chinese 125 no spark please help

[andy46]

I have a dayang 125 which looks to be based on the honda cg 12v electric start.

It was running fine but wouldn't start on sunday morning. Investigation showed either no spark at the plug or an intermittent thin yellow spark which has now dissapeared completely.

So far I have used the haynes chinese 125 manual and a multimeter to check out the ignition system.

Ignition coil primary winding shows 1.8 ohms (haynes says 0.5 to 5)

secondary winding shows 4.2 Kohms (haynes 3 to 12)

plug cap shows 5.28 Kohms (haynes "around 5")

There are no blown fuses, all relevant connections have been checked and cleaned, I am using a known good sparkplug which is properly grounded.

haynes now moves onto pick up coil:

testing accross 2 wires from pick up coil (blue/white & green/white) shows 142.6 ohms (haynes 15 to 150). I tested this at the connector to the stator and at the connector to the cdi - same result.

At this point haynes suggests a physical exam of the pick up coil which I did. I don't really know what to look for but there were no loose wires, all soldering looked OK and no obviously burned out bits.
It isn't possible to check the clearance between the pickup coil and the flywheel but both can only be fitted in one position, they were and are secure.

haynes then suggested a cdi fault with no way to test other than to try a new one, so I did - didn't fix the problem.

other tests I have done show continuity from the yellow/black wire at the cdi to the ignition coil, 56 to 60 volts AC from the red/black wire from the ignition winding on the stator (when cranking).
Also I have continuity to ground from the black/white wire at the cdi when the ignition is off - this dissapears when the ignition is switched on, a similar test shows the kill switch to be working as it should.

I have removed and dismantled the ignition and kill switches, cleaned, reassembled and refitted, both appeared OK.

The bike is not fitted with a side stand switch and I can find no evidence of one having been previously removed.

So after doing all this I thought "once you eliminate the impossible, whatever is left, no matter how improbable is what has to be" So i should order another new cdi !!

OR I could ask for some help on a internet forum, so what have I missed?

[Teflon-Mike]

You HAD a spark, just not a very beefy one, which sort of suggests that the electrics are prfobably OK... there's only the one winding in the stator to makle electric,. then the trigger for the CDi and the CDi, then the coil and the Plug itself.

So how old shagged be the plug? That's where I would start.

CDi's tend to be prttrt reliable, ditto statpors; coiuls can be a bit iffy occasionally.. I think I have had to replace ONE on one of my bikes in forty years..... clean up the earth contacts and wipe down the HT leads, replace them if you have to replace anything as first course.... after the spark plug...

Amazing how many presumed problems go away with a new plug... really is....

After that, CG ignition has make to kill off circuit,,,,, if you pull black-white wire froim the ign CDi, it sgould isolate ALL the kill switches, to eliminate all thiose possible probls nd potetial short circuits.... which are probably more likely than a fault ion the stator or CDi or coil.... so ~IF bike ruins with CDi kill wire isolated, connect one switch in turn and kook-see which one makes problems.... side stand switches are notpropuse, most oft for numpty owners tryting to by-pass them... but still....

Work systematically and logically, and remember, sparks are self exited, itrs a sumple magneto based system, its actually quite hard to turn the sparks off, so if you dont get'em, more likely something wromg in the way you are looking, than the electrics.. so look for fault in your methodology before the bikes electrics... and top tip here is to use car jump-=lead to ensure good earth for the spark-plug (As well as good new spark-plug?!)

And domnt 'sixate' on the sparks: - many reasons a bike wont rund, lack or weak sparks is but one; good odds on CGT type motor that the tappet clearances have never been tioched or have, and either way the valves are either never opening or never closing and or timing is well out, head gaskst could be gone, and cpmpression down and or the carb has been cleaned to death... of ALL the possoble reasons bike might not start, not making sparks is actually the least likely IME... and how you trying to make sparkss Good bet by spinning mpotor up on the strarter.... and remember its self exitred sparks, they dont come out the vattery,. so IF the motor no spin so quick, you no get strrong or any sparks.... try looking at the starter, cleaning up the brushes and solenoid, fitting new battery so that it does spin motor over fast enough to make beefy sparks.....

You could and are likely to be chasing a red herring, with the wrong kind of fishing fly!
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[WD Forte]

I'm giving you Karma for
testing not guessing ie swap and hope
and laying out the symptoms in a clear concise way
Well done that man !

