E: Engine Unsteady Idle

[Robert Watson]

And the state of the springs that bring the bob weights back to full retard at the idle is ?

We had some new springs wound but found them to be too strong. You could feel a definite sudden advance well up into the rev range. I stretched them and bit and that worked better. I also acquired a pile of spares co springs and all were somewhat better than the clappped out ones on that particular ATD.

You can change them through the inspection cover if required, just thread a string through the middle of the spring with both end outside the timing chest in case you drop the spring!

 

[Martyn Goodwin]

And the state of the springs that bring the bob weights back to full retard at the idle is ?

We had some new springs wound but found them to be too strong. You could feel a definite sudden advance well up into the rev range. I stretched them and bit and that worked better. I also acquired a pile of spares co springs and all were somewhat better than the clappped out ones on that particular ATD.

You can change them through the inspection cover if required, just thread a string through the middle of the spring with both end outside the timing chest in case you drop the spring!

On inspection I estimate around 10% preload at rest. I have noted though different bob weights have the spring post and bob weight pivot at slightly different centres, so on one bob weight the spring will seem fine, but on a different bob weight it will be too slack, ie no preload.

 

[BigEd]

In a recent thread about carbs I posted regarding an issue I have had with an unsteady idle speed with my Comet. With luck our moderator will spot those posts and move them into this thread.

I have copied over posts mentioning the ATD. Hope that is OK.

 

[erik]

You can use a stroboscope to observe the atd unit .Erik

 

[greg brillus]

From memory (whatever that is) most advance units that use some form of counter weight and spring, tend to be different to each other.........One will shift more easily than the other........Well that was my observation from years of working on distributors in car engines.

 

[oexing]

Many ATDs have two different springs, one rather weak spring for very low engine speed from idle. The other spring is a lot stronger with some slack at idle. So this cares for higher revs. Don´t know about the Lucas type but is it a good idea for looking for two types of springs ?

Vic

 

[Simon Dinsdale]

Many ATDs have two different springs, one rather weak spring for very low engine speed from idle. The other spring is a lot stronger with some slack at idle. So this cares for higher revs. Don´t know about the Lucas type but is it a good idea for looking for two types of springs ?

Vic

Vic
I would rephrase that to "many modern ATD's". I have always suspected the original Lucas magneto ATD is more of a switch and just bangs from fully retarded to fully advanced in one go and not a gradual advance.

 

[oexing]

But then I´d want to have it more gradual - as was found on many classiscs, at least on most bikes - and cars - on the continent. I´d go for full advance not before 3500 rpm, this simply a matter of decent springs, no rocket science.

Vic

 

[Martyn Goodwin]

Vic
I would rephrase that to "many modern ATD's". I have always suspected the original Lucas magneto ATD is more of a switch and just bangs from fully retarded to fully advanced in one go and not a gradual advance.

That was my observation of the Lucas unit. I have attempted to slow down the rate of advance by reducing some weight from the 'free' end of the bob weights. Also results in full advance being achieved at higher revs. Picture is before and after.

 

[Martyn Goodwin]

Interesting paper from BrightSpark magnetos regarding ATD performance