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CVT - Low Torque

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

  1. DR250Djebel

    DR250Djebel Been here awhile

    Joined:
    Nov 1, 2014
    Oddometer:
    310
    The axle connects behind the clutch but the side cover is one solid piece which doesn't come off unless the axle is removed.

    The drive belt snapped a few months back (difficult to check and maintain it) and when this happened one of the clutch arms broke. I eventually managed to find the only mechanic in town who would work on it. He took apart the back end and found what was damaged. I then had to order parts from Aliexpress/Ebay and he replaced the belt and clutch. All ran fine for a few months. Then it got an occasional problem getting drive to engage properly at low revs when starting off (but no problem engaging reverse). This issue seemed to resolve itself until a few days ago and now I have a problem with the wheels being driven when they shouldn't be and causing it to stall when I shift from neutral to D or R.

    I towed it back to the same mechanic a couple of days ago but found the shop shut down and nothing there but an elderly Khmer lady with a sewing machine and no teeth. She told me she used to teach French in a school until the Khmer Rouge took over and abolished all schools and also that the mechanic had got sick and gone to Vietnam for treatment indefinitely.

    As above, when towing it in drive the engine would not "bump" start when it got up to speed so the clutch didn't seem to be engaged (or the belt was slipping). Once rolling though it would start using the starter motor then drive normally so the clutch was engaged and the belt not slipping. It would then stall if I slowed down below a few km/h.

    Attached Files:

    #21
  2. DR250Djebel

    DR250Djebel Been here awhile

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    Nov 1, 2014
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    Pic from clutch change a few months ago. The mechanism between the clutch and axle is the part of the system I don't understand. I'm suspicious it could be where the problem lies but not sure how or why. I just don't get how the clutch seems to be stuck on but turning the axle/wheels doesn't cause the primary variator to turn.

    Attached Files:

    #22
  3. gumbellion

    gumbellion Been here awhile

    Joined:
    Mar 18, 2008
    Oddometer:
    634
    Location:
    Ontario, Canada
    well its about what i thought but a lot different, lol.
    It works the same as a snowmobile system with the primary clutch on the crank shaft running to a secondary clutch on a jack shaft into a gear box.
    The clutch is not a snowmobile primary but from that photo it seems to look like you are missing pieces off of several fins in the primary which could be a contributing factor
    I am still going to go with a good cleaning will have it as good as new

    The part between the axle and the primary clutch is where your secondary clutch is located. If you were to turn the rear axle that middle shaft will spin. the belt should always have some contact in the secondary clutch when the sheaves are completely closed but its not a solid connection, but when at idle or stopped the primary clutch should spin freely without even touching the drive belt.
    What it sounds like is happening is your primary clutch is staying somewhat engaged and the sheaves are applying force to the belt. Just enough to stall your engine when at idle, and when you are turning the rear axle by hand there isnt enough force to turn the motor over so the belt is slipping in between your clutches.

    I am not sure if i am explaining this to make any sense. But the engagement issue is with the primary clutch i promise.
    Have you tried to turn up the idle and put it in gear? I would just to see what happens and either with the higher idle the engine will have enough power that it will not stall from the friction on the primary, or you will be driving down the road
    #23
  4. DR250Djebel

    DR250Djebel Been here awhile

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    Thanks for your reply gumbellion. My understanding was that the primary variotor should always engage the drive belt which in turn spins the secondary variator and centrifugal clutch. Once above idle speed, the centrifugal clutch arms are forced to open up and engage the inside of the clutch bell causing the shaft to turn. I thought this is the mechanism which allows the engine to idle without driving the wheels rather than the belt being set up loose and allowing the engine to spin freely up to a certain speed before even touching the drive belt.

    There are lots of suggestions on how it should work and what potentially could be wrong but none of them seem to quite add up and when we find one that seems to explain the situation, something else contradicts it.

    I did try to turn up (and down) the idle speed to test for improvement. As it originally was the engine was making an unhealthy clunk as it shifted into drive and stalled so I only increased the idle speed a little due to fear of causing damage if shifting into drive at high revs with a potentially stuck clutch.

    In the absence of a fully cohesive explanation of the problem in theory it looks like the only step forward is to dismantle the rear end of the buggy to take off the variator and clutch cover to inspect.

    I'll get onto that in the coming days when I have a few hours free. Taking off the wheel hub and disconnecting the suspension look fairly straight forward but removing and replacing the axle are my main concern as I'm not sure how this is connected.

    Thanks again for your help.
    #24
  5. DR250Djebel

    DR250Djebel Been here awhile

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    Without dismantling the whole rear end to remove the axle I can open the variator clutch cover about 3mm. I did this to see the drive belt.

    With the engine idling the belt turns so it is not slipping on the front/primary variator.
    When shifting into drive with the wheels raised up the back wheels turn at idle so the belt is not slipping on the rear variator.

    As the belt is not slipping I would normally assume the clutch is sticking at engaged. However, when I turn the wheels manually the front variator doesn't turn so the stuck clutch theory doesn't work out.

    Unless there's some way the problem could be caused by the gears and mechanism between the shaft the clutch and rear variator are mounted on and the rear axle, the next step is to dismantle the rear end to inspect the clutch and variator.
    #25
  6. DR250Djebel

    DR250Djebel Been here awhile

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    I opened it up today. Getting the axle out was much easier than I expected. Had I known the job was not complicated I'd have been less hesitant to just dismantle the rear end and look inside. I'll know for next time.

