Works records interpretation

GBewley

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Every set of records I've seen mentions the size of the halftime pinion, usually the bearing manufacturer, and another note which I assume is a timing note, usually something like "2 degrees ATC" or similar. Does anyone here know how to interpret this last note? Is it an ignition timing note or a cam timing note or maybe something entirely different? And what is it referring to...piston at 2 degrees After Top Center, I'd assume, but I'm hear asking the question!

Thanks,

Glenn
 

GBewley

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Yes, were that the case I would think cam timing (equal lift on exhaust closing method) but I'm wondering if the works maybe had a fixture to check the actual advance of an ATD and used that to set the ignition timing without having to lock the advance, this figure (the retarded figure) being noted in the worksheets, due to variations in the ATD units.

Anyone else?
 

davidd

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Glenn,

Yes, only to say that it is more likely the cross-over point for intake and exhaust. I say this after seeing the notation:

Work Order Deviations.JPG

I don't believe it has anything to do with the ATD because this notation came from a Grey Flash Work Sheet with a manual advance magneto.

David
 

GBewley

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Dave,

Well, in that case, are we misinterpreting ATC, as the cams I've checked are all BTDC...could it be ante top center or something? Your point about the Flash makes good sense to me.

G
 

Simon Dinsdale

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If it was some sort of timing in relation to top dead centre surely it would be ATDC and not ATC ? Which opens the possibility of some other meaning like "actual timing correction" or something else.

There must be an angular tolerance for the main shaft half timing pinion key way to crank position for when the main shaft was pressed in the crank. I don't know how accurate in rotation position the timing side main shaft can be pressed in the flywheel half. Any errors can then been corrected via the multiple key way slots in the half time pinion, so is it something to do with that?

Simon
 

davidd

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Dave,

Well, in that case, are we misinterpreting ATC, as the cams I've checked are all BTDC...could it be ante top center or something? Your point about the Flash makes good sense to me.

G

Glenn,

Sorry, my Lisdexia was kicking in until I reread Tim's comment and realized I was going the wrong way. It could still be that is the low range of the retard measured on either the ATD or Manual, but it does not make sense to me with the ATC. Or maybe it does. I checked my Worksheet and it says nothing like this:

Deviations 3407.JPG

I understand that the more advanced the timing is the better the starting is, within limits. Thus, many electronic ignitions start the bike slightly advanced (from TDC) and then retard themselves for better idle. But, this still leaves you on the BTC side.

David
 

vibrac

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VOC Member
All thats worth saying is that the equal open valve method of timing at 4 degrees Before Top has to my knowledge only been in circulation for some 8-10 years But it was I am sure known to many previously and may have been prevalent in the Works even though the traditional opening and closing figures and five degree preload mantra was in all (had to be) the literature. Some indication of prevalence of this 'timing secret' is the position of the cam slot in the line drawing in Richardson
 
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