Stator.

Weak spark?

Behold and lo, the was the wimpiest pale spark you've ever seen; it would have battled to fight it's way out of a wet paper bag. That kinda makes sense - a fat spark would ignite fuel in up to the worst conditions, but a wimpy spark would need "perfect" conditions to get the show on the road. Well that's what JAW thought anyhow.

So what does a weak spark mean? If you look at a bike sparking system there is the spark plug, the ignition coil, high tension lead, wiring, the pulse sensor, the CDI and the stator exciter winding to generate the power.

The problem was isolated using another bike to swap and test parts - it had to be either the CDI or the exciter winding. But do you think you can get a motorbike shop to test a CDI? "Hey guys, have you got an XR600RJ that you can verify the operation of this CDI on?" "Oh, no no no, you must go and pay $'00 to the Master Motorbike Electricals Guru, it's all magic, only he knows how it works. Blah blah blah.

Gotta be the exciter winding...

JAW makes a bold decision: "CDIs are electronics. They either work, or they don't work. I reckon that tiny, thin winding of enamel wire that is subject to heat, oil and works from a permanent magnet is *far* more likely to be the problem.

Before you go any further, I want to know how it all works.

Okay! Firstly, there are two separate circuits on the average bike - a low voltage power source for running accesories such as headlights and indicators, and a high voltage power source (known as the exciter winding) for getting the CDI to spark. 'Crossers might only have an exciter winding - what's a headlight anyway? ;)

Now the rotor, the thing that spins around the stator, is magnetic. It has several poles, you know, Norths and Souths, on it. On this particular bike there are 6 Norths and 6 Souths. As the rotor spins around the exciter winding, thanks to some rather convenient physics current will flow in the winding. As a North pole passes, current goes up, North then swings back to neutral (ie no magnetism) current goes to zero, South pole passed, current goes the other way. Yep, it's AC (Alternating Current).

The more windings you have the higher the voltage. By winding the exciter with *heaps* of turn with really thin wire you get high voltage, low current. Using thicker wire, less turns, lower voltage, higher current.

They got clever on the lighting side of things. The four windings are just one piece of wire, but it is wound clockwise on the first pole, anticlockwise on the second pole, and so on. Why? They are positioned so exactly as a North pole is going past the first winding a South pole is going past the next winding. What does that give you? Well, the current flows depending on which way the winding was turned. So, in effect, by having the opposite pole and the opposite winding direction, the currents all add up. If you didn't alternate the winding directions, the currents would actually cancel out...

The advantage is of course that you get quadruple the output - each individual winding is good for around 3V but 4 of them gives your 12V. Note that the actual voltage is dependent on rpm but then controlled by a regulator to maintain 12V and then rectified where necessary, but that's another story.

Rewinding, a process.

On with the story. Specifications for an XR600 between 1985 and 1990 state that the exciter to ground resistance should be between 230 and 320 Ohms. My old one came in at 370.

Ripping off the old winding revealed the problem. Enamel had been baked black - but there were no short circuits. The wire in places was so incredibly thin and just broke apart as it came off. What must happen is the copper itself is getting old and sad, its resistance increases and it's ability to carry a current diminished. Hard to start but will run okay.

You'll need to find yourself a big spool of hair-thin enamel winding wire. I came across this one from the olde man in laws shed. It's 0.132mm, vintage 1978 and all...it's fatter slightly than the existing windings, so to replace the same number of turns it will be a physically bigger widning. Oh well.

Try and get good quality winding wire with the highest temperature rating on it as possible. Inside the engine can get really hot being aircooled - much hotter than a water cooled engine.

Set yourself up so the wire can freely pull off the spool and just wrap, wrap, wrap. It's suprisingly strong for hair-thin wire but don't take any chances. Do a few stress tests on it before you begin, see how easy/hard it is to snap it . You want one winding, you don't really want any joins.

After about an hour, yeah, just an hour, it was wound to where I thought it should do. A fair bit bigger than the old coil. But how do you know when to stop? Carefully peel off a bit of enamel and do a resistance check. I found a sharp knife scraping both ends enough for a connection would do the job. First check - 162 Ohms. Keep going. Second check, 250 Ohms. That will do.

Carefully solder on some flyleads and expoxy the thing in place to protect those precious little windings. I used some devcon I had lying around, it isn't conductive, it is tough as, it seemed to do the job.

Results

Okay, the winding was a success. The bike started nice and easy which was a pleasant suprise; but as time went by it got less easy. Until finally, a year later, out in the bush, it refused to start.

What could have happened?

Measuring the resistance back in the shed showed the resistance was still 250Ohms. Kickedy kick, nothing. Retest the resistance down to 150Ohms. Huh? a while later its at 210Ohms.

Right, its coming out again with two plans: (a) find out what went wrong and fix it and (b) Rewind the lighting circuit - I need some decent headlights!

First thing I did was check the how big a winding you could put in there physically. The plasticine test revealled 14mm deck height maximum.

While degreasing the winding I found out the first problem - water dropped the resistance right down. Ah ha! Must be an internal short. As I unravelled it the truth became clear. Oil and heat had broken down the enamel protective coating and caused intermittent shorts. But how did the oil get in?

I may have epoxied the windings on the outside - but oil found it way past the exciter plastic spool from the _inside_. The new attempt was firstly going to have to be plastic spool-less and secondly epoxied all the way! The Devcon epoxy used proved up to the heat and oil challenge - even though it had changed colour from gray to dark brown.

With a bare stator in my hand, first thing to do is "dunk" the think in epoxy - so that the first windings aren't sitting on metal but sitting on a protective coating of epoxy over the metal. This is how they come from the factory.

I rang around a bit and found that there was such a thing as high-temperature enamel. Normal enamel winding wire is rated to around 120-130degC. Super-enamel can go out to maybe 200 but I couldn't source any locally.

I decided to stick with the wire I used last time, but from the moment the first windings went on I was popsticking on epoxy for a "total seal". Nothing was going to get in there. This time I wound all the way out to 310Ohms. The winding was looking pretty oblong since there was no spool to neatly wind onto, but it hasn't caused a problem so far.

The lighting circuit copped some 0.8mm winding wire worth about $5 from the local electronics store. I counted 64,66,70,65 turns pulling the old wire off - I decided to wind on 75 each "since the rotor was old and the magnetism was probably not as good as it was in 1987".