explain timing lug on flywheel

[lingeringstin...]

I want to modify my ignition timing.

On things like the standard Honda CMX electronic ignition and charging system you have the inner bit with the coils and the outer flywheel that spins round them.

There's a little raised lug on the outside of the flywheel. On the inside of the engine case is a sender unit bolted in place so the lug on the flywheel passes by it. That's how the fixed timing works and in bog standard form it works fine.

HOWEVER...

My modified CMX250/253FMM-ish engine abomination has a couple of mil or more shaved off the barrels/head for reasons of different piston accommodation (more poop to the pop).

This means the cam is just that little bit closer to the crank, with the cam chain slack is taken up by a manual adjuster. This means the cam is slightly rotated backwards so it opens the valves SLIGHTLY later than normal. It's not enough to hurt anything apart from the fact that it makes starting the bike by electric starter a hassle. Cold engine and electric starter doesn't play together well with the starter clutch.

I found another flywheel that was identical in every way to the standard ones I've got apart from the fact that it's ignition timing lug was positioned about 10mm behind where a standard lug is. I figured this would retard my spark slightly and make for easier starting so I tried it and it worked beautifully. Engine starts without any starter clutch clattering or occasional exhaust expulsions, so my theory was correct. But it's now slightly too retarded ignition and I want to advance it just a smidgen, somewhere between the two.

Other flywheels for both single and twin engines that use similar stators as this have longer timing lugs or they're in a slightly different place. Why? My thinking is the position of the lug on the flywheel dictates timing, as illustrated when I fitted a flywheel with a slightly clockwise lug placement and it tamed my starter shenanigans. But why are some some lugs longer than others on engines that use virtually the same alternator, CDI and sensor placement?

I know the lug passing by the sensor is what triggers the CDI, but why are some of them longer? Does it have anything to do with the CDI capacitance loading up, which would mean a longer lug would push more charge to the CDI?

I have never fully understood the reason why the ignition timing lug on the flywheel is the size it is. Does the sensor for the CDI (bolted in the engine case) do it's thing immediately as the lug begins to pass by, or is it at the point where the lug is already going away from the sender in it's rotation? Where exactly does the spark happen, and why are the lugs different lengths?

If you ground off the leading edge of a lug would it make any difference to ignition timing, or it it only at the trailing edge where the spark takes place?

[Nobby the Bastard]

TBH I'd be looking at flashing a rom rather than mechanical solutions.
____________________
trevor saxe-coburg-gotha:"Remember this simple rule - scooters are for men who like to feel the breeze on their huge, flapping cunt lips."
Triumph Sprint ST 1050

[WD Forte]

the lug or tit carries the magnetic field from the rotor magnets towards the pulser coil
as it approaches the pulser coil it induces a voltage in that coil
which triggers a spark from the spark unit
This voltage rises and falls over a few milliseconds or microseconds depending on engine speed every time it passes.
the spark unit (cdi or TI) usually has a fixed voltage at which it triggers a spark in the coil.
lets say 2v for example

Programmable spark units like say an Ignitech can trigger at lower voltages 1.75V for example so in effect advance the timing without any mechanical mods or lug buggering.
If you put scope on it you could detect the fixed trigger voltage and
build a circuit to do advance it yourself.

Personally I'd leave it be unless there was to be some appreciable benefit from all that faffing about
____________________
bikers smell of wee

[stinkwheel]

Would it not be whichever bit of the lug is next to the sensor at the point the spark happens. I'd imagine it would take so little time as to be nearly instantaneous? So you could have a look with a strobe.

I think the standard mechanical way of slightly fucking with electronic timing by a few degrees is to use an offset woodruff key. Or there's nothing to stop you just missing the key out and locking the flywheel down wherever you feel like it (other than the obvious absolute faff of holding everything in position while you do so).

Or you could slot the holes for the pickup and move it a bit.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[lingeringstin...]

[quote="WD Forte"]the lug or tit carries the magnetic field from the rotor magnets towards the pulser coil
as it approaches the pulser coil it induces a voltage in that coil
which triggers a spark from the spark unit
This voltage rises and falls over a few milliseconds or microseconds depending on engine speed every time it passes.
the spark unit (cdi or TI) usually has a fixed voltage at which it triggers a spark in the coil.



AHA- If the voltage rises and drops as the lug passes the sensor then extending or shortening the length of that lug would make a difference to the voltage that triggers the CDI. So it's maybe not so much the actual timing that matters as the amount of voltage gets built up at that point, which makes for some interesting thinking about modifications.

[lingeringstin...]

