I'm posting this to inquire for my brother. I've personally never have had much experience with single speed bikes and my brother recently got an old peugot 9 speed bike and he took the gears off to make a single speed. He got the fixed wheel rear tire but the geared crankset stayed. The chain stays on fine but after a few mins or riding, it comes off everytime. We've tried tightening the chain, loosening it up but it falls off everytime. He's getting frustrated because he's had it for a few months and can't seem to fix it. Would the solution be to get a single speed crankset or is there something else we can try?
Also, he has the single speed chain he bought as well.
what cog is he using? A good FG cog is designed so the chain stays on the cog. If you used a cog from a multi-speed cassette, it is designed for you to easily shift gears (not good if you only have one gear).
If fixing the cabin line doesn't work, look at the chainring and make sure it not bent.
Check the spindle too -- see if the chainring wobbles left to right at all when you spin the crank. My old bike had a slightly bent spindle, and the chain went from overtight to a bit too slack in each turn of the crank.