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Yz 144 sputtering

Discussion in 'MX/SX General' started by GeorgiePorgie, Aug 17, 2015.

  1. GeorgiePorgie

    GeorgiePorgie PR Founding Father

    Location:
    Ohio the 440
    Racing Number:
    740
    sputtering in corners on low end and will miss/Pop every once in a while in the meat of the power.


    Cleaned carb.

    Moved the needle clip:
    up one. No better
    Down one. No better
    Used a 37.5; 40 and 42.5 pilot. Still sputtering. No better.

    Runs great WFO. pops in corners and misses. Would miss in the rollers and almost kill me every lap and I decided to bag it and run my (brothers) other 125 which runs perfectly.


    Help.
     
    Last edited: Aug 17, 2015
  2. MalvernMX

    MalvernMX PR Addict

    Float level.
     
  3. GeorgiePorgie

    GeorgiePorgie PR Founding Father

    Location:
    Ohio the 440
    Racing Number:
    740
    Ran perfect last Saturday at briarcliff. Blew out he countershaft bearing. Rebuilt and have had a headache since chasing richness--or so it seemed.


    Thanks to original 44 for giving me a new needle. Mine was bent somehow. And also Steve that pitted across street for giving me a plethora of jets to try....
     
  4. C Hudak

    C Hudak PR Addict

    Location:
    Lordstown, OH
    Racing Number:
    2
    Look at the reeds yet?
     
  5. ktrop251

    ktrop251 PR Member

    Location:
    Wadsworth
    Racing Number:
    251
    Timing at top dead center?
     
  6. premix

    premix PR Member

    Location:
    Westside
    Sounds like an air leak. You change Crank seals when you split the cases for the CS bearing?
     
  7. GeorgiePorgie

    GeorgiePorgie PR Founding Father

    Location:
    Ohio the 440
    Racing Number:
    740
    Im afraid...but premix is probably right.....

    Wanna try a few other things before splitting cases again.

    Vforce3 Reeds are crispy. Snap closed, look great, clean and aren't broken.
     
    premix likes this.
  8. adam63

    adam63 PR Addict

    To sum it up... it ran correct until the engine was taken apart for a out put shaft bearing replacement? After reassembly it doesn't run correct? Sounds like human error, so now how to fix it?

    What all was replaced?
    For piece of mind did you check compression? (I had one that I chased like that and it had a ring sticking)
    Very well could be a crank seal leaking.
     
  9. grantsdad25

    grantsdad25 PR Member

    Location:
    Austintown, Ohio
    Check the basics first like electrical connections, plug cap connection, etc. I had one with a plug cap that would do the same sputter/miss at low RPM or over jumps/bumps. Needle position is 60% of jetting, so if up/down adjustments made no impact, it may not be fuel related. Crank seal leak, on ignition side, usually impact WFO perfomance too, but it's possible.

    BTW most big bores will need smaller jetting than the original bore did because of the pumping effect/velocity through the same size carb will draw more fuel though the same jet size. It’s counterintuitive.
     
  10. GeorgiePorgie

    GeorgiePorgie PR Founding Father

    Location:
    Ohio the 440
    Racing Number:
    740
    It would break up before the bearing change. I went from a 42.5 to a 37.5 pilot and the break up/sputter was fixed. Disassembled motor and reassembled and now it breaks up on the bottom, sputters and misses.
     
  11. premix

    premix PR Member

    Location:
    Westside
    If I remember correctly you don't have to split the cases to replace the seals on YZ's. Can pull out with a couple of picks after flywheel and mag removed then drive back in. Just place close attention to make sure square and in as deep as old seal.

    The popping you're hearing is either a lean condition (which could be caused by an air leak). Go OEM and be back riding for the weekend.

    Just out of curiosity, what fuel and how does the plug look?
     
  12. GeorgiePorgie

    GeorgiePorgie PR Founding Father

    Location:
    Ohio the 440
    Racing Number:
    740
    Cam2+ethanol free 90 pump cut at 50/50.

    Plug looks great. Changed plugs at track. Checked spark plug cap. All good.
     
  13. premix

    premix PR Member

    Location:
    Westside
    Is the head cut for race gas w the big bore or set for pump gas?
     
  14. GeorgiePorgie

    GeorgiePorgie PR Founding Father

    Location:
    Ohio the 440
    Racing Number:
    740
    Using the athena bolt on....had to mil cases for clearance.
     
  15. premix

    premix PR Member

    Location:
    Westside
    I'm not overly familiar with the Athena kit but if it's doing it under load as you're describing it could also be detonating from not enough octane or timing being off. I would call Athena and get their recommendation for each. And spend the extra money on quality race fuel. Pro car in Akron sells it very reasonably. But once again without seeing or knowing what else is done to the motor, I'm just throwing out potential solutions.

    Start with the seal first. Pull the left side cover. Let the bike run and spray contact cleaner on the seal, if it revs up, you've found your issue.
     
    halffast and TwentyThreeMX like this.
  16. adam63

    adam63 PR Addict

    With that I assume you did not change the crank seals when it was apart?

    Have you looked at the reeds? Could have ate a piece between then and now.

    What did the piston and ring look like? Any signs of the ring sticking in the groove?

    Could always psi check the combustion assembly and not have to guess at weather or not the seal is leaking?

