Kdx200 An Even More Technical Question

[PZR]

In my never ending quest to never leave well enough alone I have another question and this is a bit more tekanogakill.
The story so far........... It was a dark and stormy night (tonight actually)
Picture my early 80s pre powe valve air cooled KDX200. I had it going really well at the family day Sunday week ago at Woodhill. Then it decieded to stop.
Ahaa !! he says an electrical fault, which sure enough it was. The plug was shorting thru the electrode ceramic so pop in a new plug and away it went. No probs so far.

Being a clever bastard I thought to myself. I'll pull the magneto off and check the timing while I am at it. Now this magneto is like my other spares bike. It seems that the old mags must crap out so it has had a new one put in but it seems they have used the old mag centre, cut off the faulty mag and welded the old centre to the new mag. With me so far?
Come on, keep up at the back there you!!

Now both bikes look like they were done by the same idiot as whoever welded them together then reassembled them on the bikes.
A/ without first tapping some threads so the next person can install a puller to get the damn thing off next time (mutter mutter mutter) but hey I have the technology I can fix stuff like that.
B/They did not line up the key way with the firing sequence ( and this is where it gets ugly for moi)
So being a clever not to mention modest buggar I marked the keyway slot before I removed the mag.

Ok so far? Everyone still keeping up. Wake up over there!!!!!

When I removed the mag the woodruff key is broken in two, but the keyway slot does not line up with my careful mocro perfect splot of marker pen on the end of the crank where the mag was sitting in the first place. Remember this bike went real well and started real well.
So clever bloke that I am (did I mention I was modest as well) I get a new woodruff key, I put it back together in the manner the factory intended i.e. with the two keyways lined up with a woodruff key holding them in line.
Would the fucking thing start. NO WAY!!!. Lots of kick back and no go.
So ok I pull it apart and reassemble it on the original mark that I made when I first pulled it apart (you remember ----Clever/Modest)

Did it go I hear you ask with bated breath???


Well yes it did actually, thanks for asking? Second kick it fired up and like any good 2 stroke and polluted the crap out of my garage but thats another story.

So to get to the point (who said thank god)
I like my machinery to be right. I would prefer to reassemble it so the keyway lines up like it should. So the keyway stops the magneto spinning on the crank. My spares bike has had the keyway hoe in the crank damaged perhaps because of this very sort of situation.
I do not have a problem cutting the old centre out and rewelding it, but I do have a problem knowing where to line it up too.
Ok here are the actual questions............

How did the last person who messed with it find what I assume was the correct place to put the mag back on so the bike actually ran, (I know I know it may well have been trial and error)

And how do I find the correct place so I can weld it back together?

The magneto has 4 magnets round the circuference with gaps between. Those gaps or one of those gaps must give a firing instruction but this is where I have reached the limit of my knowledge.

Help please, I wanna ride my dirt bike summers here
I'll check back tomorrow night for any pearls of wisdom and the occasional witticism.
Cheers
PZR

Just had a thought as I was rereading this . I will check to see if the gap between the magnets lines up with the crank keyway when it is in the "wrong" position,tomorrow evening. Its too late now
That may give the game away hopefully

[Motu]

First off - if you had a carbon track on the plug from shorting,then you have a corresponding track in the plug cap,you must replace that too.

I'm not sure what you are talking about with the flywheel,it's a long time since I've seen the inside of an aircooled KDX.I doubt that two flywheels off two completely different bikes would have the same rough cut up and welding you mention....I think this may be standard,some photos would help.

This reminds me of my KT250,when I first put it together I had to advance the ignition to the max to get it to run cleanly,and I thought there must be something wrong with the crank or flywheel,the rest of the motor seemed in pretty good nick.But many years later when I finaly had it rebored and fitted a new piston it ran perfectly on the factory marks.Remember,a 2 stroke not only compresses the mixture in the cyl and combustion chamber,but also in the crankcase.....I've caught myself out on this little fact a couple of times.A similar thing happens with 4 strokes - there is an intake stroke as well as a compression stroke,a motor has to suck before it can blow,a point very often overlooked.

So what that ramble condenses down to is - check piston and rings,and crankshaft oil seals.

[Crisis management]

I did pay attention and tried to keep up at the back, thank you, however I am not sure it helped much...

