Modern Vespa

ET/GT/LX Hard Starting, Rough Idle, or Stalling

Your scooter should generally start immediately and not need any throttle to get the engine going, although some scooters seem to need a little throttle at start regardless. However, overfilling the fuel tank (a fairly common mishap) can lead to a situation where the engine is difficult to start, idles roughly, or stalls. This problem applies to all Piaggio LEADER engines, which are used in the ET, GT, and LX Vespas, as well as several non-Vespa scooters.

The problem lies in the fuel filler neck, which 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 expansion of the gasoline on a hot day, then liquid gasoline can enter the evaporative system and stay there. This prevents the carburetor from sucking air into that vacuum port, which in turn can cause your scooter to be hard to start. It can also cause it to idle roughly or stall altogether, depending on exactly how little air is making it through that vacuum line and into the carburetor.

The solution is to remove the evaporation hose from the carburetor inlet where it is normally attached, either permanently or long enough to drain the accumulated liquid gasoline. The hose is on the front left side of the carbeurator, near the top, as illustrated by the following two images:


[Click Images For Larger Size]

If you intend to leave the hose disconnected permanently, it would be prudent to affix a small foam filter to the hose inlet on the carbeurator to prevent dust or debris from entering through this hole. Under no circumstances should you completely block the inlet on the carbeurator. Another possible solution would be to leave the evaporation hose connected to the carbeurator but to fit a hose-T inline to the evaporation hose, again with a small foam filter on the end of the T to allow air to pass into the hose.

Gasoline can seep into the air filter foam, preventing the normal intake of air through the carburetor. The solution for this problem is to remove the air filter, wring out the gasoline, and wash, dry, and re-oil the air filter as per the user manual.

It's also possible that gasoline has soaked into a charcoal canister that lies between the carburetor and the filler neck, and this too can cause the aforementioned problems. If removing the evaporative hose from the carburetor doesn't fix the problem, and cleaning the air filter doesn't either, you may have to consult a Vespa or Piaggio service center for further assistance.

After reading your helpful post, I checked out my scooter to find that the mechanic had indeed drilled a hole in the gas cap and disconnected the evaporation line to the carburetor. The scooter is still stalling at random times and is hard to start once it stalls. The scooter was recently tuned and new filters installed. I noticed that the fuel line below the gas tank that connects to the charcoal canister and continues to the carburetor has an air bubble in it. I also noticed that the in-line gas bulb filter is only half full of gasoline, could this be the problem and if so how do I get the air out and prevent it from happening again. Thanks Joe from SC

Moved to General Discussion

I want to thank you and those on this forum for the great tips. I had painted the Crank Case on my Vespa ET4 and cleaned out the air filter. In the process I reconnected a hose to the air filter that had been disconnected. Bad move. It must have brought gas into the air filter and thereby upset the air-gas balance and I could not start the my Vespa ET4 engine. fast forward.... my Vespa ET4 Engine did not start for days.

After reading online here I realize that a gas smell was coming from my air filter that I had cleaned....BINGO...it was drenched with gas from reconnecting that hose and I think the upset in the air-gas balance was insuring my engine would not start.

Solution to my Engine not starting:
1) Remove plastic piece ( I think it is called a faring) to expose easy access to air filter cover and it's screws
2) Undo screws and take off air filter plastic cover
3) Remove foam filter and soak in mix of water and dish fliud to remove gasoline. Wring out foam filter. Dip in water mix again and wring out again.
4) Start your scooter normally. Squeezing brake level and pressing starter button. You may here a bit of a cough sound and scooter should start right up...Mine did..... Ah air...we all need it and so does our bike engine.
5) Bike ran for a while and then I shut it off.
6) Tried to restart bike to make sure this was not a one time thing...engine started and then stalled out.... what happened..... Guess way too Much Air now.
7) Put back Clean foam air filter and re-screwed air filter cover on.
8) Tried to start bike again and it started right up. Great stuff.
9) Moved accelerator and bike purred like a large cat....Beautiful.
10) Went for a drive on the bike to work out the kinks and everything ran smoothly. Done..... (Hope this helps)