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R125 Misfiring And Juddering When Under Small Amount Of Throttle

Discussion in 'Technical / Mechanical Discussion' started by Arron Owen, Oct 29, 2019.

  1. Arron Owen

    Arron Owen

    Current Bike (Optional):
    2015 Yamaha R125
    License Currently Held:
    UK Provisional (CBT)
    Hello all , i have all of a sudden had issues with my 2015 Yamaha R125 , when under a small amount of throttle the bike judders ever so slightly as if its losing spark for a split second and seems to be misfiring , the bike had its valves adjusted 200miles ago and new oil and filter in at the same time and also starts straight up. It has a black widow exhaust system and a K&N airfilter installed and has covered a total of 14k miles and hasnt had a single problem from the 4 months they have been installed on the bike. Im hoping its something minor like just needing a new spark plug and not anything to do with the timing could anyone advise what it could most likely be with the symptoms im experiencing , thanks in advance!
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  3. NorthCust

    NorthCust Active Member

    Home Page:
    Current Bike (Optional):
    2008 Yamaha R125, Gasgas TXT 280, Stretched Neos
    License Currently Held:
    UK Full Bike
    heya,

    it sucks you're having that issue - I don't know what could be causing that issue but I would imagine maybe checking the state of the spark plug (and gap) might be a good start, or maybe even putting a fresh spark plug in might not be a bad idea. I'd probably also check all the leads are good and tight (e.g. HT lead). Hopefully someone else will have a better suggestion - capt slow?

    if it's not something like a loose contact, am thinking maybe battery voltage could be low? How's your battery voltage (when not running and when running)?
  4. Arron Owen

    Arron Owen

    Current Bike (Optional):
    2015 Yamaha R125
    License Currently Held:
    UK Provisional (CBT)
    the battery is fully charged as i have it hooked up to an optimiser during the night , i will be getting a new spark plug in the morning and will get a multi meter to the ignition coil , ht lead and cap , checked the HT lead and it is tightly secured to the spark plug cap , my only other guess could be something like the ignition coil or the spark plug as the day before it ran perfectly , the plug itself indicates its running well fuel/air wise
    NorthCust likes this.
  5. Captain Slow

    Captain Slow Super Member

    Location:
    Fleetwood, Lancashire
    Current Bike (Optional):
    R125, R3, CBR929RR, R1
    License Currently Held:
    UK Full Bike
    Does this sound familiar? ......

    When the bike is cold it starts without any problem but then after about 1 mile or so when you go to set off from stationary it has a little 'cough' yet doesn't stall??????

    If this is what you are experiencing then its been a unique feature of my R125 in all the years I have had it. Winter. Summer. All year round. I tried new spark plug, replaced the coil pack and HT lead only last year. New throttle body. New injector, but nothing seems to cure it so I just put it down to a quirky feature of the bike. But then again I thought maybe only my R125 did this.

    If this isn't what your finding then I would do as Northcust suggsts
  6. NorthCust

    NorthCust Active Member

    Home Page:
    Current Bike (Optional):
    2008 Yamaha R125, Gasgas TXT 280, Stretched Neos
    License Currently Held:
    UK Full Bike
    > the battery is fully charged as i have it hooked up to an optimiser during the night ,

    I would still just double check it on a multimeter - only reason I say that is that I used to leave mine on a trickle charger over the winter (because I'd had flat batteries the previous winter) and whilst it always started first time without fail. When it went in for a tune-up the garage noticed that one cell of the battery was dead which was apparently putting a strain on the electrical system...they were suprised it hadn't melted (apparently) so things may not always be as they appear to be.

    i'd have thought that an optimiser (presumably that's like a solar regulator where it lets the battery discharge a little and then tops it up, whereas mine was on a permanent, albeit allegedly "smart" charger) would be better than what I had but the voltages will tell you for sure.

    Not running it ought to be in the 12.1-12.5v sorta range depending on the state of the battery, and running I normally see 13.8v. It's probably not that but always good to rule out the basics.


    > If this is what you are experiencing then its been a unique feature of my R125 in all the years I have had it.

    interesting. I don't think mine did that, even before the PCV, though that's a while back now.
  7. Arron Owen

    Arron Owen

    Current Bike (Optional):
    2015 Yamaha R125
    License Currently Held:
    UK Provisional (CBT)
    Replaced the spark plug , took out the o2 sensor and cleaned it and resolved the issue i have had
    NorthCust and Captain Slow like this.
  8. Captain Slow

    Captain Slow Super Member

    Location:
    Fleetwood, Lancashire
    Current Bike (Optional):
    R125, R3, CBR929RR, R1
    License Currently Held:
    UK Full Bike
    It's what make mine 'Special'. It's own little cough when we start a journey.
    NorthCust likes this.
  9. NorthCust

    NorthCust Active Member

    Home Page:
    Current Bike (Optional):
    2008 Yamaha R125, Gasgas TXT 280, Stretched Neos
    License Currently Held:
    UK Full Bike
    oh ace! good job :) So what year is that then? Presumably 2014-> ?

    I didn't know the later ones had an o2 sensor. That could make modifying them much easier since you already got a slot to plug a wideband in. Hmm.

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