Modern Vespa

So, I decided to bite the bullet and get a brand new clutch and pulley assembly . Dry day here in Scotland, so off with the cover, removed the old clutch, replaced it with new clutch, torqued everything back up. Pumped up the tyres. Rolled it back about 10 meters. Started it and it drove with me on it, yippee, let it roll back again and tried to drive and the problem started. No power getting to the rear wheel , revs were fluctuating, then it would rev and drive, then stop and eventually stalled.
My engineering knowledge is limited with this scooter and I've done as much as I can with all the ideas and suggestions, I have received.My last resort is to get a diagnostic tool. But I have no idea what to purchase.
As always, I'm happy to receive all your feedback,information and suggestions
Cheers 😢😢😢

Are you sure you reassembled the transmission parts correctly — variator and clutch — and torqued them to spec?

Are you getting a Check Engine light? If not, you shouldn't need a diagnostic tool; it's no help with a transmission problem, which it sounds like you have.

Oh, is this related to the same problem you had back in July? Pulsing/ Surging
Did you find any vacuum leaks?

Yes, I have that casing off so many times, I could probably do it in my sleep!!!
My question is ,why are revs fluctuating? I'm suspicious that my problem is electrical or a faulty ECU, .
When all of this started,lots of advice about the injector, the bike wouldn't rev at this point. So, a wise man told me to get a certain injector, I fitted it , and lo and behold the engine started revving. She'll idle, but if you want to take off, she'll flaff around and eventually stall. I can walk beside her and she'll drive a little, but when I'm on her , the scooter starts misbehaving. Could it be a sensor somewhere?

When that surging started, I took the scooter to a motorcycle shop and £80 later they said that there was no air leaks, or split hoses, and they rode it around the block and said they would need further investigation to find the cause. I'm on a low disability income, and there's no way I could afford to get that done.😩

Forgot to mention, I have no warning lights on the dash.

Don't buy a code reader, if it was ECU or sensor It would put Check engine light on, I would say Transmission/final drive (mechanical) as when you sit on it and put the engine under load it does not want to have it (check your work), when you changed the belt did you change the rollers and the sliders and clean and abrade all friction surfaces with emery cloth thoroughly and clean with clutch and brake cleaner.

Put it on center stand and start it does it accelerate, rev normal and Idle without trouble.

If its been stood when was the last time it had new fuel.

On what you have described. check fuel pump working, fuel filter not clogged up , vacuum valve not damaged and check air filter not dirty and clogged up,

Longshot, could be the timing.

Last but not least, You sure you had a dry day in Scotland

not idling well: fuel, spark, air

thoughts ...

doubtful the fuel pumping is failing. maybe the injector needs a clean? robot made a video on this.

could the coil or lead from it to the plug be flakey or even the plug itself? have heard of people with bad leads having problems but think that might only be at high speed?

air. surging can be caused by air leaks. was a problem for me and I had to replace the throttle body. can check for leaks by spraying starter spray around the joins ion particular. also the seal between the two halves of the airbox might need replacing. can even leak air in around where the injector goes into the manifold (if the little oring has split). did you replace the airfilter and use correct amount of oiling (or pre oiled oem)?

since no light on the dash a code reader is of low use but can be still used for reading the o2 lambda value to check that it oscillates over the correct range (after the first 30secs of warmup)