ATD Springs

[Martyn Goodwin]

I have a "newish" Roy Price ATD that has been in use for around 5,000 miles over 12 months but lately I have had difficulty getting a reliable idle. I have deduced that what is happening is that the ATD springs have lost some strength thus at (hoped for) idle the ignition remains a tad advanced and thus the idle speed is unpredictably fast, so newer stronger springs seem to be in order. Does anyone know the specification for the ADT springs - that is free length from eye to eye and design poundage (should that be ounce-age) of the springs at 10 % extension? I hope to use this information to get some new springs made at 10 , 20 and 30% stronger in the hope of finding a suitable configuration to ensure that on idle the ATD springs move the mech into the fully retarded position. Thanks, Martyn

 

[chankly bore]

There used to be a stronger spring for sidecar users, the Lucas part no.was 416001. But at present it seems only standard strength is available. Several different people seem to supply them, such as SRM and Green Spark Plug in the U.K. and Domiracer in the Untied States. So, assuming they do not ALL come from the same backyard in Uzbekistan you may get some variation if you want to spend the money. See MPH no.82. Only standard in stock in Victorian Spares. The design is a bugger anyway- two curved ends in two parallel holes.

 

[vibrac]

There used to be a stronger spring for sidecar users, the Lucas part no.was 416001. But at present it seems only standard strength is available. Several different people seem to supply them, such as SRM and Green Spark Plug in the U.K. and Domiracer in the Untied States. So, assuming they do not ALL come from the same backyard in Uzbekistan you may get some variation if you want to spend the money. See MPH no.82. Only standard in stock in Victorian Spares. The design is a bugger anyway- two curved ends in two parallel holes.

AND the Vincent Spares Company

 

[Big Sid]

The ATD spring strength isn't critical but the length eye to eye is . The end loop must rest firmly on each end and still be able to tug against the peg , not just lie there . You can clip off the one end of the spring looking stretched and reform a loop the result being a bit shorter , to better tug nicely in the retarded position .
Done this several times to good results, a nice steady idle results . Sid .

 

[Martyn Goodwin]

A good nights sleep is (possibly?) a wonderful thing. The reason I am thinking weak ATD springs isbecause of what I observe when I am setting up the timing. Before I fit the ATD I can flip the ATD mechanismand it returns smartly to the fully retarded position – all seems OK BUT once I install it on the magneto with thetiming set fully retarded at 6 BTDC then if I flip the ATD mechanism manuallyit does not return to the fully retarded position, but seems to “get stuck” midway.

Last night (late) I replaced the ATD springs with a knowngood and new set of ATD springs (thanks Charlie) then reinstalled all – and observedEXACTLY the same behaviour of the ATD.

With hindsight I wonder if this in fact normal?? I am now wondering if what was making it appearthat way is, installed on the magneto the ATD springs – in a static positionwith the mag points just opening – are also fighting the magnetic force that isacting on the ATD armature trying to rotate it and in doing so giving me thefalse impression that the ATD springs are too weak?

Martyn

 

[Big Sid]

Might be the drag of the fiber point heel over the cam ring lobe . A light smear of HMP grease over the lobes to lessen drag here might help . Sid .

 

[Big Sid]

The ATD device itself might be stiff internally . Spray it deep inside with light oil . Sid . 3in 1 is good .

 

[Martyn Goodwin]

Today I visited Laurie Binns (think of him as an Australian Big Sid melded with a 19th century master craftsman) anyway in conversation I told him of my ATD problems and he strait way asked if there was a ridge on the magneto shaft that the ATD fits on - and yes there was. Laurie diagnosed the problem as being the rear of the ATD fouling on the case or the Magneto seal. His advise was to file around 0.010" off the boss on the rear of the ATD - I did this and like magic full ATD functioning was restored. While I was at it I also put a smear of HMP grease on the magneto cam (thanks Sid!).
All buttoned up, timing set at 6 BTDC fully retarded; 35 BTDC fully advanced the Bike starts first kick and (for now) is idling smoothly and riding well.
What a bastard that the ALTON chose today to die! At least I have a freshly restored Miller Geny on hand.
Martyn

 

[Big Sid]

Great idea , the rear rubbing . Never saw that . Sid .

 

[Ken Targett]

Good to hear you've got the ATD sorted Martyn.

The ATD spring strength isn't critical but the length eye to eye is .... Sid .

Good point, Sid. Got me thinking. A magneto ATD is very different to an auto advance in a coil ignition's distributor. In the distributor, the bob-weights are working against nothing much other than the springs, and so the spring characteristics are fundamental to the performance of the thing. In a mag ATD however, the bob-weights attempting to advance the timing are working against the magneto trying to retard it, and the springs merely provide, I'd say, a bit of 'fine tuning.' I did some experiments at the weekend, represented by this graph:

By "100% preload", I mean that the tension in the spring at fully-retarded is the same as the tension in a non-preloaded spring at fully-advanced.

Further details at http://brightsparkmagnetos.com/faqs... is the spring rate and preload in an ATD.htm

Ken.