My dad has a barely used 10 year old mountain bike that he wants to get to riding again. He complained about how it was popping. I went over and checked it out. The rear hub was *really* loose (I'm guessing it was like that from *mart), so I tightened it up enough that it still spins freely. The bb ( the kind that the crank arms are one forged part with the spindle that you have to wiggle out) was really loose too. I tightened it so that it, too still spins freely. Still popping. I took the bb apart and checked the bearings. The retaining rings looked good as did the bearings. I greased them up some more and reassembled. Still popping. However, we now notice that it feels like it only happens when the drive side crank is just past the apex of its rotation - same place, every rotation.
Any ideas of where to look next?