There's a mixture screw on later models? can you post a pic? The pacco that came on my bike only had the idle screw, like most shas and sha copies, but there is a filled hole that looks like it could have been for an idle-mix screw... I've heard of SHAs with idle mix screws, but was not sure if there were vespa style ones or just the larger 14+. Mine came with a 45 jet, which I thought was stock in all of them, but might have been rich for break in or the elevation it was sold at. How does the plug look? wet or dry? light or dark?
Anyway, it would be good to check the timing with a light, but it's unlikely to slip unless someone took the clutch off and pulled the flywheel loose.
Will it fire on starting fluid?