I was putting new tires on my 1977 CB750. The rear tire went on and the bead set without too much difficulty, but the front tire was proving more troublesome.
Previously, I had been running the Comstar wheels without tubes — something not officially endorsed until, I believe 1982 — and had used a bit of tire sealant to help the bead seat. Over the years, this liquid sealant had hardened into a concrete-like state, and despite my best efforts I believe a bit of it was impeding the process of seating the tire bead.
I tried all the usual steps to get the bead to seat. I lightly inflated the tire, then methodically bounced the wheel against the ground. I wrapped a tie-down strap around the circumference of the tire, which deforms the tire and forces the bead outward. I even tried inflating the tire as high as 80psi, all to no avail.
Incidentally, do not inflate tires to such high pressures, people really have been badly injured doing this.
After more than an hour, I gave up and left the tire overnight with around 50psi in it. The next day, I was happy to see that the bead had seated itself.
Sometimes the solution is simply to have patience.