Modern Vespa

@$15 on eBay.
Nicely painted metal gas cap/with gasket - toss that horrible plastic stock cap ...which can sometimes require two hands - and likes to cross-thread!

I first posted about this cap a few yrs ago - many have followed - with smiles.

No idea why the American Vespa parts dealers aren't offering these. But they will sell you another black plastic cap for nearly twice this price.
O.S.

Can confirm that this one little, mostly unseen accessory is the best investment for a Liberty owner.

I replaced mine on a lx that I use to have and worked fine so when I bought my son a 2020 primavera 150 fuel injected I purchased another one from Scooterwest.and had problems with keeping the engine running.After talking to the dealer he said to put the original cap back on and I have not had any issues since.It's something about the venting I believe he said.

I got a similar one for my GTS recently. Has a nice positive feel in the hand.

The cap should not be anything to do with the venting. It should be well sealed. The tank vent is a separate bit of structure. Your new cap should work just fine - if there's a venting problem, it's somewhere else, fix it.

I guess I have another problem as I tried the aluminum cap again and the engine cut off in about 3 minutes. I put the original one back on to get home.When I took the metal cap off a lot of hissing.I will have to check out further.Thanks

@jimc: Although everything you said is correct, all the caps that came on Piaggio scooters in our market, for some reason had a small hole on them. And of course, because there was no "one way valve" on them, they would always leak a bit if tank was filled to the brim. The hole disappeared around 2008...

I bet your Piaggio cap had lost its O-ring seal. That's actually an MOT failure event in the UK,

Yes, you need to investigate.

If I remember correctly, that's when all the EU vehicle 'road-worthiness' checks began to align a bit more than they had before. The eventual vent outlet had to be from the neck of the tank to below the level of the bottom of the tank so the heavier fuel fumes would remain inside as much as possible, and in the event of a tip sideways wouldn't leak.

The tank is full.The only difference between the 2 caps is the original has a flat black gasket on it and the aluminum one has a round o ring type seal.I think that is the problem.

Swap out the gasket. Remove the original gasket out of the plastic cap and, after removing the o-ring one, try it in the aluminum cap. My ebay red aluminum cap came with a flat rubber gasket. I eventually replaced it with a Piaggio flat one after the red caps original gasket started breaking apart. Wear and tear item.

I took the flat seal off of the old LX one and put on the a aluminum cap and it seems to work fine .The engine has not cut off and no hissing noise.Problem solved.Thanks