CVT - Low Torque

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

  1. DR250Djebel

    DR250Djebel Been here awhile

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    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

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    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
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  3. Wentwest

    Wentwest How's that work?

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    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
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  4. DR250Djebel

    DR250Djebel Been here awhile

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    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

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    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
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  6. Yam2Yam2

    Yam2Yam2 Adventurer

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    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
  7. Deans BMW

    Deans BMW Granpa Hoon

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    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.
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  8. DR250Djebel

    DR250Djebel Been here awhile

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    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

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    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

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    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

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    Sounds a lot like the issues I'm having with my Can-Am Maverick. Thanks for the input, guys!
    #11
  12. DR250Djebel

    DR250Djebel Been here awhile

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    This problem seemed to resolve itself after I'd driven the buggy a little more. I did notice some sluggishness and lack of power when pulling away from standstill but it would generally have enough power to pull away sluggishly then I'd often feel a little "clunk" when I got to 5-10 mph and the transmission would fully engage and run as normal.

    Because the engine revved as normal in neutral but lacked power when I moved into drive I thought maybe the gear change might be sticking a bit when shifting from neutral to drive and not fully engaging drive (as opposed to a clutch problem). It seemed to be working well enough until this week so I didn't want to mess with anything.

    In the last few days though I've developed a similar but opposite problem.

    Now the engine still idles well in neutral and revs freely. However, when I shift to drive or reverse and put a load on the engine, it stalls immediately. I tried tow starting it in drive and that worked fine. I could then drive normally on the open road but as soon as I slowed down and the revs dropped, it would labour and stall.

    My first thought was again the clutch. I thought it was getting stuck with the arms extended and not disengaging from the clutch bell when the revs drop to idle speed, causing the engine to stall. I thought that with the clutch always stuck in an engaged position, the engine was stalling when shifting to drive/reverse at idle speed.

    As it is a big job to access the clutch to remove/repair/replace due to it's location in the buggy, I wanted to test this theory first.

    The front/primary variator has an access hatch that can be taken off to manually rotate the engine when setting the timing. I took this off and made a chalk mark on the front variator to mark its position. I then put the buggy in drive and pushed it forward.

    If the clutch was stuck in an engaged position I expected the rear/secondary variator would turn as the the buggy's wheels were manually turned. Thiswould cause the front variator to rotate.

    When pushing the buggy forward in drive or backward in reverse there was no rotation of the front variator. Is my test for a stuck clutch flawed or is there something else that would cause the engine to stall as soon as the buggy is moved from neutral into drive or reverse? (Note: it drives fine if I tow start it and keep moving)

    Thanks.
    #12
  13. 1greenmachine

    1greenmachine Long timer

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    Your test is kind of inaccurate as the belt could slip in the secondary clutch as the primary holds it in place. You could get it jacked up and secured safely and then start in gear and if the wheels are spinning I'd be suspicious of the primary.

    My experience comes from snowmobiling but sounds like the primary spring is bad or something is sticking.
    #13
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  14. DR250Djebel

    DR250Djebel Been here awhile

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    Thanks Greenmachine.

    As the buggy only stalls when shifting into D or R at idle but runs and drives normally if I tow start it and keep it above 5km/h I was assuming the belt was connecting properly with the sheaves at both the front and back.

    Also, if the drive belt was slipping, I can't understand how shifting into drive would cause the engine to stall if the belt or drive primary variator just spun freely. It only stalls if it is in D/R and the engine is at idle speed.

    This is what caused me to wonder whether the clutch is actually sticking and question whether the problem might be with the mechanism that shifts from N to D or R. I've tried loosening and tightening the shifter cable. This does make a difference to the stalling when shifting into D/R but doesn't solve it so I have suspicions it may be the problem (or part of it).

    Taking off the clutch/variator cover will be a big job due to the layout of the buggy and the fact the rear axle runs through it. Before doing that I'll try the test you suggested and get the back wheels off the ground. I'll then start it in neutral and shift into drive to see what happens when drive is engaged with no load on the wheels.
    #14
  15. DR250Djebel

    DR250Djebel Been here awhile

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    A further detail just occurred to me. When I was tow starting it I had to start the engine once moving using the starter motor. Moving the buggy forward in drive didn't just turn over the engine and start it automatically.

    It seems that the belt is engaging with the sheaves because, with a rolling start, the engine runs and turns the wheels normally. However, turning the wheels when in drive doesn't turn the engine.

