Modern Vespa

BV350, caught in heavy downpour at 4000 ft. unable to accelerate when traveling at 60 - 70 mph. (I know, way to fast for the conditions)

Not talking about engine miss, hesitation or motor cutting out. I would suspect wet electrical issue under such conditions. This felt like lack of power. Slight attempt at acceleration made sensation worse, maintaining rpm or backing off a bit felt at least like engine might continue at a reduced power level. At first I thought inches of water resistance on the road was creating the sensation but it didn't react instantly to change in wheel location in section of road. This problem happened at least 3 or 4 times for short 30 sec intervals and only at the peak volume of rain coming down. As soon as it let up just a bit engine felt normal. When transitioning from the storm to drier skies there was never anything but normal engine performance and was perfect the final 100 miles of the trip.

So here's my theory that I would love you more experienced riders to consider and comment on especially if experiencing similar.

The air filter intake on this scooter, and probably most scooters, faces down and about 10" off the road surface.....and right close to the rear wheel. The way things are designed, I don't think water is getting into the filter area (there is a drain tube bulb that would show that as well as oil vapor that collects. I do think the incoming air can get so "heavy" with vapor/moisture that it effects, what, combustion, the ECU decision making process? Cars and motorcycles usually are able to locate air intake at a high point under tank or hood cover and considerably away from wet road conditions.

In addition to add complication. Poking around I found a good sized electrical connection in center of engine opening just in and forward a bit from the air cleaner. It is designed with an insulated boot with a sealing lip at the base that snaps around the plastic base of the connection. Obviously meant to protect this location from wet. The boot was completely off the terminal, left off from servicing no doubt because it was pretty difficult to put back on properly. I need to look up what this terminal is/dose. Maybe a possible cause of above experience.

Thanks for taking the time to review all this and possible thoughts.

Too many questions.
Miles on the bike? Last service? Do you saturate the air filter or use spray on? Were your changing elevation rapidly? How about a picture of said wire.

Motor was inhaling water spray/vapor. Air filter is foam, essentially a sponge. If your "sponge" is wet motor can't breathe = power loss.

Just the traction control doing it's thing.

My vote is for a small amount of hydroplaning causing the traction control to kick in.

And my vote would be to pull over and wait until the heavy rain lets up a bit. (I also think ASR is a plausible explanation)

exactly. water does not burn.