Having installed literally dozens of bbks on 139, 1p40, 157 and 152 engines, as well as being extensively familiar with all makes of scooters since I built my first Lambretta in 1988, I will now state that achievement of any flat land speed of over 5Omph on a 73cc 139, is pretty good, and about all it will do.
Mods help the 139 gain acceleration, but really don't do anything to the top end. A 73cc engine is approximately 4cid, or 1/3 the displacement of the cheapest lawnmower available at Wal-mart.
To assert the modified 139 is capable of speeds in excess of 55 mph on flat ground, is untrue, and unrealistic.
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2011 Roketa MC-23-150
4T 150cc 157 qmj
24mm carb w 115 main jet and paper cone air filter
Manual petcock w Tygon fuel lines
Scrappy Dog Scooters Retro-slash stainless straight-thru exhaust
RED spring clutch
Adjustable CDI (brand unknown, it's blue and red and works great!)
KOSO high performance variator w 12g sliders
Gates Powerlink 835-20-30 belt
GPS verified 65mph on flats.
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