I have a small issue where my year old Brompton's chain comes off sometimes when I'm unfolding the rear wheel triangle. As the wheel comes up there appears to be some slack in the chain and it pops off. Is there anything I can do to address this issue?

chain tension-er spring still working?

Take a close look if the chain is going the right route trough the chain tensioner. I have seen this problem before on a B where the chain was not properly threadad trough tensioner.

https://www.youtube.com/results?sear...hain+tensioner

Before you unfold the rear triangle, make sure that the left paddle arm is horizontal and pointing forward.

Somehow it matters, because the Brampton's pedals are not balanced, one is heavier than the other.

Tensioner is fine. I'll try the pedal pointing tip. Thanks.