JeffOYB

I've been using a 1976 Nishiki Comp for cyclocross. I was getting bad shifting with the old SunTour VGT Luxe -- seems to have body slop. So I installed a tighter old Shim Altus LT. I haven't reinstalled my barcons yet. I've been riding it on my cross course with screws holding it in singlespeed mode. The chain has been bouncing off the "untrapped" lower pulley. Just a weak spring? Any trick I can try? I went to see if I could rotate the cage around an extra time and there's a stop to prevent that, though I noticed the spring sure got tighter. Maybe I could grind off the stop. When I cross I seem to use 3 gears but I've only done it on a few courses. Maybe I should shorten my chain? Or lengthen to give more wrap? (Doubtful.) Cage is in vertical position in my fave gear for my personal course.

dsbrantjr

Are your derailleur and its hanger aligned properly? Has the derailleur taken a hit recently? Did it ever work correctly? Did you change anything else recently?

Do not shorten your chain beyond where it will fit on your big chainring/big cog combination. Even if you typically use your middle cogs, if you forget and shift into that combination you will damage your derailleur and possibly your frame.

ThermionicScott 

I'd probably switch to a short-cage derailleur that has the little tabs to prevent this. Or go straight SS.

onespeedbiker

The usual suspect is either the back cage was put on backwards, which doesn't apply in your case, or bushing in the tension pulley is shot. If the tension pulley is wobbling excessively it will throw the chain