MG MGB Firing Order: 1-3-4-2 – Definition, How to Set, Safety, Advantages & Full Technical Analysis
❓ Why Does Firing Order Matter So Much on an MGB? – Engine Physics & Reliability
The 1-3-4-2 order balances primary and secondary forces in the inline-4 layout. Incorrect order creates overlapping power pulses, excessive crankshaft torsional vibration, and uneven flywheel loading. Consequences include: rough idle, loss of up to 40% power, backfiring through SU carburetors, detonation, and potential piston ring damage. Moreover, correct order ensures the distributor’s centrifugal and vacuum advance mechanisms work in harmony, optimizing torque across the RPM range.
🧬 Types of Firing Orders (Context for 4-Cylinder Engines)
Most 4-cylinder engines use either 1-3-4-2 (like MGB, Ford Kent, Fiat Twin Cam) or 1-2-4-3 (some older flat-plane crank engines). The MG B-Series uses 1-3-4-2 because it reduces vibration by having opposing cylinders fire 180° apart relative to crank throws. This type is often called “even-fire crossplane” in inline engines. The alternative 1-2-4-3 would create uneven exhaust pulses and higher second-order vibration, unsuitable for MGB’s engine mounts and driving refinement.
🛠️ How To Verify & Set the Firing Order (Full Step-by-Step)
Step 1: Safety preparation
Park on level ground, engage handbrake, disconnect negative battery terminal. Allow engine to cool.
Step 2: Identify cylinder #1 and distributor cap orientation
Remove distributor cap. Crank engine manually (22mm socket on crankshaft pulley) until rotor arm points to the notch or mark for #1 cylinder (usually near the front of distributor).
Step 3: Map the terminals
Mark the cap terminal where rotor points (that is #1). Then, moving clockwise (since MGB distributor rotates clockwise), the next terminal must connect to cylinder #3, the next to #4, the next to #2, and then back to #1.
Step 4: Check plug wire condition
Inspect wires for cracks, resistance (should be <10k ohms per foot). Replace if damaged.
Step 5: Static timing confirmation
After verifying firing order, set ignition timing to factory spec (typically 10° BTDC at idle for 18V engines, or 6° BTDC for earlier models). Use a timing light for final adjustment.
📊 Advantages of Correct Firing Order (1-3-4-2) on MG MGB
- Smooth torque delivery: Even power pulses eliminate “chugging” at low RPM.
- Extended engine life: Balanced firing reduces main bearing wear by up to 30%.
- Better fuel economy: Complete combustion yields ~10-15% improvement over misfiring.
- Cleaner spark plugs: No fuel fouling, longer service intervals.
- Optimized exhaust scavenging: Tuned-length manifolds work with firing order to boost mid-range torque.
📉 Disadvantages & Dangers of Wrong Firing Order
- Immediate misfire & backfire: Can blow out gaskets or damage air cleaner.
- Hydrolocking risk: Unburnt fuel washes cylinder walls, dilutes oil.
- Overheating catalytic converter (1975+ MGB): raw fuel ignites in converter, causing meltdown.
- Hard starting and stalling: Engine may not run at all if order is completely scrambled.
🔎 How to Diagnose Firing Order Issues – Symptoms Table
| Symptom | Likely Cause Related to Firing Order | Action |
|---|---|---|
| Engine cranks but won’t start | Two plug wires swapped (e.g., 2 & 3) | Verify full 1-3-4-2 sequence |
| Loud backfire through carb | Order reversed or crossfire between cylinders | Check clockwise rotation, correct order |
| Rough idle, improves with revs | Possibly #2 and #4 swapped | Re-check wire routing from cap |
| Vibration at 2500-3000 RPM | Secondary imbalance due to wrong order | Immediately correct to 1-3-4-2 |
🧰 Use of Firing Order in Advanced Tuning & Maintenance
Experienced MGB tuners use the firing order to optimize ignition timing curves. For performance upgrades (e.g., hotter cam, electronic ignition), the 1-3-4-2 order remains fixed, but you may adjust the distributor phasing. Additionally, when installing a new distributor (123ignition or Pertronix), you must index the rotor to #1 at TDC compression and then follow the clockwise 1-3-4-2 pattern. Always mark the distributor base before removal.
⚠️ Is It Safe to Change or Adjust the Firing Order? – Safety Checklist
- ✅ Disconnect battery negative terminal before touching plug wires.
- ✅ Use insulated tools and wear rubber-soled shoes.
- ✅ Keep fuel vapors away (no smoking, engine cool).
- ✅ After changes, double-check each wire connection.
- ✅ Start engine in a well-ventilated area; have a fire extinguisher nearby.
Yes, adjusting the firing order is safe if these precautions are followed. It is a standard maintenance procedure for classic cars.
📚 Complete Firing Order Data for All MGB Variants
| MG Model | Engine Code | Firing Order | Distributor Rotation |
|---|---|---|---|
| MGB 1962-1967 (3-main bearing) | 18G / 18GA / 18GB | 1-3-4-2 | Clockwise |
| MGB 1968-1974 (5-main bearing) | 18GH / 18V | 1-3-4-2 | Clockwise |
| MGB 1975-1980 (Rubber bumper) | 18V (Z-S/ HIF carbs) | 1-3-4-2 | Clockwise |
| MGB GT (all years 4-cyl) | Same as above | 1-3-4-2 | Clockwise |
