Originally Posted by
FBinNY
The sprocket is almost always round. However, it's likely not perfectly centered.
Try slightly loosening the mounting bolts, turn the crank to find the place where the chain is tightest. Now use a hammer to GENTLY tap the chainring at 3 o' clock to slacken the chain. Test and repeat until you get it as good as you can, then tighten the bolts.
Note that you will never get it perfect, so adjust chain for minimum, but not zero, slack at the tightest position.
Sheldon Brown has this routine nicely written up on his website. I believe it is in the fix gear section.
My observations about this have led me to the fact that all cranksets are not equals when it comes to roundness. That the better makes and models do better and also that cranksets for single and fix gears tend to be better than the same level of "road" or derailleur bike intended cranksets. That eccentricity doesn't matter on the road. There was even a time when some believed that eccentricity improved shifting so why spend money in careful tool setup and QC?
The cranksets intended for fix gear, and especially cranksets intended for the velodrome are much better with the best being near perfect. My Sugino 75 crankset at 20k road miles is so close I rarely feel a need to improve my first guess at chain slack after spinning the cranks and it is mounted on a medium priced (but velodrome worthy) square taper BB.