Modern Vespa

Vespa ET4, 150cc, 2001, ~45,000 miles, carburetor.

For the last 10 years, I have been having problem with starting engine in rain.

These are the symptoms

1) I could ride the scooter in rain without any problem.

2) Say it rained overnight 9spring, fall, summer). Next morning, there is no rain. Scooter was parked outside in rain with cover. Engine starts easily (so starter motor is good). I let the scooter idle (1-2) minutes before riding it. Always after riding about 2 blocks, the engine stalls. I get off the bike and try to restart it. The start motor turns over strongly, engine does catch but has trouble remaining on (keeps dying after few seconds). After about (10-15) tries, engine remains on, and I can ride it.

3) Yesterday and this morning were sunny (75F). This morning, I took a spray bottle filled with water. After the scooter started, I sprayed various parts of the engine bay to recreate the problem (all sides of black plastic air filter box, all vacuum lines, large intake black air rubber boot, carburetor, fuel petco, etc). But the engine doesn’t stall.

Would anyone have a reasonable suggestion?

how's your intake manifold look?

what's the service history: last time the valves were checked, fuel tap replaced, spark plug, vacuum/fuel/evap lines?

i'm gonna roll the dice and say it's something along those lines. annoying but easy in this case.

-g

Probably the electrics, check out the connections and maybe change leads and and spark plugs. Also give the wiring a lot of squirting with WD40.
Gass silver machine gts 250 and an old ET4 that's had problems like this.