[quote="stinkwheel"]

I think the standard mechanical way of slightly fucking with electronic timing by a few degrees is to use an offset woodruff key. Or there's nothing to stop you just missing the key out and locking the flywheel down wherever you feel like it (other than the obvious absolute faff of holding everything in position while you do so).

Or you could slot the holes for the pickup and move it a bit.[/quote]


This is the old school approach I like. The offset woodruff key idea. That way a bog standard flywheel could be used for the sender and the CDI to play happy with, but I could retard the timing by that wee little bit to get it back where it was optimally before I shortened the barrels etc.

I've only taken two or three mil off the whole thing, a bit off the barrels to accommodate the pistons I'm using and a bit off the head for increased compression, and it's only altered the timing by a very small amount but it goes just fine. It's only the starting that got slightly buggered up. If it was a pushrod engine it would be easy to remedy.

If I had a kicker on the engine I probably wouldn't even care but the electric start sounds like it's trying to fly apart unless I use that oddball flywheel with the different lug placement. Unfortunately there's not an easy way to fit a kicker on this bike so I'm just electric start now. Mind you it will push start very easily so I'm not too concerned about being caught out somewhere with a buggered starter motor.

[lingeringstin...]

[quote="Nobby the Bastard"]TBH I'd be looking at flashing a rom rather than mechanical solutions.[/quote]

I doubt the primitive electronic ignition on this is flashable in any way. It's a very simple setup and the CDI is the same one they use on virtually every small engine China produces.

[stinkwheel]

Early electric start enfield bullets are very hard on starters and had a habit of destroying sprag clutches by kicking back on a cold-start.

One of the solutions they came up with was an electronic spark delay box you plugged in which made it miss the first spark on startup. This meant it had to have done at least one complete rotation before sparking so the engine was already spinning over quite quickly before it attempted to fire. I'm not 100% sure how they achieved this.

However knowing how you historically like a contrary and over-complicated startup procedure, you could probably achieve this manually with an intermittant kill switch that earths the coil that you could hold down as you spin over the engine then release when it's got some momentum up.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[WD Forte]

I recall a mate and others who had Honda GL650's complained they
could sometimes backfire like a bitch on startup hot or cold
which was very alarming and put excessive wear on the sprag clutch
and roller boss.
Intrigued and as an experiment, I built him a delay box which when the starter button was pressed, would hold back the ignition for around half
a second as I recall, to give the motor time to spin up before lighting the fires.
Worked a treat, no more huge bangs.
____________________
bikers smell of wee

[WD Forte]

re this topic
I see the CMX250 uses the CB250 motor or Lifan 253FMM
Both use the typical AC CDI ignition system

Would half a mil off the head really have so much effect that it needed ignition adjustment?
If it really does and looking at the images, I'd consider modifying the pulser bracket to shift the pulser a few degrees rather than fuck about with the lug.
That or buy a programmable cdi box.
I'd put my scope on it first though rather than swap, tinker and hope
____________________
bikers smell of wee

[virus]

Id be going for a slotted pickup sensor mount tbh, that way you can fuck around to your hearts content (and i imagine that would keep you satified for a while too) to dial in the exact perfect timing.

Shame they arent oiltight without that left hand cover on otherwise you could have easily strobed it up and had a real good nose about into the actual firing position in relation to the lug.
____________________
own: 81 xs1100g...
owned: 85 rat CG (sold), 91 GS500e (stolen), 84 gsx400f (scrapped), 81 z250 (siezed, siezed, scrapped), 83 cb250rs (sold), 84 gpz750r ratfighter (killed) 84gpz400 (sold), '80 cb650 ratfighter (wrote off) 95gsx6/12f ratfighter (killed) 91 xj900 (sold)
stinkwheel Well I just had my hands up a pigs fanny. Which makes your concerns pale into insignificance.

[stinkwheel]

Or you could slot the pickup and with an unholy arrangement of springs, strimmer cable and bowden cables, retro-fit your motorcycle with an advance/retard lever on the handlebars.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[virus]

[lingeringstin...]

[quote="stinkwheel"]Early electric start enfield bullets are very hard on starters and had a habit of destroying sprag clutches by kicking back on a cold-start.

One of the solutions they came up with was an electronic spark delay box you plugged in which made it miss the first spark on startup. This meant it had to have done at least one complete rotation before sparking so the engine was already spinning over quite quickly before it attempted to fire. I'm not 100% sure how they achieved this.

However knowing how you historically like a contrary and over-complicated startup procedure, you could probably achieve this manually with an intermittant kill switch that earths the coil that you could hold down as you spin over the engine then release when it's got some momentum up.[/quote]

That Enfield bodge actually makes sense if the early electrical system (6v?) and starters were slow to get things going. A bit like how the decompression lever for a big single kicker helps.

