Modern Vespa

Hello,
New here! quick back story of my issue. Buddy bought this not running and it sat for a few years. The previous owner had tinkered with it and it came with some parts. I'm a seasoned vehicle tech but this is the first modern Vespa I've worked on. So I cleaned the carb and put in a brand new OEM vac valve on the fuel tank. Also ended up put in a new CDI with out the immobilizer. Bike ran great for a few weeks then somehow the fuel tank emptied into the cylinder and hydro-locked the engine (also filled up the crankcase). I figured maybe there was something in the tank that lodged in the valve? I took the carb apart and cleaned it again (in my brain the needle would also have to be stuck to let the fuel through), drained the cylinder and crankcase, replaced the spark plug and it runs. With the throttle just cracked it has a very distinct missfire put your wrist into it and it runs great at higher RPM's no skip at all. I replaced the coil just to rule it out and I still have the skip, I also went through the carb again to no avail. Checked for vacuum leaks and found none. The bike starts immediately and will idle at low RPM all day. Sorry for the long winded post just wanted to put all the information up.
Vespa ET4 150 build date 04/04
Thanks!!!!!

Hi quahog and welcome to MV.

I like that you gave back story and detail to the problem and how you approached fixing it. As a tech yourself, I'm sure you know more information given usually results in better solutions.
I am sorry to say I have no expertise with ET4s. The only time I've experienced the "skip" in any Vespa I've ridden is when the spark plug wasn't properly seated.
The good news is, there are lots of folks here who know lots about ET4s. I'm sure they'll be chiming in shortly.
Best of luck in getting it sorted!

A guess, but it may be worth trying. Take the under seat carrier out so you can see the engine. Wait until it's dark and fire it up. Look around for any sparks when it's missing especially in the coil, spark plug wire area. If that does not work, check the spark plug cap, wire and coil. Good luck

Thanks Tierney, all of that is new with no change in symptoms. I'm thinking new carb may be in order but I sprayed, used a torch cleaner and compressed air. Every passage looked clear. I'm hoping something didn't get tweaked when it was hydro-locked but I didn't have this symptom before that happened.