139QMB GY6 Big Block

[bull]

fix known problems first, pull all normal maintenance, perform all your tuning, then install a BBK if needed.

BBKs come in 4 sizes:

63cc - many of these are found from factory installations when they are supposed to be 49cc

72cc, termed a BBK 80 -- from my research that is what these scooters were originally designed to be by Honda- I installed only a few of these, the best one was from valley scooters-

82cc, termed a BBK 100 - these require no crankcase boring- they are generally reliable - but there are differences between China and Taiwan made

the last does require crankcase boring, as well as a crank swap. I have not personally done one of these, but my attitude here is IF you need this amount of power, just get a bigger scooter which would prove more reliable.

49cc, 63cc,and 72cc typically do not require carb rejetting- the 82cc does require carb tuning