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Yamaha YBR 125: No Spark Diagnostic Checklist
If your YBR 125 won't start and you've confirmed there is no spark at the plug, follow this logical path to find the fault. Rule number one: Do not buy parts until you have tested the old ones!
Step 1: The Basics (Free Fixes)
Fuses: Pull every fuse out. Don’t just look at them; clean the metal tangs and ensure they seat firmly. Spray the holder with contact cleaner.
Battery Voltage: A weak battery might turn the engine over but not have enough "juice" left to trigger the ECU/CDI.
Target: 12.6V – 12.8V (Resting).
Kill Switch & Side Stand: These are the most common "hidden" culprits. Use a multimeter to check for continuity. If the side stand switch is gummed up with chain lube, the ECU thinks the stand is down and cuts the spark.
Step 2: Clean Your Connections
Go through the entire wiring loom and unplug every connector block you see.
Spray Electrical Contact Cleaner inside, plug/unplug them 3-4 times to "scrub" the pins, and reconnect.
Check for "home-made" wiring repairs from previous owners; these are frequent fail points.
Step 3: Test the Ignition Coil (HT Coil)
Primary Side: Check for 12V at the coil input while cranking the bike.
If you have 12V here but no spark, the coil or HT lead is likely dead.
If you have no 12V here, the fault is further back (Wiring, Crank Sensor, or ECU).
Secondary Side: Check the spark plug cap. Sometimes the resistor inside the cap fails. Try unscrewing the cap and holding the bare wire near the engine to see if a spark jumps.
Step 4: The Crank Position Sensor (Pick-up Coil)
The ECU needs a signal from the generator area to know when to spark. If this sensor fails, the bike is "blind."
Test the resistance with a multimeter against factory specs (usually 248–372 ohms).
Step 5: The ECU / CDI
If everything else tests perfect—the wiring is clean, the sensors are within spec, and the switches work—only then consider the ECU. These are expensive, so try to borrow a "known good" one from a friend first.
Going to do a post on not sparking , as just come across this again on a bike and id forgot everything
so had to go through all the tests again .
Id just like to say i am not really that good with electrics but i know something test first , do not buy parts and read and learn as much as you can .
First off check all easy stuff .
So -
Fuses , Check all of them , make sure they are seated corrected actually pull them out clean contacts and refit spray contact cleaner in the terminals
Battery , make sure its fully charged remove from bike and recharge dont just guess ,
and whilst doing this clean up the battery terminals and the negative and positive wiring terminals .
Test battery at the negative and positive terminals
If you have a voltage meter you can do some basic tests.
When the engine has been off for at least 1-2 hours with ignition key turned off,
100% Charged 12.60 to 12.8v
75% Charged 12.4v
50% Charged 12.1v
25% Charged 11.9v
0% Charged less than 11.8v
Connections and Connector blocks , go though all the connector blocks take them apart spray contact cleaner inside then then put them back together repeat this a few times on each connector as it will clean the contacting surfaces . If there are any home made repairs or connections redo them , its sound like a lot of work but i have lost count of how many non running bikes i havve bought and its been a faulty wiring connector .
Side Stand and Kill switch , fully confirm that these are working properly with a multimeter do not assume they are ok , do not buy any parts until you have tested these , ill do these tests on a post one day but they are simple , see the video below and the recommended book as well .
Generator test , tbh i am sure the bike would run with a faulty generator but probably not for long
so i test this now , its an easy test and at the very least confirms it working or not , see our post on this here .
HT coil , Next i would test the ht coil , first id test the primary side for 12v whilst turning the bike over , if you have got 12v here then test the coil if you have not got 12v to primary side of coil , then
its likely to be ECU ( if its not wiring fault ) , if you have 12v to here great you just elliminated all the expensive stuff , test the HT coils primary and secondary sides read this post here .
CDI , if all else has failed try a known good CDI , but prior to fitting make sure all is function
properly on the bike it might of burned the old CDI out for a reason , ill do a CDI test post one day
if i can figure it out .
Rectifier regulator , You should have found the problem buy now i have never got this far , now
the rectifier only changes the output of generator to DC to recharge battery and run lighting system so this will have nothing to do with the ignition system , but some have told me they changed this and
they got a spark and seeing as all else has failed !! .
Oh someone pointed out i never mentioned the ignition switch , which is true , i have never come across this myself but it would be worth check continuity on the ignition switch .
Here is an Excellent video on NO spark fault finding watch it all !!
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