The ignition should fire at a predetermined number of degrees before top dead centre, fit a degree wheel to crank end (I use old plastic protractors flogged from the kids school bags, they never miss them). A bit of blu tack and a fixed pointer to give a reading will do, set piston/crank at correct place and whatever triggers your spark should be lined up.

I don't know enough about 2 strokes to comment on why it would run with the ignition set incorrectly but Motu's argument makes sense to me.

Sounds like you will either pull the whole thing apart or leave well alone....how brave are you?

Well written by the way, I enjoyed that

[PZR]

[quote=Motu;773083]First off - if you had a carbon track on the plug from shorting,then you have a corresponding track in the plug cap,you must replace that too.

Sorry I did not make myself clear enough. If you hold a plug looking at the gap the spark jumps the gap normally. This one was jumping thru the electrode insulator to the body of the plug (the thread inner) meaning the ceramic insulator was cracked, not the upper outer white porcelin of the plug. Detonation can cause this to happen. Detonation is when the plug fires at the wrong time in the stoke. Ie on the way up or down rather than just next to TDC

I'm not sure what you are talking about with the flywheel,it's a long time since I've seen the inside of an aircooled KDX.I doubt that two flywheels off two completely different bikes would have the same rough cut up and welding you mention....I think this may be standard,some photos would help.

Sorry no photos available as the camera is away on holiday but trust me this is not factory. If this is still going I will try later in the week to post some photos

[Motu]

So it's the flywheel with magnets we are talking about? It's very,very unusual for them to have a problem,any trouble with a CDI ignition is normaly in the stator....the coils and things.

Do as Crisis says and check with a timing light - there should be a TDC mark on the flywheel,and hopefully a firing mark too.First with a screwdriver down the plug hole make sure TDC is really TDC....if it's not then you can toss the flywheels.

[PZR]

Do as Crisis says and check with a timing light - there should be a TDC mark on the flywheel,and hopefully a firing mark too.First with a screwdriver down the plug hole make sure TDC is really TDC....if it's not then you can toss the flywheels.

Thats what Im saying. The original flywheel (magneto) (the round thing with the magnets ) is not original. It has been modified with a non standard part. Looking around at different sites I have a suspicion that the original magnetos (Im gonna call them that substitute your own label) must fail as like I said I have two of these things with the same modification. I know magnets can loose their magnetisim over time, so someone has modified them. Or maybe its to increase the advance or to add more flywheel weight to help increase torque. I am unsure.

I just want to know
Where does it pick up the signal to say FIRE the plug. Is it in the gap between magnets or on the magnets. Anyone got one apart (doesnt have to be a KDX) and tell me what does the keyway slot line up with.
As I understand it the magneto does two things.
1/ It provides 12v electricity for lights
2/ It provides 12v to charge the coil which then gives a high capacity discharge to the plug

The CDI decides somehow how to control the spark. I know it has inbuilt mapping from the factory (primitive I grant you compared to modern bikes) but I do not know where does it get its "control "instructions from meaning speed of the motor versus when to spark. It must pick up the pulse of the on / off function of the magnets as they spin and generate on / off 12 volts. Hey I'm working this out in my head as I type here so tell me if Im wrong.
Oooops gotta go do some work. Back later

[Crisis management]

The magneto just generates power and the ignition spark is controlled by either a set of points or an electronic trigger. Both are set to the timing marks as Motu said, find the timing marks and / or trigger mechanism and off you go.
Magnetos' used to have the points buried underneath them and driven off the cam on the end of the crank. You have had this apart tho so that would have been obvious???
I am now begining to produce drivel so no further attention should be paid....

[Motu]

The KDX did run lights,so there will be a 12v lighting coil.There is also a CDI charging coil,this will be a high voltage,like 240v,mabybe more....or less.A smaller trigger coil gives a low voltage to fire the coil.The trigger is the only one that's timed,there is no advance,it will automaticaly advance with revs to a small degree.There is almost no way to find out where it really fires....you can hook up a multimeter and look at the AC voltage of the trigger but it won't tell you much.

Find TDC,mark it on the flywheel rim and crankcase,then hook up a timing light and watch where the marks are.If you get a fancy timing light with an advance dial you can find out when it is firing.

