Modern Vespa

My clutch has started making an odd whirring sound when the lever is engaged (and the clutch is hence disengaged) but only after it's warmed up -cold or early in a ride it's nonexistent. Without regard to how a clutch operates, it almost sounds like two objects are scraping against each other rather than grabbing each other.

It doesn't affect performance but since the clutch is newly rebuilt I thought I'd ask.

What clutch? What bike?

79 P200 - All clutch internals were replaced and it's making this weird noise when the lever is depressed and the clutch thus disengaged.

You need to open it up. That’s not normal…

Which side of the engine does the noise come from? Check if the flywheel is hitting the cover when the clutch is pulled.

What clutch is it? Cosa? Stock? My cosa was a particularly rattly girl when the lever was pulled.

That's definitely not normal. Something is rubbing when the clutch is engaged. Should be pretty obvious once you start inspecting.

Start by looking for side-to-side play in the crank. See if the flywheel moves toward the clutch when you pull the lever.

Did you forget to replace the clutch spacer when you rebuilt the motor? I made that mistake on my first VBB rebuild and it did something similar.

That is a horrible sound ! I hope it's something simple....
" See if the flywheel moves toward the clutch when you pull the lever. " Wouldn't it move the other way ?
If you pulled the plug and lashed the lever to the grip and turned the flywheel would you hear it or feel it ?
In any case, you're going to have to open it up. Take the clutch cover off at the least.

You're right on the direction, but if you have lateral play in either direction, you have a problem...

These are all great suggestions - thanks.

The issue doesn't seem to be detectable by hand on the flywheel side. I spun it by hand in neutral, with the clutch lever pulled, and in gear with the clutch lever pulled and sense nothing on that side. Moving into the clutch now. I'll pull it and go from there this morning.

I have to say I wasn't too thrilled with the clutch basket when I rebuilt it but it seemed quite serviceable. My first reaction to the noise was to think the clutch plates are somehow sliding against each other but I don't know.

I don't think so. That sounds like metal on metal to me. I wouldn't be surprised if you found swarf in the oil.

it's so "tinny" - it really sounds like something light-grade like the flywheel cover. But the audio is never great - you can't tell which side the sound is coming from by holding your head down on each side and giving it a go?

Identified.

When the clutch is disengaged some of the fins on the flywheel are hitting its shroud. What does that portend? I had the crank checked for true before I reinstalled it.

Excellent. I listened again and xantu was right. It did sound more external. The 'only doing it warm' is interesting. The clutch-side bearing locates the crank.

Worn bearings

They're new SIPs.

worn main bearing bushings?

Worn crank

I'll take worn crankshaft for $159.95 since it was the odd man out in rebuilding a blown motor. Here's the $64k question - what's salvageable from a rebuild with under 100 miles?

What parts would the crankshaft ruin in so few miles?

One thing you might want to confirm is that the crank is fully seated. Check that the clutch nut is tight enough. Which should be very very tight.

flywheel nut and clutch nut check out as tight and there's definitely a slight side-to-side knock in the crankshaft. I think it's gotta go.

Slight side to side is normal, never measured it but lets say 0.5mm. Anymore than that and the bearing is gone or going.
There's a chance the crank is not fully home, or the big end pin has started to separate. Could just be a bent flywheel cover but who's ever that lucky.

After yet another disassembly and inspection, things stayed the same. I then installed 1mm washers under the flywheel cover and it resolved the issue.

This told me two things: how much the crankshaft is moving out when the clutch lever is depressed (less than 1 mm), and that things are not normal with the crankshaft. The question now is how long before I must drop money into a new crank?

I don't agree with the assessment, unfortunately. The clearance of the flywheel from the cover is a lot more than 0mm to begin with. So by adding a mm you removed the rubbing, but the lateral travel is a lot more than than 1mm.