Airburst

I've been putting in a lot of miles on my fixed-gear Raleigh since I built it up at Christmas, but I've noticed quite a severe tight spot on the chain. The chain pulls tight at one point, but then has about 1 cm of up-and-down slack in it after 1/2 a turn of the cranks. I've tried Sheldon Brown's trick of loosening the crank bolts and hitting the chain when it's at its tightest, but it doesn't seem to have helped, even though the bolts were completely loose. The cranks are some SR Suntour 4-arm ones, and the chainwheel is a 46-tooth Shimano one. The tight spot always comes at the same crank postion, so I think the problem's up front, but can anyone suggest how I might fix it?

Thanks in advance...

FastJake

Sheldon's method does work, but it takes a long time and patience to get right. If the bolts were completely loose when you were doing it, they were probably moving way too much whenever you hit the chain. I've found that it takes very fine tuning to get just right. Sometimes the chainring will move such a small amount you can't even see it, but you'll notice once you spin the cranks.

Get the bolts snug to the point where the chainring is still move-able on the crank and try it again.

I suppose it's also possible that your chainring is not round, which may make it impossible to get even chain tension. But from your description I don't think that's the case.

fietsbob

more drastic.. ovalize the chainring bolt holes a skosh.

Cynikal 

Try rotating the chainring on the crank if possible. I have had several bike like this and never really found a problem with it. I find the tight spot and then tension to that spot, then the tension is never tight enough to bind. Binding will cause premature bearing wear on BB's and rear hubs.