Modern Vespa

Was having a great day...rode back roads from Lancaster, Pa across the river to York. Stopped at Liquid Hero Brewing for a while. Took the highway back, running at high speed for maybe 20 minutes. Got onto a a slower road for a while, still at about 45-50mph. Stopped at a stoplight, no indication of trouble. Light turns green, I roll on the power and it dies. Power just fades away and it stalls. Restarted it it a couple times, but as soon as I twist the throttle it dies again. I pushed it into parking lot and let it rest for about 10 minutes. Same thing.

To me it seemed like it was starving for fuel. Vapor lock or oxygen sensor (do Vespas even have an O2 sensor? I assume they do...). Either way I would have expected to see the check engine light come on. I didn't. Opened the gas cap to release vapor lock, no change. The gas light wasn't on either. The needle was low, but not into the red where I would have expected to see the light come on.

There was a Sunoco station about a block downhill from me, so, what the heck, let's put some gas in. It took about 1.3 gallons (1.7 gallon tank). I've run it lower before. Tried it, it started on the first try and ran smoothly all the way home! No indication of trouble, nice and steady.

So, what could it have been? There are a little over 7500 miles on this scooter.

One thing for certain..it was a fuel issue.

Is it fuel injected? See if you have problems when the gas is at the same level again.

What year is it, and have you had the fuel pump recall checked?

Yes, 300 Supers are fuel injected.

you said you were on an incline so maybe the gas was in the section of tank way from the fuel pickup/pump

This sounds like classic symptoms of a bad fuel pump. Stalls while hot, starts after pump has cooled down by sitting for an hour or two or adding cool gas.

I believe the problem is the impeller heats up, expands and contacts the pump housing, stalling the pump. As there is then no fuel pressure to the injector, the engine will not run.

If it happens again, check if you can hear the starter relay click when you turn the ignition on, then hear the whine of the fuel pump for a second or two and then stop. If you don't hear the fuel pump, then that is probably the problem.

I had similar issues. Fuel pump.

I had similar problem too.
Was convinced it was fuel pump, but dealer said no.

They stated the charcoal filter was clogged with fuel, and that the tank was not depressurizing (or something to that effect).
They re-ported the drain line, and replaced filter to prevent it from happening any more.
Warm weather is getting upon us, and I will be able to tell on the first long hot trip if this worked.

Good luck

I'm curious about what station you could roll down to