Modern Vespa

Hello all,

I am having a problem with my 2005 Stella that is killing me. I can't figure this thing out.

If I take off like a responsible scooter rider, all is well. I can feather the clutch and throttle for first gear and leave the line just fine.

But when I take off from a standstill using elevated rpm (e.g. for a fun launch or to quickly catch a gap in traffic) or uphill (which also requires elevated rpm), the bike lurches. I can freeze the clutch lever during the lurch and what I feel is essentially the clutch grabbing then releasing over and over again. Then ill have to bring the clutch in again and feather slower with less rpm for longer. This only happens on takeoff.

This has only been happening since my engine rebuild. I could launch no problem before. The bike has a grand total of 2000 miles. 250 miles ago, Sept 2023, the crank gave out in usual fashion. Engine wise, it was all stock except polini pipe. Now it has a mazz crank, road 2, and polini 177. All bearings, cruciform, seals replaced. Passed leakdown. Oil flinger installed.

During the rebuild, I replaced the brass bush in the clutch because it was loose and put oil between all the old plates. Only the brass bush was swapped. The OD of the bush was very tight so I sanded it till is was loose (per WhiteOneRacing YT video). Bike together and I have this lurch problem. It was my first rebuild so I figured I messed something up.

I bought a brand new Stella clutch from ScooterMercado. Using the clutch tool, I separated all the plates (while clutch was together, wanting to avoid any assembly/disassembly variable) and soaked the entire assembly for three days in oil, flipping it halfway through.

Installed in the bike, problem persisted. I then replaced the gear oil, doing two 100 ml, 5 minute idle flushes (I has a guess that maybe the wheel bearing grease I used for assembly may have some additive in it causing the problem), before adding 250 ml SAE 30 non detergent oil (bottle is 2 years old). Problem persisted.

I replaced the cable inner, problem persisted (I replaced the entire cable during the rebuild).

I have adjusted the clutch loose and tight, problem still persists.

I have no idea whats left to try. Any ideas?

Not sure if it matters but I also replaced the castellated clutch nut with the serrated washer and flange nut that ScooterMercato offers as an alternative.

I'm wondering if you changed out the springs in the cush drive? This sort of sounds like hammered/broken springs.

No I have not changed any springs. While it is possible they are bad, I havent fully accepted that could be the case (yet) as the issue wasn't there when the crank went and I have only 2k miles. I also thought that would mean there would be a lurch no matter how I feather and with all gear changes.

Everytime I've had, or seen this issue, it was a worn, or spinning clutch bushing, the bit that sits on the clutch baseplate. Replace or make sure it doesn't spin and all was sorted.

Yea so the clutch is brand new, 50 or so miles on it. It wasnt spinning on the new one nor the old one, which I replaced

Did you change the steel thrust washer? Any slight wear makes the clutch grab.

Is that the plate that the brass plunger presses on to compress the clutch? If so then no. If its the "washer" between the clutch and bearing, I dont believe I have one since its autolube, there is a gear there.

Here is a pic that i took when pulling the old clutch before putting in the brand new one. If I zoom in, I do see some kind of ring on the gear.

And from the initial disassembly

And here is the metal disk, just prior to install of the new clutch, old clutch pictured

Old clutch gear faces with new bushing

Old clutch gear faces with new bushing

On an autolube, the thrust washer is the cog. Typical on a 2 stroke, one part with at least 2 jobs.
From the picture your one is more than fkd. Worn out a long time ago.

Thank Jack, I'll order one and give it a try!

Also worth to check if pressure plate not touch clutch nut or shaft when clutch is disengaged.

Thank you, I did briefly glance at that when I took out the old clutch but I will double check. It should be clear right with some kind of rubbing evident?

Did you put a clutch plate in backwards?

Since it was a first time engine rebuild, I bought a second brand new clutch to eliminate the variable of me not knowing how to rebuild the old clutch. I have the same problem with the brand new clutch (not new clutch plates but an entire clutch)

I'm leaning towards this answer also. put me down for 3 slices of ham on that bet.

