help: confused re timing marks

[DewCatTea-Bob]

[quote= cooperplace ...

____ Once you go-ahead & install the AAU in the one position which will cause the points-cam to position it's lobe-ramp so that the points have just-become pretty-much fully opened,, then yes, that's the 'correct' timing !

" this is the one thing that still isn't clear.
SO it seems to me that both positions fit the descriptions given for the correct position. "

____ Actually, if ya pay good/close-attention to all the words in my carefully well-worded sentence, then it should be fairly quite clear exactly what I had meant !
However if that sentence of mine merely read as:
' install the AAU in the position which will cause the points-cam to position it's lobe so that the points have fully opened,', THEN your concern would've made reasonable-sense and been quite valid !
I guess I should've bold-ized all the wording which you seem to have overlooked, and posted it rather as follows...
"install the AAU in the one position which will cause the points-cam to position it's lobe-ramp so that the points have just-become pretty-much fully opened," ... You-see "just-become" means that the points have just-then opened,, whereas with the AAU reversed, the points then would've already been fully opened for a considerably more significant time ! _ And "pretty-much" -(a commonly-used American term) means 'near-about' or 'almost'', which was meant-to indicate a slight bit less than the FULLY fully-opened position found in the MIDDLE-section of the cam-lobe.



" One of these positions the corner, the opening edge, of the lobe, at 2 o'clock. "

____ That's the correct orientation, (and the "corner"/"opening edge" of the lobe is called the 'ramp') !



" The other has the bulk of the lobe facing to the right. "

____ Indeed that's certainly the case when the AAU/lobe is in the wrong-position (and the points have been long opened FULLY) !




[Quote= Jordan ...
" it's just a matter of seeing when the points are just at the point of opening.
That is a little different to how Bob described it (fully open). "

____ Actually I had stated: " 'pretty-much' fully opened", and-thus really meaning fully separated-open (rather than having achieved max.lift) !
This is because the ign.spark doesn't wait to fire-off right-at TDC (along-with the full-alignment of all the timing-marks), so-thus the points will rather have already been opened-apart and near the peak of the lobe-ramp by the time of TDC !



" You need to take into account the direction of rotation, otherwise you might use the wrong side of the cam ramp. "

____ Good point (to provide for newbies) !


Dukaddy-DUKEs,
-Bob

[cooperplace]

Excellent! Fantastic! Thank you very much!

[narrow_monza]

At the risk of causing more confusion among the newbies like myself, allow me to ask a few questions relating to the points gap.
The shop manual specifies a gap of 0.3--0.4mm for the points. When is this gap supposed to be measured exactly?
When the piston is at TDC of the compression stoke? Is this the maximum opening of the points?
The way I understand it, the points break the contact (just start to open) at the advance value listed in the published table (in my case, 21-23 degrees for a late model motocross engine, with engine still) and are fully open (0.3mm--0.4mm) at TDC.
Is my understanding correct?

Thanks

-Adrian

[Jordan]

Points gap affects dwell angle - the time available for current to flow into and saturate the high tension coil, when the points are closed. It's been worked out to be optimum at the suggested gap.
The gap should be adjusted at maximum opening, nothing to do with ignition timing or piston position BTDC.
First set that, then go on to adjust the ignition timing point.
Be clear as to whether you are adjusting ignition timing at the full retard or full advance position - different numbers and dependent upon advance range.
The former can be done statically, using a timing disc attached to crankshaft and other devices as per owner's manuals.
The latter needs the spring loaded centrifugal weights within the AAU to be fully "out". A practical way to do that is to use a timing light, with the engine running at high revs. That's if your engine has a timing mark on the clutch cover, and you have a timing tool to screw into the LH end of the crankshaft.

[narrow_monza]

Points gap affects dwell angle - the time available for current to flow into and saturate the high tension coil, when the points are closed. It's been worked out to be optimum at the suggested gap.
The gap should be adjusted at maximum opening, nothing to do with ignition timing or piston position BTDC.
First set that, then go on to adjust the ignition timing point.
Be clear as to whether you are adjusting ignition timing at the full retard or full advance position - different numbers and dependent upon advance range.
The former can be done statically, using a timing disc attached to crankshaft and other devices as per owner's manuals.
The latter needs the spring loaded centrifugal weights within the AAU to be fully "out". A practical way to do that is to use a timing light, with the engine running at high revs. That's if your engine has a timing mark on the clutch cover, and you have a timing tool to screw into the LH end of the crankshaft.

