SR125 Coil advice?

[Farmboy81]

Hi guys, hoping for a little advice about my 2nd bike, a '91 Yamaha SR125 that I've picked up for cheap so I don't drop my Fazer when riding in the ice and snow.


It doesn't run currently and the problem seems to be the coil (way off spec according to the Haynes). All the replacements I can find online have "12v CM11 50" on the side, whereas mine has "12v CM11 55".


As far as I understand it (which isn't well), the 50/55 bit relates to the distance between the posts where it mounts to the frame? The only other difference I can see is that the "50" ones have bullet connectors, whereas mine has a 2 pin connector.


I know I can file the mounting bracket to fit, and change the connection on the loom to bullets, but I don't want to do that if it's not going to work!


Any advice? (example below of the "50" type)


https://www.ebay.co.uk/itm/Ignition-Coil-12v-AC-Single-as-fitted-to-Early-RDs-XSs/302816413195?_trkparms=aid%3D111001%26algo%3DREC.SEED%26ao%3D1%26asc%3D20131017132637%26meid%3Dd83cbb58c9044c84840d26c98f3e5aee%26pid%3D100033%26rk%3D6%26rkt%3D8%26sd%3D223208672496%26itm%3D302816413195&_trksid=p2045573.c100033.m2042

[BBROWN1664]

why not just get the right one?
https://www.ebay.co.uk/p/Ignition-Coil-Yamaha-Xz550-XZ-550-550xz-Cm11-55/1837091390

[His Dudeness]

There's a very cheap one. It looks the same to me but you could ask the seller before you buy it

 https://www.ebay.co.uk/itm/Yamaha-SR-125-Ignition-coil-1993/112979408000?hash=item1a4e18f480:g:qZEAAOSwKWha8IHM:rk:16:pf:0

Are you 100% sure the coil is the problem? What tests did you do and what results did you get? Did you confirm there's power and ground going to the primary side of the coil? I'm not sure how the SR125 system works I don't have the wiring diagram but on the bikes I've seen that have two wires going to the primary side of the coil, one of the wires is constant 12V and the other is a switched ground that's switched on and off by the computer. You can check if the computer is providing a switched ground by connecting a Led test light from battery positive to the switched ground wire. If all is good the Led should flicker on and off as you crank the engine. Next test for constant 12V on the other wire by connecting the test light between battery ground and the constant 12V wire. The test light should stay on. If you've got constant 12V and switched ground and still no spark the problem is the coil, ht lead or spark plug. As I said though I'm not 100% sure that's how the SR125 works since I don't have the specific wiring diagram.

[Farmboy81]

why not just get the right one?
https://www.ebay.co.uk/p/Ignition-Coil-Yamaha-Xz550-XZ-550-550xz-Cm11-55/1837091390

Just because I'd rather buy new rather than second hand.


@ His Dudeness, You are right about the constant and the switched. I definitely have full 12v feed, and the coil will generate a solid spark at first then dwindle away to nothing. I've hooked up the multimeter and check the resistance of the coil. rough figures (don't have it in front of me) were:


Haynes
orange to red - 2.75 ohm +-10%
orange to ht - 7.5k ohm +-10%


Mine
orange to red - 3.2 ohm
orange to ht - 13.4k ohm


which seems like coil has an issue, even if it's not the only issue.

[His Dudeness]

why not just get the right one?
https://www.ebay.co.uk/p/Ignition-Coil-Yamaha-Xz550-XZ-550-550xz-Cm11-55/1837091390

Just because I'd rather buy new rather than second hand.


@ His Dudeness, You are right about the constant and the switched. I definitely have full 12v feed, and the coil will generate a solid spark at first then dwindle away to nothing. I've hooked up the multimeter and check the resistance of the coil. rough figures (don't have it in front of me) were:


Haynes
orange to red - 2.75 ohm +-10%
orange to ht - 7.5k ohm +-10%


Mine
orange to red - 3.2 ohm
orange to ht - 13.4k ohm


which seems like coil has an issue, even if it's not the only issue.

That sounds like the coil probably got hot and was damaged. If the coil is damaged it'd be a good idea to replace the ht lead, spark plug cap and  spark plug at the same time as the coil because I think if you have high resistance in them it can be the reason why the coil over heated in the first place. You could check their resistances if you want but it'd only cost a few pounds to replace them so you might do the lot to protect your new coil.

[Farmboy81]

Cheers mate, that was my thinking. I've already replaced HT, cap, and plug, but I'll check the resistance of them before i try new coil.


I've bought myself one of the £25 "50" types from ebay, figured it's worth a shot and the modification needed to make it fit should only take 10 minutes.


Fingers crossed...