Lotus Twin Cam: Ignition Timing Guide

Understanding Your Lotus Twin Cam's Ignition Timing

The Lotus Twin Cam engine, a legendary powerplant known for its performance and distinctive character, requires precise timing for optimal operation. Many owners, particularly those new to the marque or who have had their engines worked on by less experienced mechanics, can find themselves wrestling with distributor orientation and ignition timing. This guide aims to demystify the process, drawing on practical experience and established workshop procedures to help you achieve correct static ignition timing for your beloved Lotus Twin Cam, whether it's fitted with points or electronic ignition.

The Importance of Camshaft Timing

Before diving into ignition timing, it's crucial to understand a fundamental principle: correct camshaft timing is paramount. Attempting to set ignition timing on an engine with incorrectly set valve timing is a futile exercise. While the engine might start and run, it will never perform as it should. Valve timing dictates when the valves open and close relative to the piston's position. If this is out of sync, the combustion process will be compromised, regardless of how accurately the ignition sparks.

If your Lotus Twin Cam engine has been recently rebuilt, especially by someone unfamiliar with its specific requirements, it is highly recommended to verify the camshaft timing. This check can often be performed without removing the cam cover, saving time and effort.

Checking Camshaft Timing (Without Cam Cover Removal)

To perform a quick check:

1. Remove the oil filler cap.
2. Locate cylinder number four (the furthest cylinder from the front of the engine).
3. Observe the camshaft lobe for cylinder number four. It should be pointing inwards, towards the spark plug of cylinder number four.

If this condition is met, it indicates that the camshafts are likely correctly timed. If the lobe is not in this position, the camshaft timing is incorrect and must be rectified before proceeding with ignition timing. The correct valve timing positions ensure that when the piston in cylinder number one is approaching Top Dead Centre (TDC) on its compression stroke, the corresponding camshaft lobes are positioned correctly for ignition. Specifically, for cylinder number one, the lobes should be pointing away from the spark plug.

Setting the Static Ignition Timing

Once you are confident that the camshaft timing is correct, you can proceed with setting the static ignition timing. This involves aligning the distributor rotor and the engine's crankshaft to a specific point before TDC.

1. Crankshaft Position

Rotate the crankshaft until the piston in cylinder number one is on its compression stroke and approaching Top Dead Centre (TDC). The timing chest usually has marks to indicate this position. For accuracy, you can verify this by removing the spark plug from cylinder number one and feeling for compression or using a finger over the plug hole.

2. Static Ignition Timing Specification

Consult your Lotus workshop manual for the correct static ignition timing specification for your specific engine variant. For example, a "Big Valve" engine fitted with Dellorto carburettors might require ignition timing set at 12 degrees Before Top Dead Centre (BTDC).

Align the timing mark on the crankshaft pulley with the corresponding degree mark on the timing chest. For a 12-degree BTDC setting, you would rotate the crankshaft so the pulley mark aligns with the '12' mark on the timing chest.

3. Distributor Installation

With the engine at the correct static timing position, you can now install the distributor. The following steps, adapted from the Lotus workshop manual, provide an excellent method:

• Ensure the rotor arm is installed onto the distributor shaft. The rotor arm's electrode should be positioned towards the rear clip of the distributor cap before insertion.
• Insert the distributor into its mounting hole in the cylinder block. As the distributor's drive gears mesh with the camshaft, the rotor arm will rotate clockwise.
• Position the distributor body so that the low-tension terminal (where the coil wire connects) is adjacent to the cylinder block.

Summary of Distributor Orientation:

When standing facing the inlet side of the engine and looking at the fully located distributor:

• The low-tension lead should exit the distributor body at approximately the 12 o'clock position.
• The rotor arm should be pointing towards the 10 o'clock position.

Secure the distributor body by aligning the hole in its clamp with the corresponding hole in the cylinder block and fitting the retaining bolt. Tighten this bolt sufficiently to hold the distributor in place but allow for adjustment.

