1967 Corvette 427/435 Tri‑Power Tuning Manual

Version 1.0 – June 20, 2025

Table of Contents

1. Safety & Tools
2. Baseline Checks
3. Step‑by‑Step Idle & Mixture Tuning
4. Progression Ignition Tables
   • Normal Driving
   • Traffic / Hot Idle
5. Cold‑Start Strategy
6. Highway Optimization
7. Troubleshooting Flowcharts
8. Reference Tables
   • Vacuum Gauge Readings
   • AFR Targets
9. Appendices

1. Safety & Tools

• Fire extinguisher within arm’s reach.
• Eye & ear protection (you will be running the engine).
• Quality timing light capable of reading RPM.
• Hand‑held vacuum gauge with a long hose.
• AEM X‑Series or comparable AFR gauge, already installed in the exhaust.
• Flat‑blade screwdriver for idle mixture screws.
• 3⁄8‑inch drive inch‑pound torque wrench for carb base nuts.
• Progression Ignition bluetooth app installed on your phone or tablet.

2. Baseline Checks

1. Verify true TDC on cylinder #1 using a piston stop tool.
2. Check fuel pressure at idle & WOT (5.5 psi ± 0.5).
3. Inspect throttle linkage: secondary carbs must be fully closed and disconnected for the idle procedure.
4. Make sure the choke flap on the center 3660 carb is fully vertical (open) once the engine is warm.
5. Confirm spark‑plug gap at 0.035 – 0.040 in and ignition wires are in the correct firing order (1‑8‑4‑3‑6‑5‑7‑2).

3. Step‑by‑Step Idle & Mixture Tuning

1. Set Base Timing
   • Connect timing light, unplug and cap the vacuum advance port.
   • Start engine, allow to reach 180 °F.
   • Using the Progression app Static Spark, set base timing = 14° BTDC at 900 rpm.
2. Set Idle Speed (Center Carb Only)
   • Turn the 3660 curb‑idle screw to obtain 900 rpm (manual trans, no A/C).
3. Adjust Idle Mixture Screws
   • One screw at a time, slowly turn in (clockwise) until rpm drops, then back out to peak rpm.
   • Aim for highest steady vacuum. Target = 11 – 13 inHg for the L‑72 big‑cam engine.
   • Confirm AFR stabilizes between 12.8 – 13.2:1.
4. Re‑Synchronize Secondary Carbs
   • Re‑attach linkage. Verify all three throttles crack open simultaneously at 25 % pedal travel.
5. Lock the Settings
   • Tighten lock‑nuts, fit new gaskets if mixture screws were removed.

4. Progression Ignition Tables

4.1 Normal‑Driving Map

4.2 Traffic / Hot‑Idle Map

This table maintains slightly higher idle timing and speed to push more coolant through the radiator and reduce heat soak.

5. Cold‑Start Strategy

1. Pre‑prime fuel bowls with two throttle blips.
2. Crank with +10° cranking spark (Progression Cranking Timing).
3. Initial AFR will be rich (≈ 12:1); allow engine to fast‑idle at 1100 rpm until coolant hits 140 °F.
4. Progression’s Warm‑Up Enrichment: 10 % fuel trim at 50 °F, tapering to 0 % by 120 °F.

6. Highway Optimization

• Enable Vacuum Advance in the Progression app: +12° when manifold vacuum > 15 inHg.
• Target AFR 14.5 – 15.0:1 at steady 65 mph cruise for best range.
• Use leanest jetting that still yields ≤ 20 °F increase above ambient oil temperature after 10 min cruise.

7. Troubleshooting Flowcharts

7.1 High Idle

+------------+
| Idle >1000 |
+------------+
     |
     v
Are linkages
binding?
  / \\
 / No\\
v     v
Yes   Idle screw backed out?
 |      / \\
Fix    No  Yes
 |      v    v
Test   Back out    Adjust timing

7.2 Lean Surge at Cruise

+-------------------+
| AFR >15:1, surge? |
+-------------------+
        |
        v
Vacuum >17 inHg?
    /\\
   /  \\
  No  Yes
  v    v
Check fuel    Add 2° timing
filter &      or enrich jet
pump output   size by 1

8. Reference Tables

8.1 Vacuum Gauge Cheat‑Sheet

8.2 AFR Target Ranges

9. Appendices

• A: Torque Specs – intake manifold bolts 30 lb‑ft.
• B: Jet & Power Valve quick-reference for Holley 3660/3659.
• C: Sample data‑logging worksheet (printable).