Sticky AAU

Gilesy

VIP MEMBER
Joined
Aug 27, 2017
Messages
192
Country flag
Finally got my N15cs running. Engine has been completely done, a lot of the work by Norman White. The mag has been rebuilt by NOC magneto specialist Paul Wolf. New monobloc carbs, cleaned and blown out. The bike starts easily and ticks over nicely. However, when put under load it misfires and splutters, hardly able to pull the bike along. On blipping the throttle it will over run. The timing is set (and checked) at 30deg. The AAU seems very stiff and is reluctant to return to its retarded position. The chain goes tight and loose but even in its tight mode there is still slack. If I remove the chain and just put it on the mag taper it snaps back nicely, as soon as the chain is on it is stiff. I've put on new springs and cleaned it as thoroughly as I can. Any ideas? Even how to dismantle it would help.
Thank you in advance.
Giles
 
Finally got my N15cs running. Engine has been completely done, a lot of the work by Norman White. The mag has been rebuilt by NOC magneto specialist Paul Wolf. New monobloc carbs, cleaned and blown out. The bike starts easily and ticks over nicely. However, when put under load it misfires and splutters, hardly able to pull the bike along. On blipping the throttle it will over run. The timing is set (and checked) at 30deg. The AAU seems very stiff and is reluctant to return to its retarded position. The chain goes tight and loose but even in its tight mode there is still slack. If I remove the chain and just put it on the mag taper it snaps back nicely, as soon as the chain is on it is stiff. I've put on new springs and cleaned it as thoroughly as I can. Any ideas? Even how to dismantle it would help.
Thank you in advance.
Giles
I don't know if this is any use to you but I have an AAU from an 850 Commando that worked well when removed (don't know if it's the same as yours) you would be welcome to it FOC. If you want it just send a private pm.
 
Overly tight magneto drive chain will cause AAU to "stick".

Your description that chain goes tight and loose suggests the magneto armature is not properly centered.

Slick
 
I would think it is where the armatures magnet stiction is aligned within the magnets then loose prior to flipping to the opposite pole snatching the chain in the opposite direction.
The magnetic drag of a magneto creates a very heavy drag for the AA unit to overcome and causing the armature to in fact advance in opposition to the drag. I highly suspect manual minipulation may be quite deceptive to analyse .
On the other hand points/coil friction drag on the P/C AA cam is very minor in comparison.

However your description does sound like mechanical tightness of some sort since the AA springs and revolving friction and magnetic stiction should easily cause the armature to fall back and return to it's most retarded position compared to the drive chain..
ADDED: Well TS thinks it's funny, but not nearly as funny as a points/coil AAU going into a magneto....I own 10 magneto engines and rebuild my own mags. I don't need to "send them out"
 
Last edited:
Probably worth checking that the magneto flange is flush to the back of the timing case. If Norman rebuilt the engine he presumably replaced the magneto chain. A new chain shouldn't have bad tight/loose spots - unless it's defective. So wondering if the gasket or something else is keeping the magneto shaft from being properly perpendicular in the timing case and putting a load on the chain.
 
That's very kind but I think the AAU for Commando points is different from the one for a magneto, but thanks anyway.
Cheers
Giles
Even if it did not have a mag, a 20 series engine it would likely have rear points and therefore would have a CW AAU like triumph.
850 commando AAU is CCW. But you knew that....
 
Norman rebuilt the crank and put new seats, guides and valves in the head, I did the rest. It has new mag and cam chains. Cam chain also goes very tight/tight. The mag chain is loose/ looser. As soon as the chain is on it stiffens up, even when chain is loose. There is some play in the AAu shaft, so maybe that causing stiffness. Could the intermediate gear be causing tight loose issue? I'm pretty sure AAU for mag and points/coil is different. Commando AAU number is 54419344 and Mag AAU is 47508D.
 
As soon as the chain is on it stiffens up, even when chain is loose.
Curious as to what "stiffens up" is. "pulled taught"? will it pull slightly side to side if slack? or stiff flexing up and down?
Is the intermediate shaft held firmly in the cases? Why so tight on the cam chain? The top cam chain run would alway be tight if the cam is opening a valve. bottom run will be tight if a valve is closing.
 
The AAU mechanism is stiff, If I turn it to full advance the springs will not return it to its retarded position. I'll check what you say regarding valves opening/closing and its effect on cam chain tomorrow. I have checked and mag flange appears to be flat against case. I have a spare intermediate gear so thinking of trying that.
 
The AAU mechanism is stiff, If I turn it to full advance the springs will not return it to its retarded position.
If correc this is fairly normal due to the internal magnetic stiction. Normally it will return fully only by turning the engine forward and the "drag" makes it close(retard) then finally rotate with the engine once the AA has retracted/retarded.
But yes failure to retard while running, as verified with a timing light is definitely unacceptable and an indication of a problem.
 
Check out the experience one of our members had with uneven magneto chain tension.....


Slick
 
Cleaned one of my aau with penetrating oil, a bit of the brown stuff came out of the spring pivots, but was still sticking, until I realised I was holding it vertically when it is mounted on the magneto shaft horizontally, then it stopped sticking. Have you tried this?
 
If the AAU shaft is worn, I suppose even slight chain tension can cause it to bind, giving the symptoms you describe.

The UK has the best AAU repair facilities. Look up Priority Magnetos, and Dave Lindsley.

Slick
 
Thank you for all your replies, I've now got a few things to think about and try.
Cheers
 
Back
Top