Modern Vespa

After getting my P up and running perfectly, it's time to focus on my small frame.

I converted my V90 to a four-speed, and once in a while, it would slip in and out of 4th temporarily.

During the last ride, it slipped out of gear permanently. I can feel the selector going through every gear via the indents. When I kick it over it acts like the clutch is disengaged.

Before I pull the motor, does anyone have any clues about what I'm in for?

Edit: There were no unexpected noises when it popped out of gear. No grinding or banging, etc.

Did you checked loose gears tolerances and shimmed it correct. Factory values is 0.15 to 0.4
Is gear selector correct type for the loose gears? Did you changed gear selector balls and spring?
Wrong setting of gear cables also can cause jumping out.

Check that the clutch and primary are both tight, which doesn't require splitting the cases, since the gearbox is spinning freely.

Alternately, I had a cracked axle that produced a similar behavior in my GL a couple builds back.

Also realize that whatever is causing the jumping, even if you resolve this, is rapidly accelerating wear on the gear selector, so you'll be into the cases soon enough.

Yes, the loose gears were at 0.35 and all of the components were new including the balls.

That's where I'm starting due it acting like the clutch is disengaged.

That I'm not sure of. David at scooter Mercato put the kit together for me.

For once it is an easy fix. The input shaft woodruff key sheared.

Glad it was the easy fix!

It slipped because it wasn't torqued to spec. The woodruff just lets you know.

Make sure you have a good clean connection between the mating surfaces, then torque to spec.

Also, you can delete that woodruff key. I don't know why it or the clutch shaft key are there--they serve no functional purpose I'm aware of, but can smear steel onto the mating surfaces if they shear.

It's still popping in and out of gear at full acceleration, which I'm sure is why the woodruff key sheared. It works perfectly at less than wide-open throttle, though.

I'll be splitting the case in the future to further investigate the tolerances. It won't be for a while.

when you split the cases, be sure the lap the primary to the input shaft. If it's not a good, consistent fit, it'll slip and shear, same as a flywheel.

Thanks for the info.

Besides the possibility of smearing, what other advantage is there to leaving the key out?

It's one less thing to lose into the cases. It also makes getting the clutch installed a lot easier.

I think one of the purposes of the woodruff keys on the clutch side is to act as a mechanical fuse or fail-safe. They're supposed to shear under excessive load to protect the crank and keep everything from exploding if the engine locks up.

But it's a pressure fit, same as the flywheel. If that breaks loose, the woodruff isn't going to save a thing.

No, the clutch isn't a taper fit like the flywheel. It just slips on. At least that's the way I remember it. The key isn't there to keep the clutch from letting go, it's there to let it go if the crank locks up.

The key on flywheel side is there to locate the magnets relative to the stator.

Also at the front axle/hub.
The key prevents a hub from rotating against a shaft as you torque it.

Maybe it's like a cotter pin or a tabbed washer.
Adds resistance to a first slip?

We are talking about the small primary gear on the end of the crank, right? In Utahusker's picture? That one is definitely a straight shaft.

The tapered shaft is on the end of the xmas tree and the clutch spider mounts on it. I kind of figure that even if you've lapped it until it's a perfect fit, there is not much harm in putting the key in there surely?