Things that can go wrong inside a CVT

[theswede]

I tore down the entire CVT on my china-scoot to find out why I had such a high RPM at top speed now that my EFI system is finally working.
Turns out there wasn't much in there that was OK at all, almost all parts where either worn out or broken... No wonder I was having issues with speed and RPM. And I am very lucky I caught this in time.

First thing I noticed was that the inside of the CVT cover was full of rubber dust, more than I had ever seen before in a 139QMB engine.

The width of the belt was down to 15.9mm (supposed to be 18mm when new)
The drive face gear looked like this, with pretty obvious signs of a slipping belt (the black area is rubber from the belt):


The variator back plate had cracked, and one of the variator slides where competently gone, resulting in scruff marks on the guide pin where the back plate had begun to eat into the aluminum of the variator:


Flipping it over, I saw that there was actually two cracks in the back plate. I don't think it would have stayed in one piece more than a couple of miles or so...:


The inside of the variator and the Dr Pulley sliders looked pretty worn. The sliders had lost so much material that the back plate had started rubbing into the variator at the edges:


The sliders looked bad, with significant flat spots on them:


Dissembling the torque converter, I found that the grease was all gone and most surfaces bone dry:


The guide tracks where no longer smooth and even, but pretty worn:


The guide pins looked OK at first glance, but then I found they had worn flat in places and where slightly bent:


The only part that looks OK enough that I figure I can keep is the variator drive face gear and the inner drive sheave from the torque converter, everything else has to be replaced.

It looks like this kit from Enviromoto could work:
enviromoto.ecrater.com/p/6055029/ncy-139qmb-transmission-kit
Does anyone have any experience with this kit, or alternatives?

[bagga]

how many miles or km's were on the sliders? did they last longer than the rollers?

[Rebel]

NCY has a reputation for being high end parts, I have heard dealers other than Jeremy say they have been having a problem getting the parts from NCY.

Certainly looks like you may have found you performance problem. That is one sick variator.

[theswede]

bagga, I am not sure but I believe they have been in there for about 1000km or so. But the engine got pretty hot last summer when I was experimenting with the EFI, could have been contributing to the wear.

Rebel, thanks for the info. I see some other places such as Scooterpartsplace and Scooterworks listing these too. Not finding anyone in Europe though, all places seem to be in the US.
I'll contact Jeremy first.

[Rebel]

I'm sure he will do what he can for you.

It will be nice to hear how it goes with a new variator, that one couldn't have been getting you down the road very well.

[theswede]

Got a couple of boxes with parts today, hope to be able to put it all together this weekend.
I did contact Jeremy and he was very helpful as always, but it was not possible for him to make the deadline I had if I where to get a colleague bring the parts with him back to Sweden. So I ended up buying from a local dealer eventually.
Koso variator and clutch, and a "reinforced" torque converter (whatever that means). Looks OK, not the same finish as NCY obviously but this should work.
And when going thru my parts bin last night I found a brand new, unused Bando belt ;D

I am retaining the drive face gear, my custom cooling fan and a 1500RPM contra-spring. The rest is new.

[Rebel]

It will be interesting to hear how the old 50cc China scoot stacks up to your Yamaha with the injection set up correctly and the variator replaced.

[theswede]

The China-scoot actually is a pretty heavily modified 72cc, so I should see some performance improvement over the Yamaha.

[theswede]

CVT has been rebuilt, and I also installed a new 12Ah battery (this was the largest I was able to fit).
I used the variator weights that came with the kit (7g), but they are obviously too light as I am still hitting about 9000 rpm at WOT. Max speed is up though, and the engine definitely has potential for more. It pulls strongly all the way up to +9000rpm.
Now that I know the transmission is 100% OK I can start testing various weight combinations.
The NCY kit comes with a 1200 rpm contra spring and 9g weights, I am thinking I should probably be using at least 9g or possibly even heavier weights for my setup.

[Rebel]

9 gm should bring it down about a 1000 rpm I think and that sounds a lot better for a 72cc hot rod motor, you don't want to blow it up.

[theswede]

Tested 9.5g today as I had them, but it proved to be too heavy and I actually lost some top end. Have ordered a set of 9g weights which I will try instead.