    Any ideas why this could be?
    #15
  16. DR250Djebel

    DR250Djebel Been here awhile

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    Test done as suggested by Greenmachine.

    With the back wheels raised and the engine at idle speed I put it into drive and the wheels turned. This suggests to me the centrifugal clutch is stuck at engaged.

    I stopped the engine and spun the back wheels manually. The primary/front variator didn't turn. This suggests to me that the centrifugal clutch is not engaged.

    Am I overlooking something here?
    #16
  17. gumbellion

    gumbellion Been here awhile

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    I know you have said its hard to access the clutches but you really should pull them and clean them and i am sure it will fix your problem. Check the water clutch removal method for your primary
    If you really cant get them out you clean the sheave faces on the secondary and the primary. After cleaning the faces take a black sharpie and draw a line from the crank shaft to the outside of the face on theprimary and from the jackshaft out on the secondary. Start the buggy and see where the marker wears on the primary. The belt should always be in contact at the top of your secondary when at idle or when not moving. Then take it for a ride and see how much of the marker gets worn off each clutch to ensure the belt has the proper travel inside the clutches.

    If the belt was replaced because the old one blew it could easily have got some stuff stuck
    The 2 possibilities you have are

    Weights are hanging up so they keep the clutch from fully opening
    dirt/dust/rust/ buildup on the movable sheave preventing clutch from fully opening

    Take a photo of the clutches and post it up here and we can probably tell you what you have. Based on the application i am guessing there is a comet on there

    To service the secondary its dead simple and all you need is a pair of snap ring pliers. The helix can collect a lot of crap and prevent smooth opening
    Primary is a little more involved and a clutch holding tool makes life easy
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  18. 1greenmachine

    1greenmachine Long timer

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    You pretty much answered yourself in the first post, the primary is holding the belt as it slips in the secondary. Can you see if the belt moves as you turn the wheels?

    The primary's job is to compress and squeeze the belt after engagement, the secondary is forced apart by the belt being pulled towards the middle.
    This video might help explain it better.
    #18
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  19. DR250Djebel

    DR250Djebel Been here awhile

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    Thanks greenmachine. What I don't understand is: If the belt is slipping on the secondary, how/why is it engaging the back wheels and causing a stall when I shift into drive/reverse? If the belt is fine but the clutch is stuck permanently engaged and causing the wheels to turn at idle speed when I shift into drive/rev, why does the primary variator not rotate when manually turning the wheels? To me it just doesn't add up.

    1. If the belt is slipping, shifting it into drive at idle speed shouldn't make the wheels turn or cause it to stall.
    2. If the clutch is stuck on, I think the primary variator should turn when I turn the wheels. (Unless the belt is slipping. In which case go back to 1 above).

    I'm going round in circles trying to figure it out. Unless I can do so, the next step will be to open it up and inspect as gumbellion suggests (thanks for you input).

    This is tricky as it's an unconventional vehicle. Motorbike mechanics here refuse to work on my buggy because it's not a bike. Car mechanics refuse to work on it because it's not a car. It's a job I'll have to do myself and my main concern is getting the rear axle off and putting it back on correctly. The axle connects into the side of the variator/clutch housing and the cover cannot be removed without removing the back wheel, wheel hub, suspension and then taking out the axle. It's a job I haven't done before. The wheel, hub and suspension look straight forward but I'm not sure how the axle connects or how complex it is to remove/replace.

    Then, if/when I get it all open I don't have an impact wrench to take of the clutch. Unless I can fabricate some kind of tool to lock it in place without damage while I get the nut off I would have to reassemble everything so it could be towed somewhere with a compressor and wrench gun to loosen the clutch nut, then reassembled to tow home to repair, then...............

    I'm trying to reason out what is causing the problem before getting into all of this in case it isn't the clutch or a slipping belt and lies elsewhere such as the gears, gear shift, etc.
    #19
  20. gumbellion

    gumbellion Been here awhile

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    i really want to see this buggy it has me very intrigued
    The axle connects into the side of the variator/clutch housing -i am guessing the secondary clutch runs a jackshaft into a chaincase of some kind here that runs the axle

    Edit:
    If you want to try to do it as easily as possible im sure you could clean the clutches just by spraying them down with acetone and let them air dry, its sure to clean off some stuff
    Also when the belt was changed did they have to remove all this stuff to get to the clutches? it sounds like a Ruegoldberg set up
    #20