Yamaha Neos 50cc 2t bogging down

Discussion in 'Scooter Tuning' started by BadHarry1942, Sep 4, 2016.

  1. BadHarry1942

    BadHarry1942 New Member

    Messages:
    7
    Hi hope you can help with my problem. My Neos 50cc YN50 has started to bog down and go around 10-20mph when it use to top 40. I went out one day then on the way back bog down.
    I thought this may be rollers so changed the variator and snapped the crankshaft :) so total engine rebuild but problem is still there.

    So new engine, crankshaft, piston, head, variator, clutch, springs all good as is the belt. Engine pressure 170psi. Carbs cleaned and also swapped to be sure. Fuel supply good, no air leaks as best I can tell. Plug is good also swapped that still Good.

    Put the bike on the stand and it will start with one click and turn tick over nicely. Turn the throttle and it will go like hell, no problem there.
    Get on and ride it,,, 10-20mph,!!!!

    Tried removing exhaust, checked air filter all good.

    All I can now think of is HT lead\coil, did a resistance test and seems ok, as I say runs like hell in the stand.
    Then there's the Stator and the CDI.. not sure what to do here.

    What am I missing?

    Any pointers please because this it driving me nuts.

    Thanks, PS. Bike is 2015.

    Sent from my SHIELD Tablet using Tapatalk
     
  2. BadHarry1942

    BadHarry1942 New Member

    Messages:
    7
    Forgot to mention. Checked the fuel cap and the breather hole is fine. Could this be the tapcock? Suck the vacuum line and the fuel pours out.



    Sent from my iPhone using Tapatalk
     
  3. dazz

    dazz Member

    Messages:
    31
    Did you change the crank seals?
     
  4. BadHarry1942

    BadHarry1942 New Member

    Messages:
    7
    Yep. All seals and gaskets changed, but as mentioned this issue arose before the engine change and it's still there. So frustrating I bet it's something simple. Would the reeds do this?

    Sent from my SHIELD Tablet using Tapatalk
     
  5. Stevep

    Stevep Well-Known Member

    Messages:
    5,895
    Did you change the complete vario or just the rollers?
    And what clutch springs are in it?
    Are the clutch shoes worn down?
    Is the belt too long? When fitted it should be at the bottom of the vario and top of the clutch and tight. Very little slack.
     
  6. BadHarry1942

    BadHarry1942 New Member

    Messages:
    7
    Hi Steve, the complete Vario was changed and the clutch & springs plus belt in good shape. I also swapped these for a known good set and all good.
    When pulling off the revs lift and drive engages so you get that initial pull off and then everything just seems to hold. The revs don't change only slowly as it slowly picks up.
    I thing it's either lack of fuel of the spark is just not big enough to explode all that fuel under pressure. That's how it feels.
    If the bike is on the stand everything runs as you would expect. Maybe the CDI is not holding enough voltage?

    Sent from my SHIELD Tablet using Tapatalk
     
  7. Stevep

    Stevep Well-Known Member

    Messages:
    5,895
    Never in a million is it the cdi.
    Check the vario to make sure a roller hasn't popped out the channel and got stuck, effectively leaving the engine in high gear.
    Alternatively, start playing with the jets.
     
  8. BadHarry1942

    BadHarry1942 New Member

    Messages:
    7
    Cheers Stevep

    Sent from my SHIELD Tablet using Tapatalk
     
  9. BadHarry1942

    BadHarry1942 New Member

    Messages:
    7
    Guys, thanks for the help so far. Checked for leaks can't find any that I can tell. Sprayed carb cleaner around and listened for engine change, no joy. Plan to change belt and spring next week. However one thing I just picked up on. When it's on the stand it appears to run full belt BUT if I open the throttle fully the bike revs to say 3000 rpm and the is defo a pause for a second or so before it fly's on up the revs. Any further thoughts. Cheers

    Sent from my SHIELD Tablet using Tapatalk
     
  10. Stevep

    Stevep Well-Known Member

    Messages:
    5,895
    Definately sounds like a jetting problem.
    Make sure the air/fuel mix screw is at 2 turns out, it sounds like it's either getting a dump of too much air or fuel at the start before the engine is catching up with the fuel/air intake
     
  11. mrpetrepp

    mrpetrepp Member

    Messages:
    35
    Hi mate I had a similar issue to yours a few weeks ago. I found it to be the air filter, you should check inside the box to see if any of the pegs have actually snapped, if this is the case the foam inside will "flap" about and it'll cause the air / fuel mixture to be poor and cause bogging down. Try switching it with a performance air filter, you can pick up a decent one off of Pedparts for about £10-12. Even if this is not the cause of the issue, it'll definitely benefit you a lot with acceleration -- I myself made a homemade little solution with a bit of piping and some foam =P worked wonders. Give it a shot yourself mate if you can't find the issue,

    Also take a look at the reed valves, you can find a tutorial on YouTube to see the "condition" of reed valves and how to diagnose whether or not they're broken -- this could very well be the cause of your problem because 2 strokes heavily depend on reed valves and it's something which is commonly over looked by people. I have a 2004 Yamaha Neos and I needed security allen keys to remove the reed valves but it might be different for you.

    I might be wrong but it is worth taking a look at what I said. That's the issue I had and it might (or might not) be common for the Neos 50cc 2 stroke.

    EDIT: Never mind just re-read your post haha, sorry ignore the bit about the air filter!
     
    Last edited: Sep 11, 2016
  12. BadHarry1942

    BadHarry1942 New Member

    Messages:
    7
    **** Update - it was the Bloody Vacuum pipe.
    Thanks for all the advice but got there in the end. Will also look at upjetting to give it a bit more.

    Sent from my SHIELD Tablet using Tapatalk
     

Share This Page