K2F Magneto Auto advance unit

APRRSV · Nov 25, 2016

Does anyone know what the range in degrees is for the auto advance unit used on Norton twins? Specifically, Atlas and G/N15's.

Thanks,
Ed

texasSlick · Nov 25, 2016

Off the top of my head, I would guess 28 degrees. I could make a measurement on an AAU in a few days if no one else chimes in with a definitive answer.

Slick

APRRSV · Nov 25, 2016

Hi Slick. Thank you for the input. Are you saying that AAU unit, if set for 28 degrees advance at, say 4000 rpm, would be at zero degree advance with engine not running or at slow idle?

Rohan · Nov 25, 2016

Interesting question, don't think I've seen a number quoted for this anywhere before.
You always just set them for the full advance setting, and take whatever they give for retarded running....

Commando, late model (points on cam) has a stamped 12 degree advance/retard unit on it.
Being run at half speed means it gives 24 degrees of advance/retard - all being well.

texasSlick · Nov 25, 2016

APRRSV said:
Hi Slick. Thank you for the input. Are you saying that AAU unit, if set for 28 degrees advance at, say 4000 rpm, would be at zero degree advance with engine not running or at slow idle?

Yes, if my guesstimate is accurate.

I have an Atlas K2F AAU on the bench. I will measure the angle of travel next Monday. I have always wanted to know the range myself.

Slick

APRRSV · Nov 26, 2016

Rohan,
This is very useful information. Thank you.

Slick,
I would greatly appreciate your measurements. Any chance you want to sell the AAU you have on your bench? See my wanted ad.

Thank you both.

Ed

Triton Thrasher · Nov 26, 2016

The range of advance is not critical.

texasSlick · Nov 26, 2016

Surfing the Internet, I found some specifications on Lucas ATD's (automatic timing device ... also called automatic advance units AAU)

http://www.brightsparkmagnetos.com/libr ... ations.htm

From the data given, the range at the magneto varies from 11 to 18 degrees ... double that for crankshaft degrees. It is bike and magneto model specific.

Note the data includes UP TO 1957 Models.

I will post measurement on my 1963 Atlas ATD later.

@APRSV ... Sorry mine is not for sale.
I did find a site that will build an ATD to suit. You might look into it.
https://sites.google.com/site/priorymag ... s-in-stock

Update:

Measured my AAU. I get 12 degrees at the magneto, or 24 crankshaft degrees. I did not find any stamped values on the AAU body as Rohan did.

Thus, if I were to set the timing to 32 deg.BTDC as the Atlas manual so states, then the engine would fire 8 deg BTDC on kickover (32 - 24). Seems like that is a recipe for kickback, but in practice, that seldom occurs.

Slick

Triton Thrasher · Nov 26, 2016

texasSlick said:
the engine would fire 8 deg BTDC on kickover (32 - 24). Seems like that is a recipe for kickback, but in practice, that seldom occurs.

Slick

It's not critical.

You'd probably get off with anything from three to twenty degrees BTDC, for starting and idling.

Rohan · Nov 26, 2016

texasSlick said:
the engine would fire 8 deg BTDC on kickover (32 - 24). Seems like that is a recipe for kickback, but in practice, that seldom occurs.

Triton Thrasher said:
It's not critical.
You'd probably get off with anything from three to twenty degrees BTDC, for starting and idling.

Indeed, its not critical at all to the actual amount of retardation.
As long as its retarded some.

My old Enfield with a MANUAL advance/retard magdyno won't start if you give it more than half retard,
so its a long way from zero degrees, and starts quite easily with no kickback.
But if you don't give it any retard it kicks like a mule, and its quite a low compression engine.

acotrel · Nov 27, 2016

What does kick-back mean ? I used fixed timing with race cams in 650 Triumphs for years. If you don't bounce the piston off compression but come up slowly and only kick for the last bit, you don't usually get kick-back. If you do get kick-back, you should be able to hold it.

Rohan · Nov 27, 2016

acotrel said:
What does kick-back mean ? I used fixed timing with race cams in 650 Triumphs for years. If you don't bounce the piston off compression but come up slowly and only kick for the last bit, you don't usually get kick-back. If you do get kick-back, you should be able to hold it.

How much of the past century did you say you haven't ridden a road bike for -
and therefore haven't used a kick starter !!?

acotrel said:
If you do get kick-back, you should be able to hold it.

Thats a sure-fire recipe to be catapulted over the handlebars !!!
Even my mild-mannered tame old low compression Enfield can kick back like a mule,
if you don't retard the ignition before kicking.

A 500cc twin would only be 250cc per pot, and twins are notably much easier to start.
Providing the ig timing is in the ballpark...

robs ss · Nov 28, 2016

acotrel said:
What does kick-back mean ? I used fixed timing with race cams in 650 Triumphs for years. If you don't bounce the piston off compression but come up slowly and only kick for the last bit, you don't usually get kick-back. If you do get kick-back, you should be able to hold it.

Well maybe I should have been able to hold it but that didn't stop the throbbing pain in my armpit where my knee hit it.
I think "kick-back" due to overly advanced timing at start is a recognised phenomenon.
Pain is a cruel teacher!
Cheers
Rob