Tuning and Improving the Stag Ignition System

This section was written by William Mayo and Walter Holliday,
originally appearing in Issue 38 of The Vintage Triumph magazine (currently out of print)

HTML version by Tim Buja, TKBuja@insightbb.com

The importance of a sound ignition system to the running of a car cannot be overstressed. This is particulaly true of the Stag, whose high-revving V-8 and twin-point distributor create special challenges.

The heart of the ignition system is the distributor – Lucas in the case of the Stag. The Lucas distributor is as reliable as any, provided it is given proper maintenance. This is actually quite simple and will reward the owner with increased distributor life and enhanced reliability, performance, and economy.

To service the unit, one needs only a small oil can, a tube of Bosch point lube (or similar), and a thin-blade screwdriver. Removal of the unit and major service require, in addition, a 1/2″ and 7/16″ combination wrench, and a Phillips screwdriver.

Before maintenance can be performed, one must first ascertain the condition of the unit through a thorough inspection. Carefully pry back the cap retaining clips by hand or with the aid of a screwdriver. Once off, rotate the ignition rotor counterclockwise to the full extent of its travel. If the rotor does not return to the home position, or is slow to return, the centrifugal advance mechanism is probably gummed up or dirty. Lubrication may solve the problem, and as this can be done “in situ”, it should be tried first. If cleaning is necessary, removal of the unit is required. Ensure also that the rotor is tight to the shaft. If it can shift back and forth on its own, the rotor key is worn and the rotor needs replacing. The operation of the vacuum advance, retard, or advance/retard mechanism should also be checked. [See Setting the Stag Idle Speed for additional information on this topic.] On Stags an incredibly high number of these units are defective or perished. This results in a poor idle and power curve as the car is relying entirely on the centrifugal advance and has a vacuum leak as well. To check the unit, follow the hose or hoses to the vacuum pick-up on the carb or manifold and remove it at this point. Leave it attached to the distributor. Examine the hose for cracking or brittleness and replace it if perished. It is of common size and can be obtained at an American (for the Stag, “foreign” parts house. After inspecting or replacing the hose, suck on the end of it and observe the resulting movement in the distributor. One of the following results should occur: (1) If no vacuum is built up, the vacuum unit has a hole in the diaphragm and needs to be replaced. (2) If vacuum is built up, but leaks down when you block the tube with your tongue, the diaphragm has a small hole in it and still needs to be replaced. (3) If vacuum is built up, but the plate refuses to move or moves sluggishly and sticks, lubrication and/or cleaning are required. (4) If the plate moves, then returns when you release vacuum, it is functioning properly.

Next, we remove the rotor and try to move the shaft side-to-side in order to check the condition of the distributor bushings. If this movement exceeds .002 to .003″, the bushings are worn out. Worn bushings cause irregular dwell, poor idle, and poor running. If worn out, the distributor must be rebushed, replaced, or an electronic ignition may be fitted.

Assuming the distributor has passed muster, we can lubricate it. The distributor is not self-lubricating (except for the lower bushing). Lubrication should be done sparingly and often (approximately every 3000 miles). Excessive oil or grease will be spattered on the points causing them to burn. Apply two drops of oil to the spindle bushing by way of the large screw located directly under the rotor. (There may be a felt pad on top of this screw to retain the centrifugal advance weights.) Apply one drop to each point pivot pin and one or two drops to the collar where the breaker plate and the lower plate contact. Smear a bit of the point lube on the cam faces (the area where the points touch the spindle). DO NOT use Vaseline or other types of grease as they will spin off the cam from the heat and centrifugal force. Other brands of point lube (such as GM) can be used if the Bosch type is not readily available. Also, smear a small amount on the distributor cap/body interface. This will seal out water, thus preventing cross-firing or grounding in moist conditions. Wipe off any excess oil or grease and carefully examine the cap and rotor before returning them to the distributor.

Examine the firing tip of the rotor for burning, the center of the rotor (where the carbon rod touches) for wear, and the key to make sure that the rotor will not rotate on the shaft. Check the cap electrodes for burning and the cap in general for minute cracks or carbon tracking. Any of these condition can cause misfiring under certain condition. Finally, check the center carbon electrode for wear. This is a common occurrance on the Stag. The new length is approximately 1/2″. If it protrudes less than 1/8″, it is time to replace the cap. When the electrode wears to the point that it no longer touches the rotor, the rotor will develop a cratered appearance from arcing. The rotor will also have tiny arc marks on it. At this point, performance will be seriously affected. Since the cap (Unipart GDC117) is fitted to very few other cars (TR8, Rover, and some Roll-Royce), the prudent owner should carry a spare in the trunk.

If distributor removal is necessary, i.e., to replace the vacuum unit, rebush the shaft, or to install electronic ignition, this can be done after removing the coil with a 7/16″ wrench. The coil must also be removed to set the timing, unless using factory tool S349. Before removing the distributor, align the engine at TDC and mark the position of the rotor with a small scratch on the distributor body to aid reinstallation. Do not rotate the engine with the distributor removed or you will lose the ignition timing.