Modern Vespa

Hi guys. Me again! Thought I'd start a new thread as although im sure it's related, it's a different problem. Anyone who followed the last thread will know that after a few false starts, I just fitted a new dell'orto carb, and pretty much had the first good result, and got it running after some months!
I broke down the other day, and put it down to incorrect air/fuel mixture, which I thought I'd now sorted.
I just went for a loooong run to blow the cobwebs out, and the scooter was running like a dream. Nippy, fast and responsive.thank god it happened as I pulled up at home (I'd been as far as the next town, about 7 miles away!) but the same thing happened. Symptoms are:-
When going to accelerate, I notice it's not responsive, and very sluggish, so I let go of the throttle, try again, and as I twist it, the engine kinda bogs down, and dies out. It's like the throttle is doing the reverse of what it should be doing! The more I twist it, the lower it revs.id describe the feeling like it 'holding back' when opening it up. Couple of seconds later, it totally dies out, then struggles to start again. A couple of times just now, it didn't 'catch' and just made that nasty whirring sound. It started eventually, by not applying the throttle at all, but it now sounds very low pitched, and a bit rough.
I'm really happy with the way it's running ( when it is running!), but obviously taking it out twice since the new carb, and breaking down both times isn't the reliable little bundle of joy I want it to be!
I do have one thought. After testing and experimenting, the only jet I eventually changed for a brand new one, was the main jet to the same size as the original weber carb. The other two were left and are the ones that came in the dellorto.
Besides ' roll it down the hill into the river', has anyone got any suggestions? It has a new fuel tap, carb, jets, reed valve block, spark plug, and new fuel in the tank.

What size Main Jet is fitted now?

Hi Fabio. The main jet is a 76. I thought it would be wise to match it to the original weber jet, as they weren't interchangeable between carbs.

A bit more info-I had planned to investigate the other 2 jet sizes, but I fitted the carb to no avail, changed the main jet for a new 76, and as it fired up, and ran better than it ever has, I thought it best to leave it alone! I'm thinking it might be wise to try a new spark plug. It's not that old, but has been in there while I've been working on it recently. I had an old triumph herald a few years back, and one of the spark plugs kept oiling up and failing. The more I think about it, the more it feels the same. Like you're trying to accelerate with the handbrake on!!
K