How strong should AAU springs be. It seems that whenever I buy new AAU springs they are wound of much lighter gauge wire (with more winds) that the old heavier ones I take out. They don't have anywhere near as much snap the heavy springs have. The heavier springs seem to get sprung easily if one is not careful when fiddling with them. It seems everyone always says get the strongest springs possible for the AAU. Are the lighter springs even worth installing? Anyone have a source for heavier AAU springs?
Different springs were fitted depending on the advance characteristics needed for a given application. I suspect by now, many of the NOS springs you find might be obscure ones that did not sell in the old days.
Ones from new manufacture are likely made for a popular application that required full advance at low revs ... not necessarily the same as your bike and possibly not suitable.
Are your old springs broken? If not, have you strobed your ignition to see if the AAU operates acceptably? In my experience, the later style of Lucas AAU becomes unserviceable not because of the springs but because of wear.
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My present project is a 1965 T20M. I have a unused AAU I'm installing. One spring on it was fine and the other stretched out, why I don't know. I grabbed my only spare unused AAU springs and they are so light weight I could nearly get the unit to advance by spinning it in my fingers. Is there such thing as too tight/strong springs? I tend to pick up unwanted AAU's and have salvaged perfectly good springs from them in the past. On occasion I get a perfectly fine, not worn out, AAU also. Peter
There's certainly such a thing as too much initial tension in the retarded position.You can end up getting no advance until 2000 rpm or more,although it might still get full advance at 3500 or 4000 rpm. In a case like that,if you're aiming for full advance at 4000,a higher spring rate would help.With the correct tension at full advance,there would be less tension in the retarded position.
A very high spring rate,adjusted for full advance at 4000,would have no tension in the retarded position.This could get you 10 or 15 degrees of unwanted advance as soon as the engine starts.
It's much better if advance doesn't start happening until about 1200 rpm.If it's above idle speed,you get a more stable idle because the timing stays in the retarded position and doesn't vary.
Springs from large oil seals (to suit a 3" or 4" shaft) can be useful.One of these can be turned into plenty of advance springs. Leave the maximum possible number of coils on the spring for a softer spring rate.Make one with less coils for a higher spring rate.