There's actually like three or four different ways to start my bike. One way is to put a ratchet and socket on the alternator bolt and use that like a makeshift kickstart lever. I don't use that little screw-on round cover there anyway (more cooling for the alternator etc.) so it's just open and easy access. This actually works but I don't really like doing it because it's putting the pressure on the unbolt direction of the threads so it COULD lead to the alternator bolt being loosened, but not likely as it's always fitted very tight anyway.

The electrical system is purposely simple and self igniting so the bike will start easily just by pushing it or rolling down a slight incline. I do have a tractor ignition switch as a security-looking measure and it does turn on the electrical system for lights but the bike will start without a key. You have to ground the CDI to kill the engine.

[lingeringstin...]

[quote="WD Forte"]re this topic
I see the CMX250 uses the CB250 motor or Lifan 253FMM
Both use the typical AC CDI ignition system

Would half a mil off the head really have so much effect that it needed ignition adjustment?
If it really does and looking at the images, I'd consider modifying the pulser bracket to shift the pulser a few degrees rather than fuck about with the lug.
That or buy a programmable cdi box.
I'd put my scope on it first though rather than swap, tinker and hope[/quote]

I actually take around three mil off the head/barrels to accommodate the pistons I use. I physically grind off the top of the standard concave pistons so they're flat top, which takes ages by hand and ends up with about 1-2 mil off their height so I had to shorten the barrels by that much. This raises compression a bit. Then I take the head down a bit for even more compression. Sometimes (usually) I fit a 125 head for more compression. All of this helps and is necessary to make these engines do anything.

On one of my other engines I use a pair of YBR125 pistons which are lighter and also shorter and required the barrels being shortened. All of this means the cam becomes slightly altered timing. It doesn't seem to make any difference when running but that little bit of difference plays Hell with my starter clutch.

It would be nice if I could just move the sensor a bit but it's bolted into the engine case in a manner that can't be moved so I'm stuck with it.

[lingeringstin...]

[quote="stinkwheel"]Or you could slot the pickup and with an unholy arrangement of springs, strimmer cable and bowden cables, retro-fit your motorcycle with an advance/retard lever on the handlebars.[/quote]

I have actually considered this. I might even consider it further. Imagine the anti-theft quality of a bike that can be set to automatically backfire if somebody tries to start it.

[lingeringstin...]

So when I held the bog standard flywheel next to the unknown flywheel with the slightly different lugbump on it here's what I got. The difference between the two is about 8 to 10mm. The unknown flywheel's lug starts roughly about halfway along from where the standard one does (see marker line in pic).

https://i.postimg.cc/fW54yWCF/3.jpg

The lug on the flywheel (same on all Honda or clone engines of this kind, CB125T, CMX250, Jinlun JL-11 etc.) is about 17.5mm long. I'd show the oddball unknown flywheel for comparison but it's on the bike and I can't be arse to take it off, but apart from the lug position they're identical.

The unknown flywheel with an identical sized but more reversed positioned lug works better, but it's not ideal. It's slightly retarded for my liking so I want to nudge the timing forward a bit and the only way I can see to do that is to grind off about 1/4 of the leading edge of a standard "too advanced" flywheel lug.

That will shorten the overall length of the lug by a quarter. I don't know if this is going to work because I don't know when the CDI actually fires. If it fires DURING the time the lug is passing the sensor then this might work. But if it fires only as the lug is breaking away from the immediate proximity of the sensor then this won't make any different to the timing.

[stinkwheel]

[WD Forte]

Not sure it has a hall effect pulser, looks like yer typical
common or garden AC CDI reluctor type thing.
The wave forms I've looked at are usually a positive and negative spike each revolution which is half wave rectified to trigger the SCR.
The SCR only needs around 1.5V to fire
The source coils put out AC which is also rectified to feed the dump cap.

I'd at least put strobe on it to see how its timing before doing any butchery.
You'd get an idea of how far it is from the fixed timing mark then with a few calculations make adjustments with a better chance of success.
____________________
bikers smell of wee

[stinkwheel]

[lingeringstin...]

[quote="WD Forte"]Not sure it has a hall effect pulser, looks like yer typical
common or garden AC CDI reluctor type thing.
The wave forms I've looked at are usually a positive and negative spike each revolution which is half wave rectified to trigger the SCR.
The SCR only needs around 1.5V to fire
The source coils put out AC which is also rectified to feed the dump cap.

