Considering a Lambretta has Legshields (Excellent for areodynamics

), and 30+ years old, as a 175 will only cruise at about 45-50 a 125 will only do 40-45.
So a stage 4 adds between 15-20 mph, a stage 5 or 6 will only add another 10-15 mph, but is extremely tempremental.
If you want to go any higher than that you need to put a 225 TS1 engine in. Ported to the max, transfer ports matched, a reed valve 34mm D'ellorto Carb with a 42mm Exhaust, this will sit at 90mph on the Motorway all day.
The new Vespa (See the Betty Boop Scooter above) if Stage 4'd will do over 100mph!!!!! Stupidly quick for a vehicle that has only 12" wheels
