This is number one in a series of tips that have taken years of serious moped meddling to figure out. I will slowly disperse them in this thread .
If you have a vespa that will not run ,but seems to have compression , spark , and fuel , remove the cylinder and check the piston ring end gap. If it is larger than .012 on the lower end near the transfer ports the cylinder is worn out.
We have found several vespa cylinders will wear out on the lower half and because of the way the fuel charge is delivered through the tiny crank case on those motors , the pressure is not enough to draw fuel through the transfer ports to the compression side of the cylinder.