Weak spark - possible causes? [SOLVED] - [UPDATED]

I'm helping a friend who has just bought a 1987 T80.

It runs OK when cold, but when it reaches normal running temperature a strange fault happens: if you leave it idling, after two or three minutes it will suddenly cut out.  It is then difficult to restart, but after enough kicks it will start and run for another few minutes, then cut out.  The cut out is sudden - like switching off the ignition.  Some points:

• it drives OK - the problem appears when you come to a halt at a junction, for example
  
  
• the carb has been fully dismantled, ultrasonically cleaned and all passages/jets blown through with compressed air
  
  
• the fuel level in the float chamber remains constant whether the engine is revved, idling, and when it cuts out (as measured with a transparent tube connected to the drain and pointing upwards) so it doesn't seem likely to be a fuelling problem
  
  
• a new coil and new spark plug have been fitted
  
  
• checking for a spark immediately after cutting out, the spark seems incredibly weak when kicking the engine over - barely visible and won't jump more than maybe three or four millimetres
  

I've measured the ignition pickup coil resistance as 330 ohms, which is bang on what the Haynes manual says.  However, the ignition source coil resistance is 190 ohms.  This is a bit lower than the 240 ohm +/- 10% stated in the Haynes manual, but not by much.  Maybe some turns short out at high temperatures?  So, some questions arise:

1. Do you agree this is likely to be an ignition problem?
   
   
2. Do you think the ignition source coil resistance is far enough out to be a concern?
   
   
3. Or is it more likely to be the CDI unit playing up?
   
   
4. Is there a recommended flywheel puller available?
   

Thanks in advance for any help.  
PS: I've never ridden a T80 before but I think it's a cracking little bike and now I want one!

Looks like the CDI unit is faulty, cant be repaired so you will need to find another, they often come up on eBay
This is the flywheel extractor tool you need https://www.ebay.co.uk/itm/YAMAHA-TOWNMATE-T80-YFM80-RAPTOR-FLYWHEEL-REMOVAL-EXTRACTOR-TOOL-/254362646939?var=0&mkevt=1&mkcid=1&mkrid=710-53481-19255-0&campid=5338268676&toolid=10044&customid=CjwKCAjwjbCDBhAwEiwAiudBy8bYxjV9HDpZ3guhI1n_AAYIecP6yUyjNFMLYC1LweF5hfnu_DPqfRoCuz8QAvD_BwE

Thank you, JohnW.  I've ordered a new CDI unit - Chinese made ones are for sale for about £20 on eBay.  I've no idea what their reputation for quality is, but this one looks like it should work:  (Ooops, I'm not allowed to post links, but it is item number 193948727336 on eBay).

And again, thank you for digging out the flywheel puller - very much appreciated.  

When the engine is off and warm, have you checked during attempts to start the engine if it there is spark in the spark plug?

There is also a possibility that the winding of the voltage multiplier is responsible, if the winding has interrupted internally, perhaps when it heats up and due to the expansion it presents the problem again.
It would be good to measure the continuity of the voltage multiplier when it is hot and the engine does not start.

Thanks, Duckrider.  This is what's weird: when the engine is warm and being kicked over, there is just the faintest spark.  It would be so much easier if the spark disappeared completely, then it would be easier to diagnose.

How "hot" is the spark normally on these when kicking over?  Is it easy to see and hear?  If they are all like that, I might be reaching the wrong conclusion.

When you refer to the voltage multiplier, is that what the Haynes manual calls the "ignition source coil"?  If so, it measures slightly below the specified range at 190 ohms.  I did wonder if maybe some of the turns shorted out when it got hot, so the voltage was still there, but lower.

At spare parts list it called IGNITION COIL ASSEMBLY and is the No 9 in the diagram.
Ιt is the component from which the spark plug cable starts.
This coil is a dual one, containing a primary low voltage coil and a secondary high voltage coil, both coils are connected on one side to the chassis ground or to the negative terminal of the battery.
The low voltage terminal is connected to the electronic ignition via the Green / White Cable and in this coil because it has the fewest windings you should measure less resistance.
The secondary coil has many more windings and at one end it is connected to ground and at the other end to the spark plug cable.