I'd at least put strobe on it to see how its timing before doing any butchery.
You'd get an idea of how far it is from the fixed timing mark then with a few calculations make adjustments with a better chance of success.[/quote]


It's all rocket science above my head. All I know is the bump on the flywheel hakes the CDI do something, which makes the spark plug spark and makes the burny stuff in the engine go bang. So going on that theory, and the fact that I don't have a strobe, I'm just going to take an axe or a rock and knock off a quarter of the leading edge of the lump on the flywheel and see what happens. I've got like three of these flywheels so I don't care if I bugger one up.

But the mystery deepens. I have a flywheel for a Honda CB125T that looks identical in all dimensions to the Chinese ones I've been usung, but my bike would not run with the Honda one fitted. Why is that? It's got magnets in it just like the other ones. It must do the same damn thing because these flywheels are just simple devices. But the Honda one didn't seem to trigger my CDI and I can't figure that out. It's exactly the same in every way to the Chinese ones I've got but I get no spark.

Could it be something to do with the magnet polarity being different between the Honda and Chinese ones? The reason I ask such a stupid question is that my mind says that if the Honda flywheel has the opposite polarity to the other ones then MAYBE that cancels out that happens at the sensor thing because I think the sensor thing is also magnetic, or it becomes that way after use with whatever flywheel it's next to. I haven't checked the polarity between the flywheels yet but that MIGHT explain why the Honda one wouldn't fire the CDI.

Or it could just be bad magic.

[lingeringstin...]

[quote="stinkwheel"][quote="lingeringstink"]
That will shorten the overall length of the lug by a quarter. I don't know if this is going to work because I don't know when the CDI actually fires. If it fires DURING the time the lug is passing the sensor then this might work. But if it fires only as the lug is breaking away from the immediate proximity of the sensor then this won't make any different to the timing.[/quote]

The waveform from the hall effect sensor is pretty much a square wave so on the assumption the CDI triggers when a certain voltage is reached, rather than any fancy jiggery-pokery with frequencies, it should spark pretty near the leading edge of the trigger.[/quote]

I know little about about such matters, but the fact that there's a "CDI" tells me there's a capacitance playing about somewhere in the mix, and to me that indicates a buildup of voltage stored somewhere, even if it is very small and happens very fast.

Therefore I initially suspected that the bump on the magnetic flywheel built up a capacitance somewhere as it passed by the sensor and then the CDI burped it out when the trailing edge of the bump left the proximity of the sensor, but that's just a crude guess based on old ignition system understandings. It might not be like that at all. It might be that the length of the bump plays a bigger part than I thought. If you look up pics of these type of flywheels you'll see all kinds of different bump arrangements, some being quite long and others being intermittent, and I have no idea why.

I suspect foul play at the highest level.

[WD Forte]

The only 'capacitance' involved is the build up and 'dumping' of charge in the dump cap attached to the primary winding of the coil.

The rest is all done with magnets and coils and the SCR is basically a switch.

When triggered, the SCR 'dumps' the capacitors built up charge to ground which collapses the emf in the primary winding whilst inducing a field in the secondary HT winding at a much higher voltage.
This high voltage discharges itself to ground across the spark plug gap.

Honda and others used this system on many bikes and it's usually a pretty rugged and reliable system which could work well for years.
It isn't what you'd call finely tuned and it's certainly open to modding but haphazardly guessing and tinkering is a poor way to do it.
____________________
bikers smell of wee

[lingeringstin...]

After absorbing all the previous information I decided that the bump on the flywheel was probably the instigator of a square wave triggering thing, so the obvious way to retard the ignition would be to hack a bit off the leading edge of the bump.

Turns out this appears to be correct because after doing that to retard the timing a bit and subdue the shocking starter clutch malarky brought on by the consequences of my shortened top end, it all worked perfectly fine. The bike starts nice and runs well.

I still have no idea why the standard bump on a standard flywheel of this type is the length it is but I suspect it's not because of some kind of rocket science or White Man's Magic, it's probably just down to "because that's how we made it" in Hondaspeak.

So there you have it, folks. Due to the way the engine cases are designed you can't really just slot the bolt holes on the sensor and move it around very easily so the other alternative for timing adjustment is to fiddle about with the of the bump on the flywheel. Of course this is completely unnecessary on a bog standard engine that I haven't molested in unspeakable ways out of curiosity.

I suppose it's a good thing I'm not a goat farmer.



(Do ANY of these linky image things actually work?)


[url=https://postimages.org/][img]https://i.postimg.cc/gJGHkKS0/P1000815.jpg[/img][/url]

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https://i.postimg.cc/gJGHkKS0/P1000815.jpg

https://i.postimg.cc/cLQ3gn8t/P1000816.jpg

https://ibb.co/rHkWsMs

https://ibb.co/9rYgk6C

[stinkwheel]