Understanding Engine Knock
Engine knock, also known as detonation, is a harmful uncontrolled explosion of the air/fuel mixture in the combustion chamber. This occurs when the fuel ignites prematurely, before the spark plug fires. The uncontrolled explosion creates a knocking or rattling sound.
Why It Happens During Acceleration, Not in Neutral
* Higher Load: During acceleration, the engine is under more load and requires more fuel. This can lead to a richer mixture, which can be more prone to detonation.
* Increased Compression: Acceleration also increases engine speed and combustion chamber pressure, creating conditions that can trigger premature ignition.
Possible Causes:
1. Low Octane Fuel: Using gasoline with an octane rating lower than what your engine requires can lead to knock. The 1965 Riviera likely calls for a minimum of 91 octane fuel.
2. Carbon Buildup: Carbon deposits on the pistons, valves, and combustion chamber can alter the shape of the chamber and increase the chances of pre-ignition.
3. Ignition Timing: Even if your timing is set correctly at idle, it can be slightly off at higher RPMs due to mechanical issues (worn distributor components, incorrect timing curve).
4. Advance Curve Issues: The distributor's advance curve determines how much timing is added as RPMs increase. A faulty advance curve can cause timing to be too far advanced, leading to knock.
5. Spark Plugs: Worn spark plugs can cause a weak spark, leading to slower ignition and detonation.
6. Vacuum Advance : A leaking or malfunctioning vacuum advance unit on the distributor can affect timing and contribute to knock.
7. Intake Manifold Leaks: Leaks in the intake manifold can lead to a lean air/fuel mixture that can make detonation more likely.
8. Engine Overheating: High engine temperatures can increase the risk of knock.
9. Compression Ratio: High compression ratios can be more susceptible to detonation, although this is less likely on a stock 1965 Riviera.
Troubleshooting Steps:
1. Fuel: First, ensure you're using the correct octane fuel. Try a higher-octane fuel to see if the knock disappears.
2. Check Spark Plugs: Inspect your spark plugs for wear, fouling, or gaps that are out of spec. Replace them if necessary.
3. Timing: Verify your ignition timing is correct at idle. You may need to adjust it slightly. Have the timing checked at higher RPMs to make sure the advance curve is working properly.
4. Vacuum Advance: Inspect the vacuum advance unit for leaks, cracks, or proper operation.
5. Carbon Cleaning: Consider a thorough engine cleaning to remove carbon buildup.
6. Intake Manifold Inspection: Check for leaks in the intake manifold, especially at the gaskets.
7. Cooling System: Make sure the cooling system is functioning properly.
8. Compression Test: If you suspect a compression issue, perform a compression test.
Professional Help: If you're not comfortable with these troubleshooting steps, a qualified mechanic can diagnose and address the issue.
Important Note: Engine knock can cause serious engine damage. If you experience it, it's crucial to address the root cause promptly.