Ιf you have a low quality spark then CDI or the ignition coil is responsible, or even the spark plug cable itself may have a continuity problem.

It would not be bad to check the connectors and contacts from the ignition coils located outside the left engine cover, and clean them with white vinegar and an old toothbrush or a brush.

Ah yes, thanks Duckrider.  A new ignition coil has been fitted already.  I will inspect and clean all the connectors - good shout.  

The electrics on a T80 are normally bullet proof and the source or ignition coils dont normally give problems, nearly always the CDI unit. Hope the Chinese one works! sounds a bit cheap as the last price for a genuine one was £173.52 plus vat with used ones between £30 and £50 on eBay

I have a bad experience with what you wrote John.

Βefore some time i bought a voltage regulator like this one :



It was cheap and I bought it for 7 euros i think

The original was this one



To note here the following, a rectifier even from our moto with a single coil on the coil plate, a single-phase one, logically MUST contain 1/3 of the components of a three-phase motor rectifier, the duck of a newer date.
Its circuit in theory is this:



And the rectifier is what is contained in the red square.
It contains 4 rectifier diodes (left), two thyristors (in the middle that look like switches) and a voltage control circuit (regulation circuit).
Unfortunately, when I installed the cheap rectifier, the light coil burned out
So I start digging carefully, not to destroy it but to remove as many parts as I could in good condition to recognize them ....

So he started searching for the plug. Which became so from the temperature:



I started removing material on the back of the plug which had three terminals.
The one cut, I show it here:



in order to work, and from what the electrical diagram shows, an input terminal of the alternating voltage of the coil is required, one terminal is the ground and one is the output of direct voltage to the battery.

after cleaning the plug, as shown in the photo above, I found ONLY one rectifier diode of some power cracked in the middle,

Here I show with the green arrows that the passage was bet on the plug, you can also see the part of the passage that came off in the effort of the excavations





But anyone who has some knowledge of electronics definitely understands what I came across and has definitely understood what game is being played over here.

And I explain:

Normally the Townmate charging circuit is this:



Shown are the W and Y / R cables that supply voltage from the upper coil to the rectifier (two terminals) and the R supply to the battery (the third terminal)

In this arrangement in the photo below the connector in yellow is the AC input cable to the rectifier and the red with the outer casing / heatsink is the output (+) continuous to the battery while the (-) of the battery is common to the frame and metal housing of the rectifier.



So what has the manufacturer of this rectifier done?

Because the coil as you see above is dual, it has a winding (the top) for the charging system where the W and Y / R cables supply voltage from the top coil to the rectifier without having to do with ground or frame with other words, and a second winding that the Y / R in relation to ground gives to the lighting circuit.
This is the reason why the lighting on the old ducks at least (I do not know what plays with the new ones) flickers and intensifies in relation to the engine speed.

But what is happening here? the lighting coil gives without charge somewhere 28 V alternating which with the load mainly of the front lamp drops to 7 volts.
The other coil, however, produces (it would be more correct to produce) three times the voltage, ie somewhere around 55-60 volts.
and this is the reason for the necessary use of a voltage regulator inside the rectifier.
That is why it is called a rectifier or a regulator or commonly R / R.

Of course these voltages are produced at slightly increased engine speeds, I do not have a tachometer but still more than half the throttle.
So it makes sense according to the above that the larger coil produces in addition to greater voltage and more power, which is why it produces the power required to charge the battery, instruments, fuel gauge, turn signals and rear stop.
But here what did the manufacturer think and implement?
a) DO NOT CONNECT the strongest coil, if you remember in the photo above one of the plug terminals is cut internally, so one coil produces power in the air.
b) install a rectifier diode of half a euro making dld a simple rectifier in the alternator in order to get half voltage from the smaller coil.
So from the 28 volt alternator it will take somewhere 14-15 continuous.
This way, however, it charges the battery without any voltage or current control since the voltage regulator circuit is completely absent.
c) does not worry about whether more power is produced, since the weaker lighting coil is forced to do two jobs and its voltage "kneels" more directly, so what to do to stabilize it?

