Modern Vespa

Hello, I have a strange problem: When I start my Vespa after it's been sitting for a day or two, the RPM thing on my screen jumps from two bars to three and four, then back to two, then up to three or four, several times. I can feel the engine revving up, then dropping back down... it can do this for a while. When I stop the engine and restart it, everything seems to be fine. I really don't understand what's going on... it's always done this since I got it; a restart temporarily fixes the problem.

What do you think could be causing it?

thank you

dirty throttle body

I forgot to mention that it was cleaned 2 weeks ago.

Check the intake manifold for cracks.

Check your air filter too.

During my service this fall, they did air leak tests and the engine has no air leaks, so I don't imagine there's a crack...

Thank you, I will check and clean them tomorrow.

who cleaned it? and did they get the little ports on the intake side?
after cleaning was the spark plug replaced?

My local dealer. And no, the spark plug has not been changed.

Some shops will say they cleaned it, but dont actually clean it.

Exactly.

I have seen dealers cut corners on all kinds of things. or shoddy work.

oh and dumping a fuel additive cleaner does nothing to the air inlet ports that DO get dirt and debris in from the air filter.

manifold leaks are not engine leaks.

I'm pretty sure XRISTOS was talking about the short black rubber hose between the throttle body and the manifold. I've personally had to replace this three times.

a cracked one of these will cause the idle rpm to go up and down and up and down and often stall

Ah thank you for the precision. I'm going to investigate this.

I've read through my research since my post that this could be related to an Idle Stepper Motor Glitch (ECU), Voltage Drop During Cranking, Throttle Body and Stepper Port, and the battery.

I changed my spark plug yesterday. I'm going to test the battery with a multimeter.

I did this and it fixed the problem the time I tested it:
The "ECU Reset" Trick:
Instead of just turning the key and immediately hitting the starter button:
1-Turn the key to ON. Listen for the fuel pump to prime and watch the dashboard sweep.
2-Turn the key back to OFF.
3-Turn the key back to ON, wait 3 seconds, and then start the engine.

This forces the ECU to cycle the stepper motor to its zero-point twice before cranking, often preventing the glitch entirely.

Also, using a multimeter, measure the voltage while the starter is turning from cold. If it drops below 10.5 volts, the battery is weak and is causing the electronics to malfunction during starting. A new battery with a high cold cranking amps (CCA) rating often resolves this issue permanently.
It's my next step...

Kudos to you for getting in there to get it done. Also, to the members on here for helping out.