Modern Vespa

Hi just changed the belt on my Vespa. The belt that came off was only 12mm wide and I relaxed it with a 17. Since I've changed the belt it's accelerating really slow but picks up again after about 10mph. Any advice thanks.

Did you have a chance to clean everything up good while you were in there? Check the clutch for anything obviously not right (like clutch pads)? Glaze present on the variator halves? Did you replace the rollers, too? You'd mentioned going with lighter rollers in another post maybe? I don't have enough experience to spot many things being out of sorts, just trying to help you think through it.

Yes cleaned everything it was accelerating before with the old belt

Then focusing on the belt is probably the best bet. Here's the specs on the belt I used for my ET4 150: Malossi X K Belt for all Vespa & Piaggio 150's (22.5X10.5X814 MM , 30°)

and the link: https://www.scooterwest.com/malossi-et4-lx-150-kevlar-belt-m6111437.html

Where'd you get the belt from and are you sure it's the right one?

Both 12mm and 17mm for belt widths sound *really* weird. I can't even visualise how the bike would run at all!