and 5 are a sequence within the sequence. Electrical foundation first, because voltage stability affects everything. Fuel system second, because you cannot set ignition timing accurately on an engine that is not getting consistent fuel delivery. Ignition third, because this is where the engine either pulls it all together or does not.
A CJ with a fresh carburetor or a new TBI system but a tired ignition will not run well -- and the symptoms look exactly like a fuel problem. Weak spark causes hesitation under load, rough idle, and poor fuel economy. Retarded timing causes overheating and sluggishness. Before you go back to Step 4 and start adjusting the carburetor again, confirm the ignition side is honest. That is why this step exists where it does in the sequence.
The full technical breakdown -- ignition module testing, HEI installation, timing procedure specifics -- is in our Jeep CJ Ignition System Guide. This step covers the build sequence decisions: what to inspect, when to upgrade, and what to set before you call the engine done.
What to Inspect Before You Upgrade Anything
| Component | What to Check | Common Finding |
|---|---|---|
| Distributor cap and rotor | Carbon tracking, cracked towers, worn or corroded rotor tip | Carbon tracking causes misfires that appear at specific RPMs under load |
| Spark plug wires | Cracking, arcing to block or header, resistance check with multimeter | Aged plug wires arc to ground under load and disappear at idle -- easy to miss |
| Spark plugs | Condition, gap, and what the deposit pattern tells you | Plug condition is a diagnostic tool -- read them before you replace them |
| Ignition coil | Cracks, oil leakage, primary and secondary resistance | Failing coils often produce adequate spark cold but weaken when hot |
| Ignition module / pickup coil (Prestolite) | Reluctor air gap, module output, condition of wiring pigtail | Module fails when hot and recovers when cool -- intermittent no-start that resolves itself |
| Timing and distributor hold-down | Base timing with timing light, distributor clamp tightness | Loose hold-down allows distributor to rotate and timing to drift -- often unnoticed until it causes overheating |
The Factory Ignition System: What You Have by Year
Most CJs left the factory with one of two ignition systems depending on year. The 1972-1974 models use a conventional point-style distributor. Points wear predictably and require periodic adjustment and replacement -- if the Jeep still has the original points distributor and has not been serviced recently, a full tune-up is the starting point.
The 1975 and later models use the Motorola or Prestolite electronic ignition system, which eliminated the points but retained the same basic distributor design. The Prestolite system is reasonably reliable when its components are in good condition, but the ignition module, pickup coil, and cap and rotor all degrade with age. A module that tests fine on the bench can fail intermittently when hot -- the classic symptom is a CJ that runs fine until it warms up, then dies and will not restart until it cools down.
If the Prestolite system is in questionable condition, the practical options are to replace the module and pickup coil with new components, or to convert to the HEI system. Given the age of the vehicles and the price difference between a proper Prestolite rebuild and an HEI conversion, most owners doing a thorough foundation build choose the HEI.
The HEI Conversion: The Most Commonly Recommended Upgrade
The General Motors High Energy Ignition system -- typically sourced from a 1970s-1980s GM V8 application -- is the most consistently recommended ignition upgrade for the AMC 258. The HEI distributor is a self-contained unit with the ignition module built into the distributor cap, which simplifies the wiring considerably. The module produces a stronger, more consistent spark across a wider RPM range than the factory Prestolite system, particularly at the lower RPMs where the 258 spends most of its time.
The HEI swap requires an adapter to use the GM distributor in the AMC 258 block -- the distributor drive is different. Conversion kits are available and well-documented. The wiring simplification alone (two wires versus the multi-wire factory harness) makes this worth doing for any Jeep that has had previous wiring issues. A cleaner ignition harness means fewer variables when diagnosing anything electrical downstream.
The HEI system also pairs well with TBI conversions. If you went the Howell or Holley Sniper route in Step 4, an HEI conversion in Step 5 gives you a completely modernized ignition and fuel system on a platform that benefits from both.
Spark Plugs and Wires: Read Before You Replace
Spark plugs are one of the few components on the engine that give you direct visual information about combustion. Before you pull and replace them, read them. Light tan or gray deposits on a properly gapped plug mean the engine is running in a healthy range. Black, sooty deposits suggest a rich fuel mixture or oil burning. White or blistered electrodes indicate detonation or excessive heat. A plug that is wet with fuel suggests a cylinder that is not firing.
Match what you see on the plugs with the symptoms you observed in Steps 4 and 5. If one cylinder's plug looks different from the others, that cylinder has a specific problem worth investigating before you close everything up.
Plug wires on a CJ of this age are frequently original or of unknown vintage. Aged wires develop cracks in the insulation that arc to the block, frame, or exhaust under load. The misfire this causes often disappears at idle, leading owners to conclude the issue is in the fuel system rather than the ignition. Run the engine in the dark and look for arcing. Replace the wires if they are cracked, stiff, or older than ten years.
Setting Timing: Do This Before You Call the Engine Done
Correct base timing is the last step before declaring the ignition system honest. Incorrect timing -- particularly timing that has retarded -- causes overheating, poor fuel economy, and a general feeling of sluggishness that gets misdiagnosed as a carburetor or cooling system problem. The correct base timing for the 258 varies slightly by year and application, but 8-10 degrees before top dead center is a common starting point. Always verify against your specific year's specification.
Check the distributor hold-down clamp before you set timing. A loose clamp allows the distributor to rotate gradually under heat and vibration, causing timing to drift without any obvious trigger. A Jeep that ran fine last year and now feels sluggish and runs hot has often simply lost timing through a loose distributor. Tighten the clamp, set the timing, and recheck it after the first few heat cycles.
Set timing with the vacuum advance line disconnected and plugged unless your specific procedure calls for it connected. Reconnect it after setting base timing and confirm the advance mechanism is working -- the engine should pull noticeably more when you apply vacuum to the advance port. A non-functioning advance mechanism costs fuel economy and performance that no carburetor tune will recover.
What Comes Next
With the electrical foundation solid, fuel system honest, and ignition set correctly, the engine is running the way it is supposed to. The first three systems in the build sequence are complete. Step 6 moves to steering -- the component most directly responsible for how the Jeep handles on the road and on the trail.
Continue to Step 6: Build an Honest Steering Foundation →
Or return to the CJ Foundation Checklist to review the full sequence.