Modern Vespa

Im slowly testing my 300 due to an inconsistant idle. My gut feeling there is something wrong. Today i decided to test the the voltage during idle since i noticed it cuts out as soon as i twist the throttle. When i hold the throttle steady steady at 2200 rpms it will surge to 3000 rpm for half a second and go back to 2200 for ever 7 seconds or so. It will do that over and over. The other thing i noticed when i start to twist the throttle slowly the rpms increase like usual but then rpms cut out while im still holding the throttle in place.
The voltage im seeing during idle (~1680 rpms) is anywhere from 12.2v - 13v. (Voltage jumps around a lot)
At 3000 rpms its somewhat steady at 14.5v.
I now have 3,300 miles on my '18 Gts300
Is this a Stator ussue?

Those voltage readings would make me suspect the battery was iffy[1] - the voltage shouldn't dip below 12.5 to 12.6V once the scooter is running, even at idle when the stator may only just produce enough current to run the bike without charging the battery.

[1] Or a poor connection somewhere, but less likely.

I suspected the battery is the original. I just went to my nearest motorcycle shop and bought a new battery. Installed it.
The readings are 14.0v to 14.4v at idle. Definitely a difference in voltage. Bouncing back and forth. Sometimes dips to 13.9 in a flash on the screen.
The throttle experience i mentioned above still does that. I kept increasing the throttle more and more but the rpms would stay the same until i get a burst of rpms to 3000rpms. By then im 1/4th to 1/3rd open on the throttle. Doesnt seem like the cable is hanging up when i look at the throttle body while twisting the throttle. Is this normal?
Noting, the inconsistant idle is still the same.
What is the voltage supposed to be at?
Thanks for your response,

I'd be looking for a leaky inlet manifold, or a dirty throttle body idle bypass.

This is almost certainly fuel/air mixture related.

I was thinking it may be more fuel/air related. I have cleaned the idle bypass with throttle cleaner, but that doesnt mean its clean tho.
It does bog down at rare occasions at the start of throttle twist. Maybe its an early sign of something. I will pull the spark plug and see if it shows any indications. I would like to test the fuel pressure but uncertain of type of connections the vespa uses. All i have is for chrysler which is 5/16" quick release male to male. Would have to make another "T" connector.
Thanks, Jim

Wow, i finally got a check engine light to show up. In a way im happy it came on. I was checking for air leaks likerobot at scooterwest showed and cleaned idle air bypass with no go. I decided to replace my spark plug from a CR8EKB to a new CR8EKB spark plug to cross things off since its cheap. It still took long to start from cold, it actually took another second to fire up. I would say almost 3 seconds on the first try.
I could feel the engine combustion more strong. It felt nice. It didnt vibrate the whole scooter as much , just slightly less. I pulled in to my garage and noticed the check engine light. It started to run really rough. Like more than before. I turned it off. Then let it sit for 15 minutes. Started it again to check voltage or anything loose. The plug wire is on . The voltage is still reading anywhere from 14.0 to 14.4v, it still jumps back and forth frequently. Check engine light stays on during idle and while revving (it has a hard time revving cuz it wants to cutout while giving any amount of throttle)
I did notice before all this, while driving about 15mph, the engine feels like it wants to cutout, dont know what to call it, either "hesitating" or "stutter".
Cant wait to find out what the problem is when i take it to the dealer again...

could even be a bad throttle body. maybe read my thread (solved 19/May so you could skip half the thread and scroll down to then) Saw tooth idle [SOLVED - worn throttle body]

Read that all just now.
Thanks, i agree that my vespa has the same symptoms. It has no problem cruising at any speed, especially high speed.
Mine never stalled, just the idle is poor. The phrase saw tooth idle fits the description quite well with mine.
If it is the throttle body/ecu, which i suspect it is, i feel like I'd be lucky if the dealer and my extended warranty i purchased to help replace it. Nevertheless, i will take it to them in hopes it gets fixed. Crossing my fingers.
How much is a new throttle body?
And would the dealer have to install it anyways for programming? Thanks,

AU$800? I bought a 2nd hand low mileage for AU$350.

The physical installation is pretty easy. Robot / Scooterwest has good videos on youtube.

If you get a brand new one then I believe you can program it yourself at home to match you existing brown + blue keys by following the instructions in the workshop manual.

If it's 2nd hand and it's a pre HPE then it can be "reset" to match existing keys but you will need to take it to somewhere like scooterwest or af1racing. But HPE ones can't currently be "reset"

Thats not terrible. I only have 1 blue key. The previous owner lost the brown key, I suppose.
Im definitely a newb in programming ecu's. I would probably take it in to have them install.

