Just got my brand new brompton bike and was having problem with my right shifter going from gear 1 to 2. I have to press the shifter level way pass the limiter in order to engage 2nd gear. Took the bike out for a test ride to confirm since I am a newbie with brompton bike. Brought the bike to the shop and the ‘expert’ told me it was a common problem due to local climate (wtf?) and will take a few day to fix it due to the Q. He showed me a you-tube video hoping that I will do it.

Not willing to wait, I took my bike home and watch the video https://www.youtube.com/watch?v=EwYEYO3h2Po but cannot make any sense out of it. Many replies saying the video help fix their problem too. Not willing to do anything stupid, I simulated the fix and result in my mind shown in the video. To my horror, I realize the fix was to make the shifter housing fit loser by sanding the internals of the housing and the shifter ridge making the hole bigger to have enough play for the 2nd gear to engage earlier.

Not willing to follow the blind, I managed to correct the bug by solving the actual issue. I dremel (sand disc attachment) the pulley plate, removing material of the triangle so that the 2nd gear engages without the shifter go beyond the limiter.

Small amount of material were removed at a time, put everything back, check the engagement to make sure not to over do it. It took me 2 tries with a removal of about 0.75mm material from the pulley triangle. I will include some pictures in the future to make the above easier to understand.

Ever since, the shifting was superb.