Result:

a) The battery charges, marginally, but it charges, but if you turn on the lights it does not charge, you go until you run out of battery and when you stay;
you just say ... the rift has subsided, slowly, 7 euros makes I will put another.

b) with the lights on, the lighting coil due to its low power pulls a strong paddle, its current is not enough and in a little while it burns.
What will you think? the battery of course, that you put a cheap rim and burned your coils.

c) the rectifier may burn, like the one found in my hands. But again you will throw them in the battery. You will not open the lid to take out the ruffles to see the coiled coil.
Who has the tools? or money to deal with and get involved with crews;

This case fell into my hands, fused / made rectifier and burnt lighting coil.
I broke my head to figure out how?
How did the coil that is NOT related to the rectifier burn?
Cost of purchasing a rectifier; and in fact 12 volts; under 5 euros.
I was ready to make the purchase.
So let's be careful what we buy, cheap is good but it can get us in trouble sometimes.

PS: Sorry for my English , i use Google translate

The voltage regulator has no function for ignition, it only controls the battery charging voltage, again I have never had a problem with these.

JohnW wrote:The electrics on a T80 are normally bullet proof and the source or ignition coils dont normally give problems, nearly always the CDI unit. Hope the Chinese one works! sounds a bit cheap as the last price for a genuine one was £173.52 plus vat with used ones between £30 and £50 on eBay

Actually, that's a real help.  I've decided not to risk spending money on the stator plate and coils and this stage; instead I've ordered a secondhand T80 CDI unit from eBay, plus I've got one of those cheap Chinese ones on its way, which (if the secondhand one works) will do as a spare.

I'll report back on how it goes.

Duckrider wrote:I have a bad experience with what you wrote John.

Thanks for telling us that horror story, Duckrider!  Although not strictly a worry for the ignition system on the T80, it is a strong warning against buying low cost, and possibly incompatible, electronic modules.

JohnW wrote:The voltage regulator has no function for ignition, it only controls the battery charging voltage, again I have never had a problem with these.

Yes, you are right, I mentioned it commenting on the cheap spare parts of dubious origin

And I'm sorry I took advantage of this topic, normally I should open a new one.

Thack wrote:

Duckrider wrote:I have a bad experience with what you wrote John.

Thanks for telling us that horror story, Duckrider!  Although not strictly a worry for the ignition system on the T80, it is a strong warning against buying low cost, and possibly incompatible, electronic modules.

I will try to check the genuine rectifiers that have spares if they contain or not voltage stabilization
and I will report here the results.

I can not believe why Yamaha could build a rectifier without a rudimentary stabilization of the output voltage or even a limiter that limits the voltage to 7.5 volts, in my own battery I have measured up to 7.9 volts which is too much charging voltage for a 6 volt battery.

Duckrider wrote: In my own battery I have measured up to 7.9 volts which is too much charging voltage for a 6 volt battery.

I agree! 14.4V to 14.8V is a good charging voltage for a modern 12V automotive lead acid battery, and half that - 7.2V to 7.4V - for a 6V battery.

Just to let you know: I bought a genuine, second-hand CDI unit from eBay, and it all works perfectly.  The bike starts and runs just as it should.

I just wanted to say a big "thank you" to everyone who helped on this thread - very much appreciated.   Cheers, everyone!  

This is the first time I have heard of faulty CDI.

So, as you are sure that it is defective, it may be an opportunity to  find out what is wrong with it.

As I did with the rectifier, you have to remove the soft silicone with a hot air gun and a small screwdriver, but it takes patience and I do not know if it is worth it.

Thack  i really appreciate you informing us about the solution of the problem.

If I'm not mistaken you have ordered a cheap one, will you check it as soon as you receive it? I would like to know if it will work properly.

Duckrider wrote:This is the first time I have heard of faulty CDI.

I was motivated to try another CDI unit by JohnW, who wrote: "The electrics on a T80 are normally bullet proof and the source or ignition coils dont normally give problems, nearly always the CDI unit."  So, apparently JohnW has heard of faulty CDI units.    