At this point, based on answers I have received here and other sources it could be everything the clutch interacts with or the cush drive. So my plan is to shotgun the easy parts at it (autolube gear, brass plunger, brass plunger actuator thing, metal plate on clutch). If that don't solve it, then cush drive.

I hate this method but I cannot figure out a way to diagnose this issue

your plan has the benefit that the everything but the cush parts aren't that expensive, so I'd be right there with you on that.

It's the autolube cog. Yours is very fkd. Any ham bet is just robbery.

Dont worry I will report back and exchange of hams will occur

If it's the auto lube he wins ham, how is that robbery? My ham is on the cush drive.

If you insist. A haunch of Iberico will do nicely!

hey now, I'm talking some oscar mayer, Iberico is reserved for the high rollers

Couldn't take Tofurky ham off you for this one. Until that worn out autolube cog is replaced, it's the prime suspect.

Ok so I finally had time to work on the clutch. There were some surprises. The clutch on the bike was BRAND NEW. Everything came as a package from SM. It has maybe 50 miles on it. It is a stock stella clutch with no modifications. I opened the bike up because the problem persisted and my intention was to replace the thrust washer/autolube gear, brass plunger, and plunger plate. When I removed the clutch, the nut was still torqued down.

So I use the cosa nut and serrated washer to secure the clutch to the crank. I have been torquing it according to the stock castle nut number. Could this nut be causing the problem?

Here is a video of the play on the clutch when on the crank and showing the clutch seats far enough to spin the cush drive. Is it normal?

https://youtube.com/shorts/S_BnnvHtw-M?feature=share

I also noticed the serrated washer notches the clutch body but the underneath of the flange nut was smooth.

All of this damage occurred in 50 some odd miles. I'm not quite sure what my problem could be! My next plan is to get the new brass bits and go back to the castle nut. Any ideas?

I just checked the clutch that was in the bike before. The one that came with the bike and was on the bike for hundreds of miles when the bike was having the same symptoms. The brass bush is on tight as can be and the brass washer is perfectly fine as well. So those elements are not the cause but were damaged by whatever is causing my symptoms

Correct answer: Cosa nut! Got rid of it and the serrated washer, went back to the original clutch, back to stock nut and locking tab thing and the problem went away

I'm glad you're back on the road, but I wouldn't blame the Cosa nut.

I run them on half a dozen motors, all much more highly tuned than yours and two of them are even 2005 Stellas, although only one of them really counts as that.

I'm not going to take a guess where it started, but looking at your photos, you got into a feedback loop, so once the damage started, there was no going back.

The basket you posted pictures of above has notches from the tabs hitting it. That's causing the plates to not separate evenly/consistently, and it's a vicious cycle from there. Why the basket failed before the plates is a separate question (usually the tabs on the plates mushroom first), but that could absolutely cause the damage you've shown.

Also, I don't know what happened to your plunger, but it looks like it somehow splintered, which could also have been interfering with its movement, creating a situation where you were effectively riding the clutch, accelerating wear further (including the damage to the basket). Usually the pressure plate shows more obvious wear when that happens, though.

You'd be amazed how fast you can trash a clutch in a situation like this. Like less than five miles, much less fifty.

Ultimately, I'm just glad you're back up and running.

Appreciate that chandlerman.

It was a mess trying to solve this. Im not really the parts cannon kind of type. I'd rather diagnose but I couldn't figure out any tests.

The stella crank bearing finally went so I did my first engine rebuild. During the rebuild I inspected the clutch (opened it) and replaced the bronze sleeve. Put it back together and reinstalled with the cosa nut.

From first start I had problems. Because I knew the cosa nut had been used loads of times as you say, I didnt think that was the culprit. Since it was my first rebuild, including clutch, I was convinced it was my clutch rebuild. When a new one didnt solve it, nor did replacing everything the clutch interacted with, all that was left was the nut. Im even back to the first (original) clutch. All is still working. I'm not sure what the problem exactly is with my cosa nut and serrated washer but I'm sticking with the original stuff for now. Rather be riding than wrenching!