Auto advance unit

Hi
Working on a ‘73 750 with stock ignition system. The auto advance unit doesn’t seem to rotate very far, and I can’t get anywhere near the specified 28 degrees of advance, with both static timing (with AAU locked) or strobe at 3000rpm.

With the point plate removed, the AAU springs are both in place and return the weights to the closed position. However, rotating the points cam counterclockwise only moves the weights partway out…there’s still room in thh HR r weight slots for the pins to travel.

Is this normal? How far should the point cam be able to rotate?

Thanks
Marc

Are you allowing for the fact that the AAU range is 12 camshaft degrees = 24 crankshaft degrees (therefore, fully retarded is approximately 4 crankshaft degrees)?

They should rotate fully to the end of the slots. Often there is a depression or "groove" worn into the slots on the weights and they will stick at that point - either advancing OR retarding. You can smooth that out with a file. The AAU weight spindles/slots require periodic lubrication but many people ignored that, which causes the wear.

MexicoMike said:

They should rotate fully to the end of the slots.

Click to expand...

The travel is limited by the advance cam pins in the back plate slots, not the bobweight slots.
Edit + Pic.
Edit 2: Pic. update (the first was of a clockwise AAU):

Thanks to both; L.A.B, might you have a pic of the other side in the fully advanced position?

We might be saying the same thing.

MarcD said:

Hi
Working on a ‘73 750 with stock ignition system. The auto advance unit doesn’t seem to rotate very far, and I can’t get anywhere near the specified 28 degrees of advance, with both static timing (with AAU locked) or strobe at 3000rpm.

Click to expand...

If the AAU is locked in the fully advanced position and you can't get 28 degrees it means that the AAU needs to be broken loose from the taper and repositioned.

MarcD said:

L.A.B, might you have a pic of the other side in the fully advanced position?

Click to expand...

Ok, so AAU in the fully retarded position...


...and with the cam fully advanced. The cam pins about halfway along the bobweight slots.

Just realized that the advance unit isn’t keyed to the cam…are there any marks to indicate where it fits on the taper?

I’ve seen mentions in other ignition related threads that the timing index behind the inspection cover on the primary drive side is not always accurate. Can this be checked against actual top dead center at the spark plug hole?

MarcD said:

Just realized that the advance unit isn’t keyed to the cam…are there any marks to indicate where it fits on the taper?

Click to expand...

No, see manual section C38:
"Ignition Timing Basic Procedure Prior to Checking"
An alternative method is to lock the AAU at full advance and with the AAU not tightened, set the opening point at 28 degrees BTDC with the points backplate at approximately the mid-slot position.

MarcD said:

I’ve seen mentions in other ignition related threads that the timing index behind the inspection cover on the primary drive side is not always accurate. Can this be checked against actual top dead center at the spark plug hole?

Click to expand...

Yes, requires a degree wheel and piston stop.

"The travel is limited by the advance cam pins in the back plate slots, not the bobweight slots."

Thanks for correcting me on that - it had been a while since I had a Norton AAU in my hand. However, I have seen/worked on several that the slots needed filing to prevent the advance weights from hanging up. Also, several that were stuck in full advance and, a few stuck in no advance.

I'd recommend complete cleaning and inspection to be sure the weights move back and forth with no obstruction. Then lubricate the pivot points and the sides of the slots, install a new set of springs, and reinstall the unit per the earlier posts, ensuring you can achieve proper ignition advance when the weights are fully 'out.'