    I found many small issues such as broken fan fins on the front variator but the main findings of significance were:

    1. The spring on one of the clutch arms is broken

    2. 2 of the variator rollers had somehow come out of their tracks and were jamming the variator partly open

    3. The nut holding the clutch and rear variator onto the shaft was loose. It had previously been removed by the mechanic who changed my drive belt earlier this year. He hadn't used an impact wrench but had instead broken it loose with a chisel. This had caused it to be misshaped and risen parts of bent metal were protruding from its outer surface. These appeared to be making contact with the drive belt cover as a groove had been worn in it. Perhaps this contact/friction had caused the nut to become loose.

    I'm not sure whether a loose clutch nut would be the cause of the broken spring but the clutch damage is a bit suspicious as it's a new clutch replaced only 250km ago.

    I will try to find replacement rollers and clutch springs locally. If not I'll have to order from Ebay/Aliexpress and wait a few weeks.
    I'll also file down the misshaped clutch nut surface to prevent it from contacting the cover and working loose.

    I'll report back once complete to confirm whether the problem is solved.

    Thanks again for all the helpful advice. It's hugely appreciated.
    #26
  7. AlpineRAM

    AlpineRAM PartsChaser

    Joined:
    Nov 16, 2007
    Oddometer:
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    Location:
    Europe, Austria
    If already ordering parts- get a new clutch nut, this should not break the bank.
    #27
    DR250Djebel likes this.
  8. DR250Djebel

    DR250Djebel Been here awhile

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    Update: I couldn't find the correct clutch springs variator rollers locally. Rather than wait a month for parts from eBay/Aliexpress I've done a makeshift repair whith the parts I could get.

    I managed to find a scooter roller that was the correct diameter but 2-3mm too wide and got it ground down to the correct thickness. It ended up being 12g while the remaining 7 original ones are 14g.

    Similar story with the clutch spring. No way to get the correct replacement or even a set of three matching springs of a similar spec. I ended up finding a mechanic with a broken clutch from a Honda Airblade scooter with 2 springs remaining so bought one of those.

    I've reassembled it and it seems to be running fine but I'm not confident the clutch with mismatched springs will be a reliable solution. I've ordered a new set from Aliexpress. No way of knowing if they're the same strength as my original ones but it least they'll be 3 of the same type.

    I think the buggy may still be a little sluggish pulling away from standstill but it doesn't seem to be a big issue.

    As the variator had been jammed by the out of place roller the belt had only been operating on the outer area of the primary sheaves which were visibly clean and polished while the inner section was dull and unpolished. Perhaps the belt isn't moving freely into the center to give a small "front sprocket". I cleaned out the variator when putting the rollers back in but perhaps the rollers aren't moving in and out freely. I'll keep an eye on this and see if the problem fades with use. If not I can order a new variator and/or rollers if the lack of torque when pulling away turns out to be a problem.

    I suspect that whatever caused a couple of the rollers to come out and jam the variator in a high gear ratio in the first place may have happened again since I put it back in. Not sure what would cause this to happen though.

    Thanks again for all the help and advice.
    #28
  9. Steve_h

    Steve_h Been here awhile

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    Sep 15, 2016
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    Allowing it to expand whilst being reassembled or before the belt is installed causes that.
    #29
  10. baloneyskin daddy

    baloneyskin daddy bikaholic Super Supporter

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    southcentral PA.
    If the rear variator sticks just momentarily after you let off the throttle it will cause the belt to get slack in it as the weight or rollers in the fr variator are no longer being forced against their ramps by RPMs . This will allow the rollers to jamb up as they fall out of position. I read where you mentioned hearing a bang every so often that could be the rear variator unsticking and slamming shut.
    #30
  11. fast1075

    fast1075 Not a Lemming Supporter

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    Dec 28, 2013
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    A little research into how a CVT actually works is in order. The clutch is in the driven element. A worn belt runs lower in the variator and higher in the secondary. It will rev to the moon with plenty of go power unless it is so worn it slips and won't go at all. Moving the rear wheels should not move the variator. If it did, you couldn't sit at idle. The clutch is centrifugal, spun by the belt. The problems are from rusty/stuck/worn out/improperly assembled/lubricated parts in the variator/driven element.
    #31
  12. DR250Djebel

    DR250Djebel Been here awhile

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    I found that after reassembling and driving the buggy the low torque problem recurred. After taking it all apart again I found one or two the rollers had again come out of their tracks and jammed the variator half open.

    The effect is that I am permanently stuck in "higher gears", hence difficulty pulling away but running fine once up to speed.

    Another consequence of having a large "front sprocket" and "small rear" when starting up is that the ratio is such that the secondary variator and clutch spin much too fast at idle even though the engine is idling at the correct speed. This is the likely cause of the damage to the recently installed clutch and the clunking/juddering when shifting into gear at idle.

    The primary variator is the source of all the problems. I'm not sure how the rollers are now able to come out of their tracks when I drive. They have some flat spots but don't seem in terrible shape. They or the variator could have been damaged when the drive belt snapped a 250km ago.

    I can't buy the correct parts locally. Rather than order online and wait a month or two I tried to solve the problem by placing a washer about 1mm thick on the drive shaft behind the variator.

    I'm hoping this will prevent the variator from opening far enough to allow rollers to escape their tracks. Obviously this will also limit the top speed, hopefully not by too much.

    After a few km testing, all now seems to be well. The buggy is a lot more responsive at low speed and doesn't judder when shifting from neutral to D or R.

    Further driving will be needed to find out whether this washer is a permanent fix but I am now suspicious the buggy may have had a washer behind the variator originally and it was not reinstalled when a mechanic replaced the drive belt and clutch not so long ago.

    If so I would be wasting my time and money ordering a new variator or rollers.
    #32