Modern Vespa

Hi all - I'm getting this sound, audible only at idle, coming from somewhere behind the flywheel of my PX. It's this sort of pulsing/chattering squeal. It can't be reproduced turning the fan by hand. I pulled the vape fan/rotor off, fully expecting to find the stator plate had cracked and the rotor was rubbing on the pickups etc, but there's no evidence at all of anything rubbing.

The crank's flywheel stump meanwhile has no more play than is usual for an engine with 3 or 4 thousand kms on it since the last bearings refresh. (I usually overhaul it every 2 years /15,000km or so).

So yeah - am fishing for any ideas of what might be happening in there that wouldn't require splitting the cases to figure out. I suppose I could pull the stator off and the fly side seal and eyeball what little of the fly side bearing is visible. Apart from that, I'm out of ideas.

TIA for any advice!

Mark the flywheel and then remove it see if there is any rubbing.

vape bolts for the fan to pulley are all still tight?

Vape fan bolts are in there real tight, haven't budged.

As for rubbing, I didn't mark it up to test, but some macro photos of the flywheel rotor shows a totally pristine surface. I'm pretty sure there's no contact there.

Everything was fine earlier - this came on all of a sudden. If I stop and think about what changed, I'd earlier been messing with the mixture screw, and had turned it in a quarter turn - a little less than 2 turns out from full stop. That ran great around town and the idle settled nicely. But then I'd taken it on a WOT rip down the highway, and with the throttle closed at speed there was a lean stutter down through the revs and a hunting for idle. Later in a parking lot I'd turned the screw back out a quarter turn, but not before I noticed the noise for the first time. Opening the screw a bit did solve the closed-throttle deceleration stutter at least.

In other words I'm wondering if too little mixture at idle can lead to a dry fly side bearing. Obviously yes, as it gets all its lubrication via the 2T. But can such a little change make a bearing whine? And if this is indeed the bearing making that noise, is it possible that my monkeying with the mix screw just exposed a bearing on its way out?

Of course I was in a hurry and didn't get any audio of the thing and also didn't think to have a long listen to it after opening up the mix screw and in the meantime I already have my carb and flywheel off for some other fixit jobs. Argh. Am about to pull the engine and swap the swingarm mounts, see. And I know I'll be staring at the lump on the workbench, hemming and hawing over opening it up and swapping the bearing out. Which would suck because it's an otherwise healthy, non-leaky, settable engine.

While it's on the bench, maybe better to check. The fly side bearing and big end bearing both squeal when damaged.

Rundown stutter is more to do with atomiser/diameter than anything else. If the pilot jet has a 1/4 turn effect on stutter, it's running right at the lean limit. Going to feel great to ride.

Thanks Jack. Time to source a new flywheel bearing and some case gaskets hereabouts, I suppose. Here's hoping it's just the flywheel bearing and not the big end bearing -- if it's a bearing at all.

With the pilot jet in there right now, the current spread between lean (rundown stutter) and rich (bogs when blipping the throttle) is about half a turn of the fine-threaded Spaco needle. In other words: 2 turns=too lean and 2 1/2 turns=too rich. So sensitive with this pilot jet in. If I get you correctly, going up a size on it would yield more room to manoeuvre with the mixture screw.

Check what's happened inside the crankcase. If nothing is damaged, it's just cost you time and a gasket.
If there is any bearing damage, it's most likely been caused by the jetting. Which might explain why the pilot jet is feeling so delicate. If nothing is wrong inside, then the jetting is not so bad, just needs more work.