British Motorcycle Electrics Advice

My 1965 Norton Atlas (applies to many other bikes) has three wires coming from the stator and the replacement you have sent (part no 47205) only has 2 wires. I must have ordered the wrong stator.

The three wire single phase stators used on many 12 volt bikes in the 1960′s are electically exactly the same as the later 2 wire type. On the early stators the wound coils are visible, on the later ones they are encapsulated for protection. These stators are not to be confused with the 3 wire 3 phase type which were not fitted as original equipment before late 1970′s Triumphs. It is easy to tell the difference – single phase stators have 6 wound coils with 6 groups of steel laminates visible, 3 phase units have 9.

The way these early three wire stators worked (take a look at the wiring diagram in your workshop manual to confirm this) is that AC output across the green/white and green/yellow wires only has 2 of the 6 stator coils producing voltage and these go directly to the rectifier. This is quite sufficient to keep the bike running and the battery charged up as long as the lights are not on. The green/black wire goes right to the lighting switch. When the switch is turned on it performs two distinctly different functions. Firstly it connects the battery to the lights and secondly it connects the green/black wire back to the same rectifier terminal that the green/yellow wire is already connected to. This has the effect setting all 6 stator coils into operation to produce maximun output. In order to control this output the zener diode was fitted.

It was later decided to connect the green/black and green/yellow wires internally in the stator so that all 6 coils could produce maximum voltage all the time. Most of us ride with the lights on all the time and even if we don’t zener diodes or modern solid state power boxes have proven to be reliable.


Do Zener Diodes ever fail of if they get packed with dirt would they be unable to dissipate heat and cause the battery to boil?

Lucas Zener Diodes are generally pretty reliable and it is unusual for one to fail however they do need good air flow around them and their heat sinks. Problems are more likely due to poor grounding. If the current can’t flowe through the diode, usually because of a loose or dirty fitting, then the excess current has to go somewhere – eg coils/Boyer/bulbs/battery.


Boyer Ignition System Thoughts

We are asked on an almost daily basis for our thoughts on various makes of ignition systems, other than Boyers. We have sold more than 15,000 Boyer kits over the last 20 years and in all honesty have had less than a dozen which could be considered genuine warranty problems (other than customer induced faults). We had one customer make a claim on a burnt out box and then claimed again a few weeks later on the replacement. On investigation we found that he didn’t consider it necessary to have a zener diode on his bike and therefore had no voltage control. From personal experience I don’ like to run a Boyer without a battery. While it should be OK if the alernator etc is in really good order, starting can prove difficult, although fitting an additional capacitor can help. This may well be improved with the new Mk4 kits, we have still to do some testing. Ther only other occasional problem occurs, usually with Commando’s, where the wires from the pick up break inside the insulation within the timing cover. This is caused by vibration so the wires need to be well supported in there. The problem with a voltage drop on electric start bikes appears to have been corrected with the new Mk4 kits whifh will continue to operate OK at much lower voltages.

We frequently get Boyers returned by customers as warranty claims, which is why I built a test bench.There’s nothing wrong with 90% of the kits returned and we then assist the customer to solve the problem.

I have noticed one of the competitors has been making derogatory comments about other manufacturers’ products (Boyer kits) in his ads. I really don’t like this practice (GM are doing it right now as well) and feel if your product is good, featuring it’s own benefits and features should be sufficient.


Boyer Kits With Magnetos

Kirby Rowbotham in the UK has been making conversions to fit these for many years and has a good reputation. You retain the original magneto, so the bike looks “correct”, but you still have to find a location for the coil/s and need a battery to support it. My preference has always been to get the mag properly rebuilt and stay original but if there’s enough demand from customers I’d look at carrying the conversions. I chat with Kirby about this every April at the Stafford show in the UK and am inpressed by the quality of his products.


I’ve owned a 78 Bonneville for many years and it now won’t idle on one cylinder.Once it gets up to around 2,000 rpm it runs OK on both cylinders. I think it is electrical, it went OK with original ignition system but now has a Boyer and I think this may be the problem.

Always difficult to answer these kinds of questions by email but there’s some things which should be looked at. Any fault with a Boyer will will always show up equally on both cylinders. This type of problem comes up regularly and the most common cause is a blocked pilot jet circuit in one of the carbs. The first thing to do, if running with a Boyer, is swap the plug leads over – just take the caps off and put the left on the right plug and the right one the left plug. If the problem moves to the other side, the cause must be a fault with the coil, HT lead or the plug cap. If the problem stays on the same side, it must be something wrong in the carb or a mechanical problem connected with that cyclinder.


My bike runs fine most of the time except under hard acceleration or pulling hard uphill.

Could be a lot of reasons and one could be fuel mixture at high throttle openings however it should be borne in mind that the plug needs more voltage to spark when the compression in the combustion chamber rises (as would be the case in the situation described). Also too large a plug gap requires more voltage and becomes a problem first evident under load. Low battery voltage or coil problem likely with coil ignition, loss of magnetism in magneto is possible with mag ignition.


Higher Output Alternators

We are frequently approached by customers who feel they need higher output alternators because their batteries are going flat. While high output 180w single phase and three phase stators are readily available, in 90% of cases the problem can be solved by fitting a new magnetic rotor. There’s only one type available (either genuine Lucas or Oriental reproduction) and it works with all three types of stator. For a quick check, a single phase 120w stator (Lucas no 47205) with a new rotor kicks out 20V AC at 1750 RPM. When we get them in for testing a lot of them are only kicking out half that figure.