[Ed in Tx]

Originally Posted by xargos

For the fun of it, I decided reinstall the old arm but with the new wheel on it (the wheel and tire combined are the right size, but the new tire doesn't fit the old wheel). Oddly enough, that solved the problem. Perhaps the new idler arm is a little bit off somehow. No big deal, though..

I was going to suggest that but you figured it out. Ran into the same problem several times with aftermarket VCR parts, either out of dimension slightly, warped, or who knows. When I serviced VCRs for a living I refused to use anything but OE parts on customer sets I had to guarantee would work.