4. HT Lead Orientation

With the rotor arm pointing to approximately 10 o'clock, you can now determine the correct routing of the spark plug leads from the distributor cap:

• The lead from the terminal the rotor arm is pointing towards connects to the spark plug of cylinder number one.
• The distributor rotor arm rotates anti-clockwise.
• Following the firing order of 1-3-4-2:
• The next terminal in the cap (around the 8 o'clock position) connects to cylinder number three's spark plug.
• The subsequent terminal (around the 4 o'clock position) connects to cylinder number four's spark plug.
• The final terminal (around the 2 o'clock position) connects to cylinder number two's spark plug.

Install the distributor cap, ensuring it is securely fastened with the clips, typically located at the 9 o'clock and 3 o'clock positions.

5. Fine-Tuning the Static Timing

This method allows for accurate static ignition timing:

1. Turn the distributor body anti-clockwise by approximately 10 to 15 degrees. This moves the rotor arm away from the No.1 cylinder contact point in the cap.
2. Remove the high-tension lead from the ignition coil to one of the spark plugs (usually the lead going to cylinder 1). Bring this lead close to a good earth point on the engine block, but not touching.
3. Turn the ignition on.
4. Carefully grasp the distributor body and slowly rotate it in a clockwise direction.
5. Continue rotating until you hear a distinct 'click'. This click signifies that the contact points (or electronic ignition module) have opened, causing the coil to discharge a spark through the HT lead.
6. Once the spark is observed (or heard), stop rotating the distributor body and tighten the clamp screw securely.
7. Turn off the ignition and reconnect the coil HT lead.

Your static ignition timing is now set. The engine should start and run.

6. Verification with a Timing Light

After starting the engine, it is essential to verify the static timing using a timing light. Connect the timing light according to its instructions and aim it at the timing marks on the timing chest and crankshaft pulley. Check that the timing mark aligns with the specified BTDC figure (e.g., 12 degrees). Furthermore, check the ignition advance at higher RPM (typically 2500 rpm) as specified in your workshop manual. The timing light will reveal if the centrifugal or vacuum advance mechanisms are functioning correctly.

Troubleshooting Common Issues

Several factors can complicate the timing process. Here are some common issues and their potential resolutions:

Fuel Injection as an Alternative

For those seeking the ultimate in engine control and performance, modern fuel injection systems offer a significant upgrade over traditional carburettors. A typical fuel injection package for a Lotus Twin Cam can include a comprehensive management ECU, crank and pulley sensors, electronic ignition packs, and throttle bodies with fuel injectors. While carburettors often involve a compromise in jetting to suit various engine conditions, fuel injection precisely delivers the correct fuel amount for every scenario. This results in a smoother driving experience, improved throttle response, and potentially better fuel economy. While a base kit provides essential components, additional parts like fuel lines, swirl pots, and throttle linkage kits may be required depending on the specific application. Crucially, even with a base map provided with the ECU, professional fine-tuning on a dyno is recommended for optimal results.

Frequently Asked Questions (FAQs)

Q1: Do I absolutely need to check my camshaft timing before setting ignition timing?
Yes, it is a critical first step. Incorrect valve timing will prevent the engine from running correctly, making ignition timing adjustments futile.

Q2: My engine starts but runs poorly. What could be wrong?
This commonly points to either incorrect ignition timing or, more likely, incorrect camshaft timing. Revisit the checks for both.

Q3: Can I use a timing light if I have points ignition?
Yes, the static timing procedure described is applicable to both points and electronic ignition systems. The timing light is used to verify the accuracy of your static setting and check the advance characteristics.

Q4: What does "static timing" mean?
Static timing refers to setting the ignition timing with the engine stationary, based on specific marks on the crankshaft pulley and timing cover, and usually involves finding the point where the points open (or an electronic signal is generated).

Q5: The HT leads seem to exit the distributor cap at odd angles. Is this normal?
While the exact angles can vary slightly with different distributor caps, the relative positions should correspond to the firing order. The key is that the rotor arm points to the No.1 terminal when cylinder 1 is at TDC on its compression stroke and the ignition timing is set correctly.

By following these guidelines, you can confidently set the ignition timing on your Lotus Twin Cam, ensuring its legendary performance is realised and maintained.

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