Modern Vespa

I have a 2016 150cc GTS with about 80,000 km on it that occasionally dies. Typically this occurs after braking and hitting a speed bump, or going off a five inch curb, but it does die when simply coming to a full stop as well.

Turn the ignition switch off and on and it will start again. Doesn't appear to be starved of fuel, and the battery has been replaced.

Does the 2016 model have a sensor that cuts the electric? Anyone have any suggestions?

Thanks.

Does it have a side stand that acts as a kill switch like my 2019?

Yes, it has the side stand kill switch and I suggested that to the shop, but I've no idea if they looked at it. I'd left it there and when I returned they said they couldn't find anything and sent me on my way without charge.

At 50,000 km the bike would die sitting at red lights. I'd walk it to the side of the road, wait a few minutes and it would start again. This turned out to be the alternator.

I've also had a three way switch go at 20,000 km, and the usual battery issues, but this is a real puzzle.

Certainly sounds electrical. Loose connection somewhere between the ECU and spark plug?

My big Beo started having idling difficulty after being warmed up. After a fruitless quest for fuel problems, it turned out the spark plug wire was loose - not the side that connects to the spark plug, which of course is pretty easy to look at, but the side that connects to the coil, which is buried under the footrest.

Who knew both connections were important??

I always suspect ground issues with any mysterious electrical vehicle problems. I remember reading somewhere a study of the loss when the ground is using the frame or body of the vehicle instead of a direct connection.

Here is an informative youtube video link that may or may not apply to your situation but is it possible that a main connection on either side could be loose in such a way that it makes connection most of the time but is momentarily disconnected with a jarring bump?

The negative terminal on the battery is the ground on any vehicle. Sometimes a wire is run from the negative to the frame or body to complete the circuit as shown is the video mockup with the bar and the battery. This is a weaker connection than having a copper wire all the way back to negative on battery, the frame or body does not conduct as well as a copper wire.

Also it could be that the connection is faulty and intermittent in the battery itself. I once had a vehicle that would run and drive but had mysterious electrical issues over time that were very hard to diagnose. Similar to finding a slow leak in a tire it had to get worse before I was able to find it.

At one point driving the vehicle on the highway at speed the engine would miss when I turned the turn signal on and the miss coincided with the blink! That problem was a faulty battery, the battery would work, it was not old, the car would start, run and drive and the wiring connections to the battery were good but an internal battery connection was faulty intermittently. Not revealed by load test until it worsened enough to be detected.

A new battery solved the problem and yes I know only hack mechanics just replace parts hoping but I have personally had a battery that while the battery worked it caused intermittent electrical problems that disappeared with a new battery and it was because of how vehicle electrical systems rely on negative of battery for ground.

Good information.

I'll be travelling for two weeks and will be taking the battery out and on a trickle charge soon. I'll check for loose and corroded connections at that time.

That last sentence is totally incorrect. True, a steel wire of the same diameter as a copper wire would have more resistance - but the frame is the equivalent of several thousands of such wires in parallel. Extremely conductive!

A direct connection to the battery negative can make sense in some situations, where ground loops due to faults can cause other equipment to malfunction. No such situation occurs on our scooters.

OP - I'd be looking at the coil/spark plug/cap connections first. It's very common for the spring inside the spark plug cap to become weak, and make poor contact. This often doesn't matter when revving, but come to a stop especially with the rear brake applied and the vibration (rear suspension locked) can cause the bike to stall.