[T.W.R]

??? your saying that the Magneto rotor has been cut from one mounting plate onto another?

The outer hub containing the magnets is machine press rivited onto the base spline

The magneto rotor containing the magnets spins around the magneto stator at a set distance, the magnets are set at a pre-determind +/- , the pulse of these passing the stator windings induces a signal current that travels to the CDI to produce the spark.

The advancing & retarding of timing are done by moving the stator plate (the mounting holes for the screws should have a pre-determind amount of allowable movement ie; be slotted) the Rotor can't and shouldn't move on the crankshaft

The KDX is a 6v system so the output pulse signal should be around 25-30v

The rotor and stator have to match to produce the correct pulse signal otherwise interesting things happen.

Been there with a IT250 which uses an identical system. Changed the stator without changing the rotor, ran perfectly but in reverse , tried inverting the stator windings only to get spark breakdown through the plug cap and when it managed to run more farting & banging than after a night on curry.
Problem was only solved once the rotor and stator were matched up together as new/new not old/new

[T.W.R]

Part #21007 (magneto rotor) has been modified? then it's just a case hit n hope as to where the magnet housing should sit on the hub.

The hub has a key-way that matches the key-way on the crankshaft via the locator (woodruff key) #92038.

The magneto stator has slotted mounting points for advancing & retarding the timing of the pulse signal.

The large winding on the leading edge & the inner winding on the trailing edge are for the ignition and the small outer winding is for lighting.

[PZR]

Thanks for the advice TWR I appreciate it.
I got the camera back so here are some photos of what I have got to work with. The cunning plan at this stage is to put it back to where it does run and mark it. Then cut it apart and move the keyway to line up with that mark then reweld it. I should then be able to put it back together with the keyway slots lined up which will then stop it rotating whenever it wants to which of course could cause all sorts of damage. I then have the normal factory adjustment slots to help with any advance or retard. I am curious about setting advance on a two stroke. I have done heaps of 4 stroke cars but I am still finding 2 stroke bikes to be a bit of a black art. I can tell when a 4 stroke pinks but this thing makes so much noise I am having trouble seperating the various sounds. The engine seems to be in pretty good nick but for me there are a few unknown variables especially once you get into rings, piston side seals etc etc

[PZR]

??? your saying that the Magneto rotor has been cut from one mounting plate onto another?

See the photo yep it sure has

The outer hub containing the magnets is machine press rivited onto the base spline

Not any more

The advancing & retarding of timing are done by moving the stator plate (the mounting holes for the screws should have a pre-determind amount of allowable movement ie; be slotted) the Rotor can't and shouldn't move on the crankshaft

And therein lies the problem. This thing has been assembled wrongly and put back on approx 20 to 30 degrees away from the keyway slot in the crank with a broken woodruff key. Whoever did it went to a fair bit of trouble to put a new magneto on but I cannot fathom why they did not do the timing part right. I mean it would not have run so they had to pull it apart and then through trial and error find the correct place so it would run. So much simpler to do it right the first time.
But waht the hell, I like a challenge .

[barty5]

just a quick thought that if your carefull when you cut the welds given the original age it was when the cock up was preformed you may find that the surface under neath wasnt cleaned all that well and there may well be wear marks or stains from were it was sitting from new just a thought worth a try. PS not the most techo answer but you never know have had simalar thing happen in the past.

[Ixion]

My guess would be that either the flywheel or the crankshaft on your bike has been swapped out.

I suspect that this (modified) flywheel came off a bike where the keyway in the crankshaft had crapped out. So someone had a new keyway machined, in a different place on the crankshaft. Then had to modify the flywheel to match. Later either the modified flywheel has been switched onto your (unmodifed crank) bike, or a new crank has been fitted to your (modified) bike.

Does the modded flywheel line up correctly on your SPARES bike (on the long shot that that one might have the modded crank) ? Why can't you use the flywheel from that?

[PZR]

just a quick thought that if your carefull when you cut the welds given the original age it was when the cock up was preformed you may find that the surface under neath wasnt cleaned all that well and there may well be wear marks or stains from were it was sitting from new just a thought worth a try. PS not the most techo answer but you never know have had simalar thing happen in the past.

Good thoughts I ll check that out for sure thanks barty5