School Me On Ignition Coils.......

[spokerider]

I'm trying to sort through a spark problem on the 84 TY 250 mono....

1] Had the bike 4 weeks now, two rides on it, and a bunch of time working on wheels, carb, etc. When I got it, it started on the 3rd or 4th kick. It ain't like that now.

2] Noticed it's getting harder and harder to start, each time I try to.....needs more kicks. Had a plug fouling [ over rich gas ] problem which didn't help. Eventually it needed a new plug just to start it when cold, a lightly used plug would not spark.

3] Went for a ride today, as usual, was a bugger to start. With a brand new plug it ran however. Then I tried crossing a river that looked 1.5 ft deep [ was last week ] ......wrong, it was deeper....much. Got swept over a drop off and submerged the bike. 2 hrs drying it out etc, and with a new plug it ran again. No problems for the 4 miles back to the truck.

4] Back home, doing the oil changes to get the H20 out, and even with a new plug it won't start. I can *barely* see a faint spark when I shade the plug tip from daylight.....even then it's tough to see. When I hold the plug and my hand is the ground, the shock I get is very anemic. Ever since I got the bike I thought the spark was very small and weak....

5] The coil to frame ground looked fine, but I cleaned it up anyway. Wires all seem fine and intact around the coil and that other little electronic "gizmo" just up front of the coil. Haven't checked the wires coming from the magneto yet. Tried to unscrew the high tension lead from the coil to check for appropriate resistance, but I don't think it unscrews?

6] Does this sound like a coil problem / fix? Other? My stihl chainsaw just quit the other day......it needs a new coil.......grrrr. These two strokes are making me mental.

7] Thanks for your insight.

[lineaway]

Start by pulling of the flywheel cover and drying everything out. XCheck all connections. 99.99% it is not the coil. It could be the wire or cap or stator, but not the coil. I worked at a yamaha dealer for years and can not remember a coil going bad. Lots of CDI`s and stators, but the only coils we sold was to backyard mechanics.

[dond]

I have worked as a motorcycle mechanic for 40 years and I have rarely seen an ignition coil go bad. That said, I did have one go bad on my 1993 Yamaha TYZ250. It got proressively harder to start and after checking everything else, I replaced the coil (I had one to try) and that solved the problem. However, as was previously said, that is a very rare fault. I would recommend that you get the specs for all three coils (exciter,trigger,ignition) and measure them. Replace any not within specs. Then if the problem is still there, replace the CDI box. The best thing is if you can find someone with a similar bike who will let you swap parts one at a time until it works. Then you will know which part to buy. DonD

[spokerider]

Thanks for sharing your knowledge you two guys, it's helped me get a better understanding of the problem I'm trying to solve.

I found an online manual for the TY 350 [ specs should be the same as a TY 250 mono ] and have read the electircal section on troubleshooting a no-spark situation. It has also provided the resistance specs for the electrical components, and I have just finnished the step-by-step testing with a multimeter.

Here are some specs from the manual and my test results:

Source coil leads spec 240 ohms +/- 10% Mine tested at 157.4 ohms.

Pickup coil leads spec 4.0 ohms +/- 10% Mine tested 4.1 ohms.

Ignition coil primary resistance specs 1.0 ohms +/- 15% Mine tested 1.1 ohms.

Ignition coil secondary resistance specs 5.9K ohms +/- 15% Mine tested 5.9K without the plug cap on the high tension lead and 11.6K with the plug cap on [ not sure if the plug cap reading is significantly high?? ]

So it seems that the source coil is low resistance..........would this be the cause of the weak spark?

Just what is the "source coil"?

I haven't been able to remove the flywheel yet as I don't have the Yam specific flywheel puller tool.

Thanks.

[stork955]

G'day, you have found your problem, the source coil is shorted internally. This coil provides all the power for the ignition system. It typically makes about 50 volts AC at kickover, rising to 100 + volts AC running. If it is weak in its output the whole system falls down. It typically fails through vibration and heat. Sometimes weak magnets in the flywheels can also cause a low output here as well, but your issue is the coil itself. The resistance measurement effectively checks the length of the coil and if it is low it means that the coil is touching itself (no pun intended...) somewhere inside and bypassing some of the windings resulting in a low output. The actual wire is just copper wire coated in a varnish to act as insulation and this is what has actually broken down. The only cure is a rewind or replacement.

It would pay to check if your bike is meant to have a resistor plug cap also, many early CDI bikes never used one and this can also cause issues with the CDI if incorrectly fitted. 5K is a typical plug cap resistance.

HTH,

Cheers,

Stork

Edited April 15, 2009 by Stork955

[spokerider]

Stork,

Thank you for the informative explanation on what a source coil does, what has exactly broken down with mine [ causing the low resistance ] and the effects of the dysfunctional coil on the running of the bike. I now understand it.

I have ordered a new source coil from the local Yam dealer and it's coming right from Japan, no less........ just have to hurry up and wait for it to arrive.....

The spark plug cap is the original piece. It appears that it IS a resistor cap. I bought a NGK non-resistor cap today, and was going to install thinking " more juice at the plug" is a good thing. Would you recommend against that idea? May damage to the CDI result with a non-resistor cap?

Thanks,

Spoke......

[andy.t]

I hope you have found the problem, I would like to point out that all the resistance readings given in manuals are not always correct, lets hope your diagnosis is correct.

[stork955]

Gday Spoke, stick with what is specified originally for your plug cap - the system is designed to work reliably with this in mind. More volts wont make the bike go any better but it can damage the CDI or break down insulation in the ignition coil causing failures here. In almost every case what the manufacturer did originally is best for the bike. Resistor plug caps are most commonly used on virtually everything today so they should be cheap and easy to get if you need to replace it. A good tip here is to put a squirt of dielectric grease onto the end of the lead from the coil to help stop water getting into the screw connection and causing corrosion problems which can haunt you down the track.

Cheers,

Stork

[spokerider]

Well the source coil came in from Japan [ they are still available unlike the cdi units ] and I installed today.

The spark at the plug is now a nice fat, blue spark, just what I was hoping for! The bike went on the first kick, so, it appears that the problem is solved.

Now, I'll see if the improved spark has any effect on the rich-fouling spark plug problem with the next ride out. Would be nice if that problem was corrected too!