Hi Saltlick,
There are a lot of good suggestions here, from everyone.
I'm pretty sure your setup is similar to the one on my '76 400F - there are two bulb sockets in each gauge. All four instrument bulbs operate on the same circuit/fuse. The bulbs are tungsten filament, single contact, BA9 type affairs with a wattage of 3.4 watts at 12 volts. Each has two small pins on the base sides - one 180 degrees opposed from the other. The pins are of equal distance from the bottom (as opposed to other, similar bulbs that have one pin slightly higher than the other). The pins are for the push-in, spring type mount, and require the bulb to be lined up properly when you put it in. The pins are required to install/uninstall and to seat the bulb in place and hold it there relatively firmly.
Any bike store should have them. In Canada I've found I can get similar bulbs at auto supply places, although often the bulbs are cheapish in construction and not as durable as I would like.
Can I ask - do you have a multi-meter or a voltage tester?
And to add to what others have said - when I first started changing those bulbs I was pretty hard on the little rubber housings while I was removing them from the actual gauge housing. (They are designed as a sort of "squish fit" type deal anyway, so they're supposed to be pretty tight to keep out water etc.) It's kinda hard to describe, but if you do go ahead and change the bulb(s) try and ease that rubber piece out very gently. Don't just push from one side, but give it a more sensitive, mini-champagne cork kind of removal - nudge it up from at least two or three sides alternately. Just go in tiny increments and resist the temptation to twist the thing - on such old rubber it's generally pretty easy to rip if you twist. By gently rocking the rubber back and forth you'll be able to work it out without damaging it. (I've got to the point where I take them out with my thumb and first two fingers in a kind of straight smooth rolling pulling action.) Regardless, I think the important thing here is not to twist or unscrew them. And if they haven't been out in a long while it'll be even more important to put light, equal pressure on them to get them out because they'll likely be a little stuck with degraded rubber and whatever else has found it's way into the seal over the years.
After I get the rubbers out I clean them with Mother's "Back to Black". This preserves the little suckers and it also makes them a lot easier to reinstall and to get out the next time.
Finally, if you do remove the bulbs, clean the connector/socket. I use a little bit of fine sandpaper gently around the inside of the socket, blow it out, and spray or wipe it with electrical contact cleaner or alcohol, and coat it with dielectric grease. Before the grease I make sure the bulb fit is tight, and I use a precision screw driver to slightly bend the unattached side of the singe contact in the base upward a bit. (Often in these bases I've found that this little contact is the real problem, and not the bulb at all.) If you're careful, and a bit adventurous, you can actually push the two wires through the rubber bootie without tearing it and get to the basics - like in the picture. I clean it all up real well, expand the spring slightly (if it needs it) and bend out the little, single sided pole contact a bit. I have a feeling that adjusting the spring and pole like this makes a real difference by significantly increasing the reliability of the fitting.
This all sounds a little over wrought I suppose, and I'm just telling you because I didn't like the feeling I got when I ripped up the rubber the first time I changed an instrument bulb.
PS I have yet to attach a picture to one of my posts, so please forgive me if all you get is some letters saying [img] or something like that. I'll figure it out eventually. Good luck!