I jsut re-read your post:

I didn't have the voltage issues that you do. I was consistently above 14 within a second or two of starting (not matter how rough the idle was).

I did have a voltage problem, but a new vattery seemed to resolve it. The battery was 4 years old and leaking on top. Figured i just replace. The vespa went to the dealer today and explained what ive seen it do. Wish me luck.

The dealer doesnt see any check engine light. And nothing in the history. They see the idle is normal and cant find anything out of the ordinary.
Im starting to think im crazy
Im uncertain when i can ever test drive another GTS. Im usually busy when my group of GTS300 buddies schedule an event.
Until then, ill just ride it and enjoy.
Thank you everyone

Other things to look at:

Is the positive battery terminal in position to arc against the side of the frame?
Is there a break in the positive lead where it goes around the frame?
Is there a break in the spark plug cable where it goes around the frame?
Is the little clip still in the spark plug cap, and the spark plug clicking into place?
If you give the spark plug cable a bit of a tug from under the bike, do you end up with the entire thing in your hand?

Hm, i would say battery terminal is not close to the frame for it to arch, i could say the battery is sitting in the tray correctly and the tray is in place. I took the battery tray out cuz it got moved while placing the battery in. And positioned the battery tray in correctly as far as i could tell, then placed battery on top carefully. I know that for sure.
I havent checked for any breakage on positive lead nor spark plug wire. I did make sure the spark plug clicks into the cap. I did see the "clip" is in fact in the cap.
I never tugged on the wire as a check, i tried to make sure the plug wire is on by pushing on it. Does the plug wire twist on at the coil side or does it slide in. I know it twists on at the cap.

If it was arcing, you will often see little black marks on the frame near the wire. The wire twists into the coil. A faulty coil will also give some of the same symptoms you are experiencing.

Right, I dont think i saw anything like that. But as soon as i get her back home i can start inspecting the wires and connections.
Is there a way to test the coil? Like checking the ohms?

are you sure that it's still bad since you replaced the battery which you said fixed the voltage? maybe you're imagining that there's still problem, ie ya sure that didn't fix it?

I didnt notice a difference at all when i replaced the battery.
I believe i was wrong in thinkjng it would be a statir problem.
The only thing that made a difference in changing the idle was a new sparkplug. The combustion felt stronger/more pronounced with the new one. I snapped a pic of old spark plug. I see the electrode has been eaten away on one side. Sparkplug looks normal to me. After 3 mintues with new sparkplug, the check engine came on and it was running real rough like a sawtooth idle (every stroke was very aggressive).

I may just be imagining that there is something wrong with the idle. Like i said before, it never stalled on me.

When im sitting at a light whether im holding the handle bars or not, my whole body vibrates with the scooter, including my head. But maybe this is normal. I just thought fuel injection would be smoother. But IT IS a single cylinder 300cc.

For awhile now, ive been watching youtube videos of other 300's, and trying to focus on how others idle.
But its so hard to tell, cuz they all seem like they dont vibrate the steering handle/mirrors/gauge cluster or small windshield. On one video i may have got a glimpse of the mirror vibrate during idle but that was a gts300hpe.
All other 300's i hear sound like "clockwork". Mine does not sound like that.

They do vibrate a lot when idling. Some cameras sorta reduce the appearance of vibration due to their image stabilisation systems.

That was my thought. Lol
On a side note, im learning and getting prepared how to pull up check engine light codes if it ever comes on again.

Thanks steelbytes

Are you holding the rear brake in while it's idling? That will make it feel a lot rougher.

No, i have a bad habit of not touching, holding or squeezing the brakes while at idle.
When i squeeze the rear brake, it does transfer the vibes up my arm as expected. I dont really like holding the rear brake while idling cuz of that. Front brake does nothing.

Today i removed the whole intake and physically checked for leaks.
Nothing i can see or manipulate to expose cracks. Before i assembled everything back together. I cleaned the idle air bypass for the second time. I used a whole can of throttle body cleaner this time. I got some black flakes out from somewhere as seen in pic.
After a ten mile ride from cleaning that. The idle feels noticeably better (smoother). Still settles in a lumpy idle when sutting at a light for a mintute. BUT it is LESS aggressive now. I would say it still vibrates and has this little hunting idle. Other than that, its good.
I do hope the cold start gets a little better too. But im thinking that might be the valve clearance, although i dont hear them when engine warms up.