If you swap your variator weights out with 6g FLAT sliders, and properly tune your carb with correct jet, you can get a 139qmb to easily reach about 45mph (gps verified, on flats, "handsome boy" Roketta rider 200lbs), without a bore kit. I HIGHLY do not recommend bbk on the 139, it's inadequate cooling makes the bigger bores more prone to overheat on load.
A tuned correctly 157 should get the same rider to 65-70 without a bbk. The 157 can handle up to about 180cc before heat becomes an issue. Surprisingly, a 180cc isn't much faster or stronger that a 150cc
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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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