In all seriousness, I was satisfied that the spark was extremely weak when the symptoms were occurring, and really there are only three feasible options: the ignition source coil, the CDI unit, and the ignition coil.  (I don't think it's feasible for a faulty trigger coil to cause a weak spark - it would either trigger or it wouldn't.)  I'd already replaced the ignition coil, so that left the source coil and the CDI unit.  John W's view (and mine, to be honest) is that an electronic failure is more likely than an intermittent low output from the source coil.  Luckily it turns out to have been right!

Duckrider wrote:As I did with the rectifier, you have to remove the soft silicone with a hot air gun and a small screwdriver, but it takes patience and I do not know if it is worth it.

I might do, if I get time and it hasn't been thrown away.

Duckrider wrote:If I'm not mistaken you have ordered a cheap one, will you check it as soon as you receive it? I would like to know if it will work properly.

That's a good idea, and I will report back.  The owner of the bike is a friend-of-a-friend, so it might get complicated.  But if he is willing to co-operate I will definitely check the Chinese one and let you know.

Thack wrote: ]

That's a good idea, and I will report back.  The owner of the bike is a friend-of-a-friend, so it might get complicated.  But if he is willing to co-operate I will definitely check the Chinese one and let you know.

Ok, I will expect results.

A technical summary:

The symptoms were intermittent cutting out after the bike had been running for a few minutes.  Restarting took many kicks.  Quite often it would idle, but cut out as soon as the throttle was opened (as if to pull away).

Many people think that a fault that is triggered by opening the throttle must be a carburation fault (e.g. running weak), but in fact a low spark voltage can cause exactly those symptoms.  That's because opening the throttle increases the pressure in the cylinder, and that increases the voltage required to jump the spark plug gap.

The formula for calculating the voltage required to make a spark at the spark plug is:

 breakdown voltage V = 4.3 + (146 * P/T) + (324 * P/T * Dg)      Pashley et. al 2000, SAE paper 2000-01-0245

where:

V = spark voltage (kV)
P = cylinder pressure at moment of spark (bar)
T = cylinder temperature at moment of spark (K)
Dg = spark plug gap (mm)

So you can see that increasing the pressure P will increase the breakdown voltage V of the spark gap.  That's why a weak spark might be OK with a closed throttle (low pressure), but fail when the throttle is opened (higher pressure).

Just thought you'd like to know that!  

@duckrider, et al

I promised I would report back on the Chinese CDI unit.  Well, it works perfectly.  In fact, the owner reckons it starts better on the Chinese one than the genuine item, which I suppose is possible, what with electronics having come on in the thirty years since it was made.  He did a long(ish) road trip yesterday, and it worked perfectly.  He plans to leave it on the bike, keeping the genuine one as a spare.

This is the item I bought, for just under £17 (free postage):

https://www.ebay.co.uk/itm/233930331042

I should say, though, that the same item is available from numerous other sellers, all for around the same price.

So, I can recommend these cheap Chinese CDI units, and they're so cheap it's probably worth buying one just in case you need it one day.  They take three or four weeks to arrive from China.

And once again: thank you - from him and from me - for helping us out.

Thack

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Thack wrote:@duckrider, et al

I promised I would report back on the Chinese CDI unit.  Well, it works perfectly.  In fact, the owner reckons it starts better on the Chinese one than the genuine item, which I suppose is possible, what with electronics having come on in the thirty years since it was made.  He did a long(ish) road trip yesterday, and it worked perfectly.  He plans to leave it on the bike, keeping the genuine one as a spare.

This is the item I bought, for just under £17 (free postage):

https://www.ebay.co.uk/itm/233930331042

I should say, though, that the same item is available from numerous other sellers, all for around the same price.

So, I can recommend these cheap Chinese CDI units, and they're so cheap it's probably worth buying one just in case you need it one day.  They take three or four weeks to arrive from China.

And once again: thank you - from him and from me - for helping us out.

Thack

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Presumably you did not have any import duty to pay when it was delivered?

Nope, no import duty. I don't think they bother with low value stuff like this. I buy quite a few electronic bits and bobs from China, and have never had to pay any excess charges.