The "b" variant is theoretically the fastest gy6 150cc engine. Are you certain it has a "b" variant, as the "b" engine is used EXCLUSIVELY with 16" wheels?
If you have good compression and are not leaking/burning oil, then I see no reason to buy a new longcase. 4K is really not bad as far as mileage goes, I know of a bike with a 157 that is at over 15k and runs like a champion.
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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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