Originally equipped with a magneto, instead of getting a full 180 degrees of crank rotation for full coil saturation, only 50 degrees passes before the next plug fires, thus making it
imperative the magneto be in perfect working order to produce a spark in these relatively high compression for the era rich running motors. Having been quite happy with magneto’s on old BMW’s, elected to continue with same in this application rather than experiment with the variety of battery powered electronic ignitions used on my other machines. Was nominally satisfied with the original Lucas magneto/ATD configuration until installing a BT-H on my newly resuscitated Shadow in December of 09.
More info regarding the Lucas KVF magneto as used on Vincents can be found here at thevincent.com
- Lucas magneto: Whether you’ve bought a basket case or an old rider, regardless of the seller’s claims of this instrument’s health, have it rebuilt by someone reputable. It can lead to a legion of woes, hard hot/cold starting, plug fouling, misquided symptoms of carb leaness. In the U.S., Doug Wood can rebuild both your Lucas magneto but Miller D6 and Lucas E3L generators. Click here to generate an email to him.
- ATD: Like their hydraulic shock absorbers, Vincent was one of the early developers of automatic timing using the now tried and true method of pivot weights and springs to advance/retard the timing. Unfortunately, unlike a relatively light distributor assy, a magneto presents a far greater load at the shaft
for the ATD to control because it is basically a generator. The existing unit has no margin to accommodate wear before accuracy is lost generally resulting in stepped advances through the rev range and the unit hanging on decel leading to elevated idle from too much advance. Replace your springs and ensure the pivots are not worn.
For those desiring greater tuning accuracy, this is a single fire system and, unconventionally, the points attach to the magneto armature and rotate in a slip ring attached to the body. It is not uncommon to have the two lobes on these slip rings out of phase to one another. Short of reshaping them to exactly 50 degrees apart from one another, a quicker remedy is to split the timing difference between the cylinders by checking point opening in relation to advance timing setting with a degree wheel.
- Points: Unlike battery ignitions with constant high voltages across the points, magnetos are comparatively easy on theirs, if you can at all find original Lucas platinum points for this application, do so. With diligent greasing of the point block, they can be nearly life of the magneto unit itself.
- Spark Plug Caps/Wires: Original sparkplug caps (MO39-PR37) are being faithfully reproduced. If you will be using these, and especially with a high powered BT-H magneto, to avoid "flashover" at the first hint of moisture wherein the spark jumps from the terminal to the metal jacket anchored in your head you will need to make some rubber seals that fit tightly to between the cap and porcelin.
As both HT leads anchored to each other and in turn to the oil feed line at the head, especially with a BT-H, you want as heavy and exterior casing as possible to avoid crossfire as well as copper core with no resistance to dampen spark. This wire can be acquired in raw length from vintage American motorcycle dealers as well as shops catering to old cars.
- Spark Plugs: At the risk of delving into the oil subject where everyone has a favorite, you want several things out of your plugs: extended tip for good projection into these hemi heads which at this small a displacement obscure flame fronts in single plug applications on domed pistons, little resistance and if your machine an oil burner, self cleaning as provided by multiple ground plugs as well as fine wire for lower voltage demands on the original Lucas. A standard starting point is the NGK BP6ES and BP7ES which has become hard to find. BT-H has some modern alternative recommendations here.
- Modern Magneto Alternative (BT-H): Better than any battery powered electronic ignition in this application requiring a more robust/reliable charging system, there is now a modern alternative absolutely superior to the original and made by BT-H in England. Not only do you get even ignition strength on both cylinders, being capacitive discharge (CD) the spark is stronger and improvements in the generation II unit provide more powerful spark at lower rpm for ease of starting and results in cleaner plugs in low speed urban riding. This unit controls advance electronically which means elimination of the troublesome ATD, replacing it with a nylon-type or fiber solid gear.
Being more efficient, less total advance is required, thus best results have been obtained with settings at 34.5 - 35 degrees BTDC. In addition to getting it directly from BT-H, you can order it through the VOC Spares Scheme. Be sure to consult the BT-H site for the part numbers for the fixed gear drive wheel assembly as it’s not included with the magneto. You can review a customer's documented install process here which can be performed in less than an hour without modification to your machine. These drive gears come in both plastic and fiber like the original, I run one of each in my two machines and have noted no wear differences to date. Note the coil anchorage beam on the left, being secured only at one end with the mass of the coils at the other can amplify any motor vibration. As CDI coils operate at much higher voltage than inductive, it is imperative the small primary wires not rub on the engine cases - gingerly bend the spade terminals - nor the secondary wires foul your side stand springs. Post 2007 ignition coils have threaded prongs for turning your plug wires onto. Make a small pilot hole before turning these wires on to ensure they are centered and the wire cores making good contact.
With heads in place, the much preferred method for timing this device is using a degree wheel with a plug stop. Pictured is a plug stop made from a gutted spark plug and a 1/4 inch carriage bolt whose cylinder insertion depth is controlled by adjusting the stop nuts (lower one turned off round for clearance) on either side of the plug body. The item on the left fixes the timing disc to the main shaft inserted bore after removing one's oil quill. Using a tapered plastic shaft, drill through a commonly available tapered rubber stopper
and insert the shaft,
drawing the disc up the taper clamps it tighter to the plastic shaft. I mount my pointer to the bottom side of the kicker arm securing bolt.
A word on ignition systems as it relates to today's lower recommended total advance timing on your Vincent. A traditional battery coil or magneto system is inductive. As the points close, the primary voltage is fed into the secondary winding of the coil to create a spark. The amount of resistance (compression the dominant factor here) determines when/how strong that spark is at the plug. The coil "pushes" against that resistance "swelling" till it's overcome and the spark occurs. Other influences include your condensor which is rated to the coil whose voltage input it is controlling from gapping the points prematurely before closing (rated in microfarads). A capacitive discharge ignition (CDI) on which the BT-H is based works much differently. Besides eliminating inefficient mechanical switching, it generates a very strong primary voltage which is fed to the coils which in turn amplify - so to speak - this voltage with a far more rapid voltage rise-time before delivering a very strong quick spark. CDI is ideal for engines tending to run rich, typical of older designs like our Vincents. Some CDI's, mimicking transistor assisted ignitions (TAI) produce multiple sparks during the event much like you would strike an arc with a welder.
Thus, between fuel formulation changes over the last 60 years, the availability of a very quick firing CDI ignition for our application and possible unknown compression ratio/induction changes to your machine original timing specs in the manual are merely a rough baseline. Ultimately, only a constant load dyno to determine max torque will indicate where perfect timing is achieved. As most Vincent owners are more concerned with tractability on engines not in a high state of tune, more conservative timing settings will greatly ease cold/hot starting, low speed operation and help soften the effects of worn carb slides in particular, thus, the settings stated above.