A few questions:
Is there in ideal RPM to fine tune the idle setting? What mark am I looking for in the window at IDLE? "S"?
Then when I rev it, what RPM should I start seeing the "F" mark at?
If the "F" mark is low in the window at higher revs (where the advance kicks in), what would be best way to adjust to get the "F" centred in the window? Rotating the whole body of the magneto, or only the plate that holds the points?
I know that on /5-on Airheads, you're probably looking at around 3000-3200 RPM for full advance. Probably something in that range or maybe 2800-3000. Note that once you see the F-mark (the mark, not the F) it should not move for any more increase in RPM.
The flywheel rotates clockwise when viewed from the front. That means that looking in the window, the marks will appear in the top of the window and move to the bottom. Test that out by putting the bike in 4th gear and turn the rear wheel in the proper direction. If the F-mark is low in the window, they the ignition is retarded...the spark is happening too late.
To change the timing, do not touch the magneto or the body once you have aligned it. If you read that written down anywhere, don't do it. There are two timing steps...the magneto and ignition. To change ignition timing, you'll want to change the plate that opens and closes the points. Sometimes you might have to change the gap if you rotate the points plate and you're still not properly timed. Two cylinder engines can stand quite a bit of point gap range...0.016" is nominal, but you could go to 0.012 or 0.022. A small amount of gap change makes a bit difference in ignition timing.
'78 R100/7 '69 R69S '52 R25/2
Search the forums for differential timing and get to know what it is. When the motorcycle is idling ideally you should see the S line clearly in the middle of the window. In reality what you will see is two lines, and many people get confused with this. What you are seeing with two lines is the timing on one side versus the other. If the two liens are apart too much (see the Barrington manual for more info) then you will need to "adjust" the differential timing.
See my recent thread about my R69S starting and you will see that my advancer lobes are quite worn and I see both the S and OT lines in the window which means spark in one cylinder is VERY retarded while the other is quite good. I am replacing my advancer unit.
The F line should be fairly steady in the window when the advancer reaches full advance. If it bounces around or is not clear you may have another issue. Again, read the Barrington manual.
1967 R69S, 1966 R50/2, 1965 R27, 1963 R27, 1952 R51/3 and 2018 R1200GS
I started the procedure again from the top -even taking off the magneto rotor and resetting to as vertical as possible;
Matched the notch in the V;
All while flywheel on the Mark just above "S".
Adjusted breaker gap;
Then adjusted the breaker backing plate just to the point that my Buzz box triggered.
Also put new spark plugs in for the hell of it.
Purrs like a kitten at slow speed
When I gave 'er some acceleration, I got some backfiring.
Haven't done the dynamic timing yet -maybe tomorrow.
I'll pay extra attention to see if I have the differential timing going on.
Last time I tried dynamic timing a few months ago, I could not see the S mark at idle -was very frustrating -until I lowered the RPM to 900 ish.
I was wondering if there is an ideal RPM at idle, at which the reading should be taken?
make sure your valves are correctly set FIRST. Once this is done, do the ignition timing just as Kurt says. Its not the S or F you have to align, its the little mark above the character which should be aligned to the little punched mark on the left side of the window. I usually paint the marks on the flywheel with a color to make them more visible. (3 different colors for OT, S and F). Dip a small screwdriver in paint and mark the small punched line.
OT marks Oberer Totpunkt (which is where the piston JUST turns)
F means "Frueh" means advanced
S means "spaet" means retard
I adjust my idle at 800 to 900 revs and I can clearly see the mark. If I go higher, the mark moves out of the window.
At 3500 latest you should see the F mark in the window. If it is on the lower part of the window, then you are slightly on retard ignition. I personally would not bother with that since today's gas reacts different to what they had back in the days. 2mm means roughly 1 degree in timing ...
Important is that once you increase the revs further, your F stays in the window. If it disappears you will overheat and eventually damage your engine. You will have to examine your advance unit ... probably the little stop spring is missing or not working properly.
In general, advance timing is more important than retard timing. At idle, you can accept anything between OT and S ... no worries (after OT is not good, though). F is the mark you want to make sure is properly set. NEVER more advance than F!!!
If you have differential timing (as James mentions), make sure your earlier timed cylinder fires on F. So that the delayed cylinder fires before the F mark.
I am afraid You will see 2 lines with the strobe light. The only time I see just one line only is when the differential timing is so far off that you actually only see one cylinder within the window firing. As Kurt says, some differential timing is acceptable.