Brompton help please

[rower40]

When I fold my Brompton, I need to turn the cranks backwards by 90 degrees or so, to put the folding pedal at the 10 o'clock position, and the non-folding pedal at the 4 o'clock position, nestling under the front wheel.

But when I do this, the rear sprockets stay stationary, the chain tensioner just curls, and the 'top' side of the chain (by now underneath) goes totally slack.  If I let go of the cranks, the chain tensioner pulls tight again, putting the cranks back to the ten-past-8 position.

If the back triangle is in the "unfolded" position, then I can turn the cranks backwards as much as I like, and the chain pulls the rear sprockets backwards against the freewheel ratchet.  But when the back wheel is folded under, turning the cranks backwards just causes the chain to start to come off.

What's going on?  How can I get the cranks to where I want them when the back triangle is folded under?

(Apologies to anyone who hasn't got a Brompton; this'll read like total madness.  Go find something less boring.)

[duncan]

Is it all nice and clean and well oiled? I think I had this problem once when mine was absolutely filthy. I think on mine it was that the middle of the three cogs at the back was stuck up. What gear setup do you have?

[border-rider]

It can also happen if the chain is stretched/too long/dry cos the chain isn't pulling through the tensioner straight.  Mine tends to do just the same when the chain is a bit tired.

[duncan]

What I was thinking is if it is a 6-speed, then you can get those sorts of problems if the little jockey wheels aren't moving sideways. I guess that is something like what Mal Volio is talking about.

[rower40]

Three speed hub (some oriental pale imitation of Sturmey Archer's Finest epicyclic gizmo, don'cha know?) and a two-speed derailleur.  It's always in the lowest-of-low gears, 'cos I live at the top of a hill, and haven't taken it back to LBS to get a smaller chainring fitted.  (Forgot to ask for lower gears when I bought it.)

It's middlingly clean - no squeaks and crunching noises coming from the chain or gear hub.  Chain is new and well-oiled.  I'll lubricate-and-then-some the hub, then report back in a day or two.

Jockey wheels appear to be in line with the sprocket that has the chain on at the time.  It appears to be some kind of stiffness in the hub itself.

Thanks for the suggestions.

[Fi]

Same thing happened to me with my L6 intermittently over a few weeks.  I never did work out what was wrong and, more to the point as I have no patience and my husband has more than enough for both of us, could he.  However, following a you know what, I thoroughly cleaned the jockey wheel while the wheel itself was off and it has never happened again.

[nuttycyclist]

Mine does this sometimes, and I don't know why.

I just turn the cranks the other way (as though pedalling forwards) to put the pedal ni the correct place.  You do have to lift the main tube slightly to allow the rear wheel to turn though.