    All new gaskets on reassembly?
     
  17. hershey

    hershey PR Elite

    Location:
    Ozone Layer
    Racing Number:
    01
    Is it popping or missing ? 2 different things. If you were jetting dead on before the heat and it is popping then you are rich on the needle and pilot. It is hotter this past month than it has been all year.

    If it is missing or going flat then you are lean and have an air leak issue from putting it together wrong.
     
  18. ahrma479

    ahrma479 PR Member

    Hello.

    All the above suggestions have merit.

    Here are some of my suggestions you can try.

    1) Unless you did a throttle specific chop IN the throttle position/fuel circuit you are testing plug reading is suspect.
    Simply removing the plug and looking at it is not an indication of AFR plus, you need to look at the spark plug base ring to determine AFR. Plug reading is complicated.

    2) Ambient air temperature/air density have been super high lately, this causes a rich condition. Are the needles you have/tried the same p/n?
    If not, use a micrometer and write down the diameter top to bottom of the two.
    http://www.sudco.com/Carburetor/MikuniCatalogPg32.jpg
    Larger diameter = less fuel = leaner.
    Smaller diameter = more fuel = richer.
    The I.D. size of the needle jet vs the O.D of the needle is the amount of fuel that can flow through the needle jet to the main jet.

    3) I would do a throttle specific to fuel circuit test to determine the exact fuel circuit to change.
    0 ~1/4 = air screw/pilot circuit.
    1/4 ~ 3/4 = needle position.
    3/4 ~ WOT = main jet.
    BUT,,,, ALL fuel flows through the needle jet to the main jet (not counting off idle where the slow speed circuit comes into play) so don't forget that the main jet DOES come into play in the 1/4 ~3/4 throttle position.
    The needle plays a HUGE part in tuning which is why there are so many variations of needle/clip position (s).

    4) Do you set the sir screw each time riding? On both 2 and 4 stroke (although 4 strokes have a fuel screw and the slow speed metering system is completely different) you do need to set the air screw or fuel screw each time you ride to the
    highest smoothest idle once the machine is up to normal engine temp, then re set the idle to where you like it.

    5) Pump fuel is always suspect if you are looking for stability. I did dyno testing when I was at SuperTrapp/Kerker using different local fuels and found a huge difference in quality and performance.
    A dyno room is supposed to be temperature/humidity controlled and have a high air change rate per hour thus a stable testing atmosphere to give consistent results.
    I have done allot of testing in poor dyno rooms where the exhaust gasses remain in the room giving false powercurve readings making fuel re calibration a nightmare. You also get a nice headache.
    Sunoco was the best pump fuel batch to batch.
    Additives in todays pump fuel make it harder to get a proper AFR than 100% race fuel.
    ALWAYS use race fuel if possible, race fuel is consistent batch to batch, pump fuel is NEVER consistent batch to batch in quality and additives. Always, always test ANY fuel for water.
    I use Kolor Kut:
    http://www.grainger.com/product/KOLOR-KUT-Water-Gauging-Paste-8UNU1

    6) Keep a log of carb circuit settings per track and note the ambient air temp and water grains. Sand tracks/muddy conditions are of major concern. Sand/mud taxes the machine increasing engine temperature and throttle position.

    7) Crude ways to check AFR: do a more air less air test. Simply taping over the airbox inlet will show a quick result.
    More air = less fuel =leaner.
    Less air = more fuel = richer.
    Hot ambient temp/high water grains = more fuel = richer.
    Cold ambient temp/low water grains = less fuel = leaner.
    For all the 4 stroke racers: Innovate has a fantastic AFR you can use to keep tabs on your machine. Summit racing has them.
    http://www.innovatemotorsports.com/products/MTS.php
    I have a AFR on my drag bike combined with a weather station I can predict how my machine will run in different ambient conditions.

    8) 2 strokes not only have to pass fuel through the fuel circuits but they also have to pass the lubricating oil through the same fuel circuits.
    A change overnight in oil manufacturer can change the AFR.

    9) Usually, once you set up a fuel system (2 or 4 stroke) you normally do not need to make huge changes spring, summer to fall. The setting of the air/fuel screw is the exception.
    So, if your machine is running poorly it is probably something else you did to the machine and not the fuel system.

    10) Dirt bikes are not racing where TIME is measured in your 60 foot, or your elapsed time or index. Your set up in the morning may be totally different than the heat of the day where a weather station and AFR monitoring
    allow you to make changes to your dial and or fuel re calibration so you can meet your prediction or index. Dirt bikes are forgiving, we do not "feel the difference" in minor ambient changes.

    Good luck and I hope this helps.
     
    TBO, hershey, MX955 and 4 others like this.
  19. sturd

    sturd PR Addict

    A nit to pick

    Air temperature has been super high and air density has been super low. Still causes a rich condition
     
    hershey likes this.
  20. ck1racerx

    ck1racerx PR Addict

    Location:
    Iron Chef Chili Kitchen
    Racing Number:
    1 My Chili Trophy tells me so
    479... WOW
    Some of you guys that turn wrenches amaze me. A rider tells you it does "drrrr-drrrrr-pop-brappp. and you come to the that. bravo.
     
    GeorgiePorgie likes this.

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