Forum

[Les H]

Hi Will. I thought I had better come back again on this one, as I didn’t have time to go through it all yesterday. I am going outline some extra considerations that you should/must take into account. The method of connecting both sets of coils in the alternator will provide the highest output which is required to enable the coil to produce a decent spark for starting. However the output is far too much for continuous running and there is a very good chance that you will burn out the coil. The zener diode was introduced to control the rising output v revs of the alternator and limits the voltage rise to about 12.7v and then shorts the excess current to earth. Without the zener, the voltage will rise far too high especially if the coil is the original 6 volt one. I would STRONGLY recommend you buy a 12 volt coil and a 12V zener diode, fitted to a good heat sink, this will take care of the excessive voltage/current. This is going to cost around 60-70 quid? (not much I reckon in motorbike part terms) If you do want to skimp on costs, I would recommend that you wire in another switch to disconnect the GREEN/YELLOW wire from the alternator (HIGH O/P COILS) from the cross connection point as shown on my diagram. Once started you must operate this switch to disconnect the high o/p coil. This will leave just the LOW O/P coil (Green/black) feeding the coil which should be sufficient to power the coil. To me, this seems a lot of bother especially as you are likely to forget and eventually burn out the 6v coil. It is obviously up to you, but I would recommend using a 12 volt coil and adding the diode all as per diagram. BTW use a decently thick wire for the zener connection to avoid voltage drops along its length and inaccurate voltage regulation.

[The_Italian]

Thanks Les H and thanks Will.
I will try and let you know.

ride safe

[will]

Once again many thanks. Will take all the information on board, Not quite ready for final wireing and kick over. but very near. Many thanks.

[Les H]

Slightly modified to include earth polarites and 12v zener as NOT an exta.

[will]

Once again great diagram. Final question manual shows earth from positive side of battery. so is bike positive earth, and if you do not have manual how do you know. Thank you the thick one.

[Les H]

You can keep it the same, with a Positive earth, most British classic bikes used positive earth and the original zeners were only available, with Pos earth. Nowadays you can get zeners with negative earth at the same price, so you have a choice, but as long as you get the polarities right on the capacitor and zener, there no advantage apart from most modern electronic items being neg earth but really and it won't matter which you choose.

[will]

Many Thanks LES H you have a Wealth of very useful knowledge. thank you.