I had a serious problem with my (then) stock 150cc GY6 tricycle that was doing over 40km/h (unacceptable). After a bit of research, I found that the stock fuel pump was starving the engine; and boy was I right! I still choke a bit around 50mph, but it probably has more to do with carbs. In the future I plan to buy a Mikuni brand carb depending on how cool that pump is. Highly recommended, I don't see what can go wrong with this.
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