My poor Dax suddenly started burning oil and smoking last week, so on Friday we disassembled the cylinder head, and had it checked. It turned out that the intake valve's guide had cracked, letting the seal move with the valve, letting oil in the cylinder.
The shop told us it was non-repairable, that new guides would not fit and be loose, so I asked George, the mechanic, to get another used head and fit new guides. He, however, tried another shop, where they told him to get genuine Honda replacement guides and get back. He did so, and in 3 hours we had my original cylinder head good as new! I also changed the sprocket to get lower ratios, it was screaming going 80Km/h.
I am now using the Dax instead of the Vespa for everyday commute, to run it in again...