Modern Vespa

the engine starts and idles but when you give gas the engine stops.
what could be the problem?

The good news is that it's starting. So, you know it is getting air and spark. So, that leads me to believe it is a fuel delivery problem.

I would assume that it is a blocked fuel line, or you have a dirty carb or debris in the carb. If your able/inclined, pull off the carb, remove the float bowl and check the jets/internals for blockage, spray it out with carb cleaner while your in there. Button it back up and see if that cures it.

Clogged main jet.
Happened to me during my commute home this fall.

Also could be water in the fuel. That happened to me on my ET4. Had to drain the tank and put in new fuel and that solved the problem. Hope it is a plugged jet and you don't have to drain the fuel.

This link might provide some helpful information if you want to try carb or jet cleaning:

Carb and Jet Cleaning 125/150/200 engines.

Let us know how things go. Good luck, and merry Christmas!

~Deborah

Even if it is a clogged jet, you should still drain the fuel. The clogged jet was likely caused by fuel that was either contaminated or stale.

Before having the carb cleaned you may want to try some easier solutions. If you get a siphon (a pump one is about $5) and drain the gas, you can try an additive called Seafoam (also about $5 or so at an auto parts store) with a tank of fresh gas. Run it through the system for a while. If the clog or gunk is pretty minor, it should clean it out and you've spent less than $20.

Seafoam is a great additive to use if you're not riding frequently, storing your scoot for the winter, or encounter an issue like this one.