I think I'm starting to see the light (no pun intended). So as I understand it now, the specified gap (0.3 -- 0.4 mm) is the maximum opening of the points regardless of the position of the piston or crankshaft. So I should adjust the gap when the AU cam causes the gaps to be wide(st) open.
After that, I should adjust the static timing using a degree wheel. Namely the position of the piston relative to when the points break the ground circuit. And this is done using the 2 screws that hold the plate behind the points(?)

Am I on the right path to enlightenment? Or going down the road to confusion land?

Thanks

-Adrian

[DewCatTea-Bob]

[quote= narrow_monza ...
" The shop manual specifies a gap of 0.3--0.4mm for the points. When is this gap supposed to be measured exactly? "
When the piston is at TDC of the compression stoke? "

____ There's actually no particular 'exact' spot atop the cam-lobe for setting the gap...
When the engine is at that TDC, the points cam-follower should then be riding close to the peak of the lobe-ramp (of the points-cam), and that particular spot on the lobe is as good as any other place atop the lobe. _ And although the lift-height of the entire lobe's lift-track may vary slightly from place to place,, it really doesn't matter exactly where the gap is set at, anywhere along that area/section (of the point-cam's lift-track), just so long as the resulted gap-setting at-least meets the minimum of .012" ! - (However, I recommend setting at a maximum, between .016 to .018".)



" Is this the maximum opening of the points? "

____ Well once you've set the gap (within the intended range),, the exact lift-height still may possibly vary merely-just a few thousandths higher at some-other spot along the lobe's lift-track, but that irrelevant-variance doesn't matter and makes no real difference.
Hopefully this answer covers whatever you actually meant by "this".



" The way I understand it, the points break the contact (just start to open) at the advance value listed in the published table
and are fully open (0.3mm--0.4mm) at TDC.
Is my understanding correct? "

____ I don't recall if the points cam-follower has actually fully reached to the very tippy-top of the peak of the cam-lobe's lift-ramp by the time of TDC, but I'd say your "understanding" is quite close enough.
Just keep in mind that while the points' chosen gap-setting can certainly vary the ign.timing, you're actually supposed to set the timing rather by moving-around the points-plate's set location.


Hopeful-Cheers,
-Bob

[DewCatTea-Bob]

[quote= Jordan ...
" Points gap affects dwell angle - the time available for current to flow into and saturate the high tension coil, when the points are closed. "

____ That statement leads me to assume that you're more-so a multi-cylinder car-mechanic rather than primarily a Duke-mechanic. _ Cuz with a single-cylinder, the relatively small difference that the points-gap setting has in relation to the entire dwell (in this case), is quite insignificant !
__ Furthermore, on top of that, we're not dealing with a battery-powered type of ignition-system (which much depends on 'saturation' of it's ign.coil) !
Rather, in this case,, the limited DC.pulse (directly from the mag) is what really sets the actual duration-time of the power-feed to the ign.coil (when connected in series), and naturally supersedes the points-cam's dwell-duration (to control the DC.power-feed's duration-time).
And still furthermore,, when the relevant ignition-system has it's ign.coil connected-up in 'parallel' with the points (as Ducati intended !), then there's absolutely no coil-saturation involved in the ignition-process, whatsoever !
__ So in THIS case, the specific amount of the gap-setting is totally irrelevant to any effect of dwell-angle/duration, (since either the DC.pulse is much shorter in time-duration - [with series-mode],, or, the dwell-result is rather dumped to ground - [with parallel-mode], instead of passed-through the ign.coil) !



" It's been worked out to be optimum at the suggested gap. "

____ That's no-doubt quite true with points-cams which include multiple breaker-lobes (for multi-cylinder engines [that employ a straight-DC.powered ign.coil]), but the rather long duration of the single dwell-valley -(located either side of the cam-lobe), makes the importance of the gap-size's effect on dwell-time, pretty-much irrelevant !


Dukaddy-DUKEs,
-Bob