Modern Vespa

Started my 2025 GTS SS today (only 200mi on it), the engine turned over fine then there was a bang sound and the engine shut off. I'm thinking it was a backfire. I waited a moment, flipped the kill switch off/on and tried again and it started okay, no bang, no check engine light. Any ideas or experience like this? It's really hot and humid here this weekend and I'm wondering if the excess moisture is somehow to blame as it's been stored in a very humid garage with no airflow???

It's a brand new bike and I haven't modded or touched the mechanicals at all. Checked the oil and air pressure today before starting her to prepare for a long ride with a passenger. Now I'm afraid to take her to far from home.

This is the best idea if you want to enjoy your Vespa for a long time. Not!

Easy there mate. It's Wisconsin in the US. It's hot for about one week a year here. Not exactly the tropics, lol.

Have any " extras" been fitted, bar end mirrors ( fouling throttle tube) indicators ( wiring issues) different exhaust?

In the US you have different emissions systems to us in Europe, could it be unburnt fuel in some " capture" system has been re ignited?

Not really. I replaced the turn signals about two months ago, but that seemed to go off perfectly. I added a a side stand relay about two weeks ago as well but that also went smoothly. Otherwise all mechanicals are stock. The only thing I had done right before starting was checking tire pressure and engine oil level.

I did notice after checking tire pressure (I was adding air for a second passenger) that my hand held electronic pressure gauge malfunctioned. Which I believe is because some moisture from inside the tire found its way down the tube into the unit. It was just so bizarrely humid yesterday here in Wisconsin. I think it may be responsible.

Well, checking the tire pressure would not be responsible for the bang (unless you blew a tire, in which case you would surely have noticed the source of the sound! ) I can't think of any reason that high humidity would cause a bang from the engine. My guess is that the engine backfired once. Unless it continues to do so, I would call it a one-off event and not be too concerned about it.

What is that?

It's the kill switch for the side/kick stand. Scooterwest offers a bypass for those who want to remove the side stand. Sorry if "relay" isn't the proper term.

From my dealership/shop this morning...

"that should not happen, however, if the engine started up, runs well, and there's no check engine light it might have been a build up of fuel vapor. I've noticed that some of these plastic tanks swell with heat and humidity increases. Try releasing the fuel cap to vent the fuel vapor pressure in the tank next time before starting."

bypass is a better term

I recommend watching this vid
https://www.instagram.com/reel/DKzaYh4RhNn/?igsh=cjI2Mng2NXQxYm1u

Sorry I think there's some confusion here. You've posted this warning to me before in my last thread where I did a side stand removal walk thru. However I clearly state in both that and this thread I removed the side stand altogether, so your warning video doesn't really apply. There's no side stand to accidentally leave in the down position.

I get your warnings if someone is removing the side stand kill switch while still using their side stand. But mine is literally in a box in my basement. So there's little chance of it dragging on the pavement under me.

Whoops, didn't realise that was your thread. Sorry