Flat spot on a front wheel.
I've just picked up an old front racing wheel with a 700 x 23 tire. When I spin it, there is an obvious downward movement at one section of the wheel. It's not a side to side wobble, just downward. When I remove the tire and tube, the wheel spins fine, at least to my eye. However, when I replace the tube and tire and pump them up, I can clearly notice a flat section about 4 inches long, almost as if the tube is sitting too deep in the rim. Both the tire and tube are fairly new and seem to be in good condition. Similarly, the rim doesn't appear to have any obvious kinks in it. Can anyone help? I've never come across this kind of problem before.