My Vespa has been neglected, so after being gone all day on a difficult task, I went for a short ride. It is not running well. I started it up for the first time in at least a month and a half, and thanks to my trickle charger, it roared to life.

It was idling fine, but as soon as I applied throttle, the engine died. So I let it idle for about 20 minutes (see below). It seemed to work, as it no longer died under throttle, and ran great on a short ride. But tonight it is sluggish, but I have some ideas.

I am actually excited, as the Vespa and its LEADER engine is really fun to take apart and play with. It is wonderfully simple, and being forced to care for it when our local Vespa dealer closed gave me the confidence to try bigger wrenching challenges. It is a wonderfully simple and durable engine that has me still contemplating a 190cc cylinder swap and some go-fast upgrades this summer.

Of course, it is Italian, and is thus not without its challenges. Certain things, like spark plugs, are in impossible to reach locations as design dominated over practicality. It can be temperamental, and its air cooled engine does not like our hottest summer days. But has a weird thing I have not seen elsewhere.

These Vespas are designed in such a way that it’s very easy to overflow the gas tank while filling up. You can try, but it’s inevitable that you will at some point wind up with an overflow problem. Some forums suggest a similar problem can occur when the scoot sits due to evaporation and condensation in the evap system.


On carbureted models like my ET4, overflowing gas causes in difficult starts, rough idle, or intermittent stalls. The kind of issues I have been having. They recommend running it to burn off gas.

The fuel filler neck has a hose to vent evaporated gases from the gas tank back through a closed system and into the carburetor. The purpose of this system is to reduce emissions by making sure that gasoline evaporation does not escape into the atmosphere, and is instead recycled into the engine.

If liquid gasoline reaches the evaporation hose inlet in the filler neck, either through overfilling or the evaporation, then liquid gasoline can enter the evaporative system and sit there. This prevents the carburetor from sucking air into that vacuum port, which reduces airflow through the carb.

Most of my research says to remove the evaporation hose from the carburetor inlet where it is normally attached. But I like the idea of this weird system. I think I am going to pull it apart and try to clean it all this weekend. We will see.


Sorry for the Vespa nerd explosion.