I've had a lot of problems with the starter on my '76B, and I found this troubleshooting procedure online that was pretty helpful:
1. Turn off the ignition key, and put the transmission in neutral.
2. Locate the starter relay. It will have four wires attached to it: Black, Brown, White w/ red stripe, and white with brown stripe. The brown wire is hot at all times, key on or off, and does not have a fuse. This is true of ALL brown wires in your car.
3. Without removing any wires from the relay, use a short piece of wire, 18Ga or larger, and jumper between the brown wire and the white/red wire. Touch the wire to the terminals just below the connectors. If the starter works when you do this, there is either a bad connection in the W/R wire between the key switch and the relay, or the key switch is bad. If the starter does not work when you do this, go on to step 4. If it does work, go to step 6.
4. Jumper from the brown wire to the white/brown wire. If the starter works, the relay is bad. If not, go to step 5.
5. At the starter, you will find the other end of the white/brown wire, and the cable from the battery. Jumper from the battery lead to the white/brown wire terminal It doesn't matter if the white/brown wire is still connected or not. If nothing happens, you have a bad solenoid. You should hear a loud, solid clunk.
6. If the starter worked when you performed step No. 3, use a multi-meter test light to check the terminal on the back of the key switch where the white red wire is attached, with the key to the start position. If you have voltage present when you do this, the switch is good, but you have a break in the wire.