CVT - Low Torque

Discussion in 'The Garage' started by DR250Djebel, Aug 12, 2019.

  1. DR250Djebel

    DR250Djebel Been here awhile

    Joined:
    Nov 1, 2014
    Oddometer:
    251
    Hi,

    I have an off road buggy with a Yamaha YP250 motorcycle engine. It has CVT transmission. I understand the basics principles of CVT but have always owned manual bikes in the past and have never had to repair a CVT transmission so have limited understanding.

    Recently when setting off from stand still, the buggy has been sluggish and unresponsive at low speed but fine once moving. This problem has been gradually worsening and now the buggy will sometimes not pull away from stand still if parked on a slight slope or slightly stuck in mud.

    If I start on a flat surface with no obstruction/resistance it pulls forward slowly until I gain a little momentum. It then runs fine after the initial slow start.

    I am looking for problems with the transmission but as CVT is new to me I'm looking for advice on what to check. The centrifugal clutch and drive belt were replaced a few months ago so should be fine (in theory). If the weights in the variator were sticking, would this cause sluggishness at low speed or is there something else I should be checking out?

    Thank you in advance for any input.
    #1
  2. broncobowsher

    broncobowsher Long timer

    Joined:
    Apr 21, 2010
    Oddometer:
    4,006
    Location:
    Baking in AZ
    Too long of a belt, belt stretch, or adjusting the shafts too close to each other will give less than ideal ratio to start, and limit top end.

    Don't overlook engine issues either. If the engine isn't making proper low speed torque there is nothing a CVT can do to make up for it.
    #2
    DR250Djebel likes this.
  3. Wentwest

    Wentwest How's that work?

    Joined:
    Feb 19, 2007
    Oddometer:
    2,732
    Location:
    Northern California
    There are two rotating parts - the centrifugal clutch and the variator. The variator is probably your problem. When the belt was replaced the variator was probably partially removed. Chances are good the outer plate is not bolted down tight now so the whole assembly has loosened and the rollers inside the inner are jammed in a mid-range position. You have to take the variator off and open it up and check the rollers to see if they are worn out with flat spots, or maybe jammed in crossways and torn to pieces.
    #3
    DR250Djebel likes this.
  4. DR250Djebel

    DR250Djebel Been here awhile

    Joined:
    Nov 1, 2014
    Oddometer:
    251
    Thanks for your tips. As the bike engine is mounted in a buggy frame it is difficult to access and I have to dismantle a lot of things to get to it. It's good to have some guidance on what to look for in advance so I don't strip other things down unnecessarily.
    #4
  5. kantuckid

    kantuckid Long timer

    Joined:
    Nov 29, 2007
    Oddometer:
    8,721
    Location:
    Kentucky-Eastern that is!
    It would be best to have a shop manual diagram that shows the parts positions in their proper place (in case somethings wrongly assembled) and the torque value for the main nut. Beyond the belt there can be spacers/washers missing or out of position and the belt width specs. Leaking lube onto the belt?
    #5
    katbeanz and DR250Djebel like this.
  6. Yam2Yam2

    Yam2Yam2 Adventurer

    Joined:
    Jul 21, 2012
    Oddometer:
    98
    Every snomobiler will tell you this is a classic case of a worn-down belt. The thinner belt effectively causes the CVT to be in 'second gear' (the primary clutch is at a larger radius than it should be).

    Many clutches allow you to adjust where the belt sits at idle, to adjust for this, but you should replace the belt, **then adjust the clutch**, so things work as intended. Duckduckgo snomo belt adjustment for details.
    #6
    concours and DR250Djebel like this.
  7. Deans BMW

    Deans BMW Granpa Hoon

    Joined:
    Feb 17, 2004
    Oddometer:
    6,536
    Location:
    Copperhill, Tennessee
    You have NOT mentioned how the engine sounds. Does it sound as robust as it sounded before? How does it sound in comparison as it used to. If the engine revs up more than it did before, then either the belt or clutch, other wise the engine needs looking at.
    #7
    DR250Djebel likes this.
  8. DR250Djebel

    DR250Djebel Been here awhile

    Joined:
    Nov 1, 2014
    Oddometer:
    251
    Thanks again for all the tips. I'll check all those things out when I open it up. The drive belt was replaced a few weeks ago so, unless it's very poor quality, shouldn't be worn. There hasn't been any noticeable change in noise and it seems to drive as normal once I get moving. It's just slow to pick up the first few km/h and sometimes won't even get rolling at all if there's anything causing resistance such as uneven ground or a slight uphill slope.

    I'll report back once I've had a chance to open it up and check inside.
    #8
  9. concours

    concours WFO for 47 years

    Joined:
    Dec 2, 2008
    Oddometer:
    6,934
    Location:
    Kensington, NH USA
    The wrong (length) belt, as little as 1/4” circumference can do it. Check it against the old one. A flat seamstress tape measure works.
    #9
  10. Chillis

    Chillis Long timer Supporter

    Joined:
    Aug 25, 2005
    Oddometer:
    10,719
    Location:
    Las Vegas
    Sounds like the clutch may have broken its spring(s). Grabs too early in the rpm and doesn't allow necessary rpm climb before it catches.

    CVT transmissions can be hell. I'd try and get it to baseline settings.
    #10
  11. DingDangKid

    DingDangKid El Lechero

    Joined:
    May 27, 2009
    Oddometer:
    4,343
    Location:
    Tioga County, PA
    Sounds a lot like the issues I'm having with my Can-Am Maverick. Thanks for the input, guys!
    #11