Hi, need a little help here from the Brompton experts. I have just upgraded my Brompton 2-speed to a 6-speed. The 3-speed BWR hub is working fine, but I can't get the 2-speed derailleur to run smoothly in the lower gear (larger sprocket). The shifting from the small cog to the big cog works ok, but during pedalling it feels rough. I could feel vibrations through my feet. It doesn't happen in the small sprocket though.

I have already tried adjusting the derailleur, and even tried out a newish one, with no difference. It was working well before I upgraded to 6-speed. Does anybody have the same experience, and if so, how did you solve the problem? Any suggestion will be most appreciated. Thanks.