Need Help with Ignition Advance Unit - Coverting to 12 V

[tonyduc]

All
I have a 67 250 Mk3, I just purchased the Electrix 12V Conversion kit for NC. It requires either a battery or a power pack (not sure what that is). It's not mentioned in the instructions, but to convert from the magneto bikes with 18 degree advance units, do I need now need the 28 degree unit (from battery bike)? I am also not exactly sure which advance my bike came with - I bought a timing cover case from 175CC bike (pic below), and the unit cam lobes from the 250 and 175 look the same? So perhaps I already have the 28 degree advance ?

Also, I am assuming that the degrees from TDC that I set on the flywheel now is now also different, for the MK3 magneto bike it is 32-36 degrees, so I would now use a setting for a battery type bike? The Electrix flywheel pictured below does have these markings.

Thanks for your help as always !
Tony

[veloduke]

Those units are from a battery ignition system (28 degrees). As you mention magneto ignition, can I assume you have no battery at present?
I don't have a MK 3, but the workshop manual specifications say magneto ignition, and the electrical schematic also does not show a battery.

Do you have 3 wires coming from the original alternator? Two for the lighting and one for the ignition coil? In this case, flywheel positioning is important.
The ignition relies on a pulse of energy from the alternator to the coil in order get a spark, so the flywheel has to be set up so this happens at the right time.

The battery ignition system does not need specific flywheel timing as the energy to the coil is supplied by the battery, not the flywheel.

To use the Electrex unit, which comes with a regulator/rectifier, I think you will need a battery and different ignition coil, and do some rewiring.

The fully advanced ignition setting would remain the same.

[tonyduc]

Thanks, Max the units being 28 degrees is good news. What’s unusual is that the alternator had 3 wires and the 2 coil stator which is typical for a no battery bike - I bought the bike when it was a basket case so it’s hard to know what was original. But it did appear to be a magneto MK3 minus the 18 degree advance unit. But that’s all moot now since I have the kit, it was important to know what advance unit it had.
Thanks
Tony

[Bevel bob]

Brit MK3 NC bikes may have had normal battery ignition like the Mach 1 . flywheel position wont matter. Ign timing 36 full advanced or 8 static . Most likely a 66 bike registered 67. The factory set them up with more advance but it makes harder starting and easier to knock out big end. Keep motor above 3000 and below 8300 if you want the big end to last.

[Eldert]

Hi Tony

are you sure about AA 369 ? could that be a AA 359 ? anyway it advances 28 degrees .

the AA unit that advances 18 degrees is the AA 367 B . the breaker cam is also diff shaped to saturate the AC coil .

Eldert

[Bevel bob]

I think Eldert is right (he usually is!) those units look fine. If you find setting the timing difficult ,the 8 degree static position works well for me. The ability for the bike to give a feeble kickback seems to be around 5 degrees adv. which gives a very slightly lazy motor and less stressfull starting.

[veloduke]

Yes, 359 can be seen stamped into one of the units in the photos.