Originally Posted by
Lascauxcaveman
I believe you may just need a
single-speed chain; wider across. AKA a 1/8" chain. The teeth on your sprockets are too thick (not spaced wrong) for the chain you have to drop down all the way over the teeth.
I ran into this problem when I built my fixie; the cog on back had teeth too wide to work with a multi speed chain.
[MENTION=280039]Lascauxcaveman[/MENTION] got it exactly right. We recently had this same problem rebuilding a kid's single speed at Bike Works in Seattle where I volunteer, and using a wider single speed chain as he describes was the fix.