I have a 1982 Westy. There comes a time (about a month or so) when the
engine loses power... I get into first or second gear, try to pick
up speed and it does, until about halfway up the power curve, and 
then the power goes into a nose-dive. It's annoying. I checked what I
could and have found that I think what causes the lack of power is that
some white deposits (carbon?) accumulate on the metal contacts in the 
distributor cap. It kinda cakes onto the contacts and I have to scrub
them off and sandpaper the contacts. Then the engine performance
gets back to how it should be.

What causes these deposits to get there? What can I do to avoid getting
these deposits to accumulate there?

(I've changed the rotor/cap and it still gets these deposits. I've
sealed all the vacuum hoses; I've put Split Fire plugs [works good, too];
I use 92 octane gasoline; I've changed the spark plug wires/coil wire)

BTW, please keep the responses at layman's level. I'm not a "real"
mechanic.

---roy