I assume its an AC-CDI type ie powered from a stator coil not 12V?
The 50-60 VAC you're getting onthe Black/Red looks low to me, I'd expect more and suspect the source coil isnt supplying the CDI with enough power.
The AC gets rectified in the CDI unit so you'll get a lot less output and CDI coils need to run at much higher LT voltages then 12v ones

Re the CDI itself
With known good source coils!
In the absense of a scope, a quick and dirty CDI output test on early CXs with similar AC CDI ignition was to
Disconnect the CDI to coil wire
put the digital meter in the 250VDC range
crank the motor on the button and read the voltage pulses
between the CDIs coil ouput terminal and ground
A good system would give say 150-200VDC reading whilst cranking
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bikers smell of wee

[Easy-X]

My bet is the spark plug coil. Regardless of the "normal" readings you get it might still misbehave when you attempt to put it into operation.

Shouldn't cost much. Also, did you buy an "ordinary" CDI or some fancy "racing" one 'cos from personal experience I know the latter can be a waste of time
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Husqvarna Vitpilen 401, Yamaha XSR700, Honda Rebel, Yamaha DT175, Suzuki SV650 (loan) Fazer 600, Keeway Superlight 125, 50cc turd scooter

[andy46]

Tef the elderly shagged plug be very eld n shaggled, not at all pleasing to the eye with rusticus and other carp but both it and it's noo replacelment give luverly big phat bloo sparkies when tested in a differential boike.

WD - This is where i was wanting to head next but I couldn't find any figures for what the output from the red/black should be, If as you say the cdi output should be in the hundreds VDC then an input of 60VAC isn't going to do it.

i have now tested the cdi output as you suggested and the old one shows about 1VDC while the new one shows about -1VDC. Probably fair to assume they aren't getting the required input.

I'm thinking this could be a buggered ignition winding on the stator or it might be weak magnets in the flywheel.
To help me try to work it out, what sort of AC output should I be getting from the other windings on the stator? (two thickish wires 1 pink 1 yellow)?


in other news. a proper compression test has already been done - 145psi which aint brilliant but it should run, and possible fuel issues have been bypassed by a quick squirt of easy start. I really believe that the bike will not run untill I can sort the spark problem, regardless of what else could be fixed or made better. i also tried a different ignition coil a few days ago - not a new one but one kicking around in the shed, possibly from an old innova, it gave similar resistance readings.

[WD Forte]

Don't get confused
AC from the source coil (open circuit) and pulsed DC after the CDI
the magnets a possibility but its usually windings.
Had to fix lots of bad windings but dont ever recall finding stator magnets
so bad as to need replacement.

The AC from the source coils is rectified in the CDI box to charge up a capacitor until a pulse from the trigger coil
turns on an SCR which 'dumps' the caps energy to ground collapsing the coils EMF and so on.

Tldr:
The point is, low energy in = low energy out

I've rewound some myself usually for the lads bikes but they're so damn cheap I wonder if its worth it
the one below for instance is only £23

https://www.ebay.co.uk/itm/125cc-Motorcycle-Stator-for-Chinese-CG125-OHV-156FMI-157FMI/161163725711?epid=18003267730&hash=item25861b4b8f:g:kkIAAOxyLN9SmeLk
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bikers smell of wee

[andy46]

Yeah I've just ordered that, the one saving grace of these chinky bikes is that parts are cheap.

Many thanks for everyones input - I'll post back when I've fitted it with results.

[andy46]

Well the good news is that the bike is running and running well, starts easily etc.

But it needed the new cdi AND a new ignition coil (I pinched a coil off another running bike in desperation after fitting the new stator and still not getting a spark).

So top marks to Easy-X for being 100% correct.

it still needed the new (bog standard) cdi tho - no spark using the old one.
New stator/source coil gave the same readings as the old one.

Is there a way that either the cdi or ign coil could fail and take the other out with it? Fitting a new or known good one of either of them didn't produce a spark without fitting the other and for both to fail at the same time is a bit too much of a coincidence.

I think this is one of those rare occasions where random fitting of new (or KNOWN good) components would have worked better than testing with a multimeter. But it's ALWAYS going to be a good idea to check wiring continuity, earths and switches as a first call.

Lesson for the day: Don't use some old shit part you find in the shed without being 100% sure it actually works (not just appears to test OK). Every day's a school day - never did like school.

[Easy-X]

Glad you got it running
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Husqvarna Vitpilen 401, Yamaha XSR700, Honda Rebel, Yamaha DT175, Suzuki SV650 (loan) Fazer 600, Keeway Superlight 125, 50cc turd scooter

[Tankie]

A good informative thread, with a good conclution
I've had cdi's destroyed by kill switches before, going short, so maybe a coil going short will make the cdi work too hard and pop!
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